Is a 30ohm load safe for my amp?

[scott1568]

I am trying to find out because I am thinking of buying the Ultimate Attentuator that has a 30 ohm load.. I have a 4 ohm amp and an 8 ohm amp, and I don't want to ruin them

I am confused as to why THD and others insist on impedence matching for the safety of your amp, whereas the UA has a one size fits all and seems to be an excellent performer.

[sidehatch]

as long as the speaker load is the same or HIGHER than your amp is puting out you will be ok.

Think of ohms as a water hose fitting. You Cannot hook a faucet to a smaller hose without blowing the fitting and spring a leak but you could put a bucket under the faucet. Thats what you'll be doing with a 30ohm speaker load and an 8 ohm amp.

It might not sound as good as an 8 ohm load but the science behind it is ok.

However, Never hook a lower speaker load up to an amp. IE: 4 ohm speaker and 8 ohm amp. I think of that as forcing water into a much smaller hose or fitting if you will.

[mark norwine]

Going "higher" would cause me some concern because of flyback voltages.

[hasserl]

as long as the speaker load is the same or HIGHER than your amp is puting out you will be ok.

Think of ohms as a water hose fitting. You Cannot hook a faucet to a smaller hose without blowing the fitting and spring a leak but you could put a bucket under the faucet. Thats what you'll be doing with a 30ohm speaker load and an 8 ohm amp.

It might not sound as good as an 8 ohm load but the science behind it is ok.

However, Never hook a lower speaker load up to an amp. IE: 4 ohm speaker and 8 ohm amp. I think of that as forcing water into a much smaller hose or fitting if you will.

WRONG!!!!!!! Sorry dude, but your analogy is all "wet", so to speak.

In this case, the output of a tube amplifier, we have power tubes working against a required load; the load needs to be within a range, not too high, not too low. We don't know what amp the OP has, but since he mentioned one has a 4 ohm load, let's take a chance and assume he has a two speaker Fender, maybe a Pro or a Twin or a Bassman, with two 6L6 power tubes. Those amps are designed to work with approximately 4000 ohms load at the power tubes. The output transformer transforms a 4 ohm speaker load to the 4000 required by the tubes. If you instead put a 30 ohm load on the amp, that 30 ohm load is almost 8 times as great. So let's see what happens.

If z1 * x = z2, and z1 = 4 & z2 = 4000 then x = 1000 right? So then 4 * 1k= 4k, right? So replace the 4 ohm load with a 30 ohm load and you get 30 * 1k = 30k :yikes: Not good, not good at all!

That load WILL result in very high flyback voltage to the tubes, very probably leading to arcing at the tubes and/or sockets. We've actually seen oscilliscope screen shots here showing this. Also, the load will result in increased screen grid current draw, leading to shortened tube life and possibly failed screen grid resistors.

In this case THD is right and UA is wrong, very wrong.

Besides, that analogy is all wrong anyway, increased ohms means increased resistance; or to use your analogy you would be trying to force the same amount of water thru a smaller hose, which aint gonna work toowell. Reduced ohms equals reduced resistance which equals a larger hose. Your analogy is backwards.

[fullerplast]

as long as the speaker load is the same or HIGHER than your amp is puting out you will be ok.

Think of ohms as a water hose fitting. You Cannot hook a faucet to a smaller hose without blowing the fitting and spring a leak but you could put a bucket under the faucet. Thats what you'll be doing with a 30ohm speaker load and an 8 ohm amp.

It might not sound as good as an 8 ohm load but the science behind it is ok.

However, Never hook a lower speaker load up to an amp. IE: 4 ohm speaker and 8 ohm amp. I think of that as forcing water into a much smaller hose or fitting if you will.

Sorry, some bad analogies and incorrect statements here. :nono

For tube amps always preferable to have a lower impedance load than a high impedance load. That's why many amps have shorting jacks on the speaker. It's actually safer for the output to be shorted than to run no load.

Generally, a 2:1 mismatch is safe in either direction. The closer you get to a high impedance load, the more unsafe the operation due to flyback voltages, as Mark said. Damage to the output transformer is the biggest risk.

As a general rule of thumb, I'd consider a 30 ohm load OK for a 16 ohm amp, marginal for an 8 ohm amp, and flirting with disaster for a 4 or 2 ohm amp. YMMV.

[scott1568]

Thank you all for those detailed responses-and the education.

[paolojm]

Can anyone comment on the fact that a speaker doesn't exhibit its rated impedance at all frequencies.
In fact it's impedance is considerably higher than the nominal value given at certain frequencies within the guitar range.
Is flyback voltage at these freqencies less dangerous for valves and OT insulation than at other frequencies?

[hasserl]

Can anyone comment on the fact that a speaker doesn't exhibit its rated impedance at all frequencies.
In fact it's impedance is considerably higher than the nominal value given at certain frequencies within the guitar range.
Is flyback voltage at these freqencies less dangerous for valves and OT insulation than at other frequencies?

The impedance is frequency related, but a 4 ohm speaker would not present 30 ohms impedance within the range of frequencies produced by a guitar, and an 8 ohm speaker only present a 30 ohm impedance at the extremes of a guitar's frequency range (around 80 to 100 hz, & above 5kHz). And of course, this is impedance, not resistance.

[jay42]

I have it on some authority that UA will make them with the load of your choice. You have to deal directly with them, not a store or dealer...and I think the guy has to be in the mood.

[phsyconoodler]

A customer came by with a boutique amp that he bought on E-
bay and his ultimate attenuator.The problem was it kept blowing fuses with the attenuator.I checked out the amp and made sure there were no tube,socket or other issues to cause the fuse to blow and couldn't find anything.Without the attenuator it worked fine at any volume level.No blown fuses.As soon as the attenuator was added into the output and turned down to low levels the fuse blew every time.
Now I didn't design or build the amp in question,but it has zero issues without that damned Ultimate attenuator.
I advised he use VVR instead.He subsequently sold the amp and attenuator.
30 ohms is quite a load in my opinion,and can cause troubles in some amps.I don't like the ratio at all.But that's just my experience with them.

[paolojm]

The impedance is frequency related, but a 4 ohm speaker would not present 30 ohms impedance within the range of frequencies produced by a guitar, and an 8 ohm speaker only present a 30 ohm impedance at the extremes of a guitar's frequency range (around 80 to 100 hz, & above 5kHz). And of course, this is impedance, not resistance.

Sorry, but that doesn't answer the question at all.
80-100Hz is not an "extreme" of guitar frequency, and the impedance usually starts rising steadily from 2k onwards too, which is also well within guitar range.

[jay42]

Is flyback voltage at these frequencies less dangerous for valves and OT insulation than at other frequencies?It makes some sense...plus the resonance frequency is open air. Once you couple it to a non-infinite baffle, things change a bit. I would suggest that the greater the upward mismatch, the greater the occurance of beyond-spec flyback viewed both as snapshot and cummulative.

It sounds like you're familiar with the Z curve on a speaker. These things are all woofers in truth, so the curve increasing with frequency has fixed limits both in the mechanical ability to respond and the lack of those frequencies being sourced by a guitar or bass.

Years ago an ME said something to me like, "Nominal is best." Well...it should be best, if the design is right. Anyway, tubes and transformers are designed for certain characteristics and straying from those is not for the cash-poor. Apparently, the guy who designed the UA is of the opinion that it sounds the best that way.

[hasserl]

Sorry, but that doesn't answer the question at all.
80-100Hz is not an "extreme" of guitar frequency, and the impedance usually starts rising steadily from 2k onwards too, which is also well within guitar range.

Low E = 82hz Maybe instead of "extreme" I should have said at either end of the frequency range for guitar. Keep in mind however, that a speaker is a reactive device, not resistive; and as such it interacts differently with an amp compared to a resistor. However, mismatch your speaker load too highly and flyback voltage is an issue. Many an amp has failed the OT due to running a mismatched load.

You can doubt this if you want, but we've seen the oscilloscope screen shots right here showing the high flyback voltage when mismatching an amp into a fixed resistive load of 4 times greater than optimal. That would be equal to running an 8 ohm amp into a 30 ohm load. Running a 4 ohm load into one is even worse.

[paolojm]

Low E = 82hz Maybe instead of "extreme" I should have said at either end of the frequency range for guitar. Keep in mind however, that a speaker is a reactive device, not resistive; and as such it interacts differently with an amp compared to a resistor. However, mismatch your speaker load too highly and flyback voltage is an issue. Many an amp has failed the OT due to running a mismatched load.

You can doubt this if you want, but we've seen the oscilloscope screen shots right here showing the high flyback voltage when mismatching an amp into a fixed resistive load of 4 times greater than optimal. That would be equal to running an 8 ohm amp into a 30 ohm load. Running a 4 ohm load into one is even worse.

I understand perfectly well a speaker is reactive and that's why I ask the question.
I'm not pretending to understand all of this, but I don't understand why we don't see large flyback voltage from these specific frequencies?
Surely because of the reactive nature of a speaker, and it's increased impedance at certain frequencies, we must see corresponding flyback voltages, or am I missing something?

My opinion, from what little knowledge I have, is that with a mis match these high impedances at certain frequencies become more problematic, and obviously at some point cause meltdown.

If the UA is a purely resistive load @ 30 ohms, then I don't see how it is any worse than connecting an 8 ohm speaker to an 8 ohm amp, except that the normal flyback experienced at certain frequencies is now being experienced at all frequencies? Perhaps this increased frequency range at which flyback occurs causes cummulitive flyback which is more detrimental for valves and winding insulation?

On your test; did you also check for flyback in the 80 - 100Hz range from a perfectly matched speaker? How about the 3-4k range?

[phsyconoodler]

A tube has a load that it likes to see.The load lines are developed so the tube is in it's more or less optimum area of operation.The speaker provides that load and the plates of the tubes are connected to the OT and then ultimately to the speaker.When those loads are altered,the tube,being a rather archaic device,reacts.Either in a positive way or a negative way.Electrons are tripping around inside that glass bottle and too much load makes the tube do weird things.
same as not enough load.Then the OT tries to compensate and can't,so it saturates the core and heats up and the voltage is spewed all over the place.Fuses blow,tubes expire,transformers melt.
Tubes are the stumbling block here,not the speaker impedance or load from an attenuator.

[paolojm]

A tube has a load that it likes to see.The load lines are developed so the tube is in it's more or less optimum area of operation.The speaker provides that load and the plates of the tubes are connected to the OT and then ultimately to the speaker.When those loads are altered,the tube,being a rather archaic device,reacts.Either in a positive way or a negative way.Electrons are tripping around inside that glass bottle and too much load makes the tube do weird things.
same as not enough load.Then the OT tries to compensate and can't,so it saturates the core and heats up and the voltage is spewed all over the place.Fuses blow,tubes expire,transformers melt.
Tubes are the stumbling block here,not the speaker impedance or load from an attenuator.

Ok, but you understand that a speaker exibits different impedance values at various frequencies, and so presents a much different load line at these frequencies, right?

[phsyconoodler]

Yes,that is true,but the load requirements are matched to the tubes and output transformers.
I look at it like a water line.The amp is cranked and the water pressure is at it's highest.Then a valve gets put in the out line and it gets shut off.The pressure gets high on the feed side and looks for a way out.If there are any weak links,it finds those and exploits them.
Electricity follows the path of least resistance.Why do so many amps blow up when using the Ultimate attenuator? Because they can't handle the pressure.
There can be a number of reasons,like underdesigned transformers,poor quality tubes,etc.But the fact is,a tube amp is an old design and it was never originally meant to be subjected to the punishment they get from attenuators.
Yes I realize that the impedance load changes with audio speakers but when using parameters that are beyond design limits,somethings gotta give.

[paolojm]

Yes,that is true,but the load requirements are matched to the tubes and output transformers.
I look at it like a water line.The amp is cranked and the water pressure is at it's highest.Then a valve gets put in the out line and it gets shut off.The pressure gets high on the feed side and looks for a way out.If there are any weak links,it finds those and exploits them.
Electricity follows the path of least resistance.Why do so many amps blow up when using the Ultimate attenuator? Because they can't handle the pressure.
There can be a number of reasons,like underdesigned transformers,poor quality tubes,etc.But the fact is,a tube amp is an old design and it was never originally meant to be subjected to the punishment they get from attenuators.
Yes I realize that the impedance load changes with audio speakers but when using parameters that are beyond design limits,somethings gotta give.

My point here is that, assuming the UA is a purely resistive load, then I can't see how it would do any more damage than a regular speaker.
A regular speaker rises above 30 ohms impedance for a sustantial range of frequencies within the guitar range, even when matched perfectly to the amp.

[jay42]

A regular speaker rises above 30 ohms impedance for a sustantial range of frequencies within the guitar range, even when matched perfectly to the amp.No it doesn't. This is what I was trying to convey before The free air resonance, which could be 55Hz, 75Hz, who-knows-what-hertz, isn't exactly that when in a speaker cabinet of any design. The rising Z you see above 5KHz, isn't really important, as the the mechanics of the speaker can't respond to it and the amp isn't putting that much out at those frequencies anyway.

[sidehatch]

Since we have some knowlegible amp guru's on the line here please set me straight on this. I dont know much about the UA but the info that have read many books states you can go higher in a speaker mismatch. IE: 4 ohm amp can have a 4 , 8 or 16 ohm speaker cab. Heck, I have been running a 2 ohm super reverb with 4,8 and 16 ohm cabs for years.

I have several books on amp maintenance and repair but I'm no repairman by any stretch. I know that I should never run my 8 ohm amp into a 4 ohm cab and have never tried it because of the info I've read.

So my question is: what is the cutoff point on where it becomes harmful to the amp?

I read the faucet theory in one of my books(can't remember which one and not assuming the author is right).

I've never heard of a 30 ohm load. In my experience its 2,4,8 or 16 ohm . Is this specific to using an attenuater or general amp knowledge?

Is there any chance of Damage using my 2 ohm super reverb witha 8 or 16 ohm cab? I've gigged it hard for years like that but I know that doesn't really mean anything.

Lets, clear this up for all the guys like me that have been reading perhaps false info.

Thanks

[jay42]

So my question is: what is the cutoff point on where it becomes harmful to the amp?Nominal is best...or another rule of thumb...don't mismatch any amp with EL-34s. I wouldn't recommend what you've done to your Super Reverb. Running it at 4 ohms is fine, but beyond that is a bit like asking if a plastic part will break. All plastic parts break...the real question is when, not if.

The 30 ohm load is unique to the Ultimate Attenuator. 32 ohm speakers are used in Ampeg SVT 8-10" cabinets, but are generally not found elsewhere.

[SatelliteAmps]

So my question is: what is the cutoff point on where it becomes harmful to the amp?

Is there any chance of Damage using my 2 ohm super reverb witha 8 or 16 ohm cab? I've gigged it hard for years like that but I know that doesn't really mean anything.

Lets, clear this up for all the guys like me that have been reading perhaps false info.


Anytime there is a mismatch, there is a possibility of damage. Bigger mismatches mean bigger chances of damage. It doesn't matter which way you go, there is still a possibility. Fender's USUALLY are ok with a one time mismatch, meaning running a 4Ω amp with an 8Ω or 2Ω load is ok. Some amps are much more sensitive to these mismatches.


The 30 ohm load is unique to the Ultimate Attenuator. 32 ohm speakers are used in Ampeg SVT 8-10" cabinets, but are generally not found elsewhere.

I just got corrected about this. Fender used 32Ω speakers in some of their PA's.

[paolojm]

No it doesn't. This is what I was trying to convey before The free air resonance, which could be 55Hz, 75Hz, who-knows-what-hertz, isn't exactly that when in a speaker cabinet of any design. The rising Z you see above 5KHz, isn't really important, as the the mechanics of the speaker can't respond to it and the amp isn't putting that much out at those frequencies anyway.

So are you saying that, in actual practise as opposed to theory, a speaker exhibits a fairly constant nomimal impedance across the guitar range of frequencies?
Not trying to have a go, just trying to understand this better.

[jay42]

So are you saying that, in actual practise as opposed to theory, a speaker exhibits a fairly constant nomimal impedance across the guitar range of frequencies?Just trying to convey that it doesn't vary as widely as you supposed, based on whatever graph(s) you must have seen. It does vary.

[pdf64]

Celestion's Dr dB has sent me an impedance/frequency plot for a V30, he says it's basically the same for any of their speakers. At resonance 70Hz, on an open baffle (ie like a regular combo), it's around 100ohms (8ohm nominal). By 3kHz, it's 16ohms.
In a closed back cab, the resonant frequency will be pushed higher, and the resonant peak will be higher.
You can test this yourselves easily with a signal generator, series resistor (1k) and voltmeter. And a speaker.
But running an amp designed for a 2ohm load into a 16ohm speaker, other than at domestic levels, is a really bad idea. Where did you read that was ok? Peter.

[SatelliteAmps]

I think a very crucial piece of information is being ignored in this discussion. What the output wants to see is relative to what the speaker is getting. Neither is completely static. By trying to force the impedance value of the speaker at a specific value, is ignoring what the OT is doing at the same time. Yes, at different frequencies, the actual impedance load of a speaker will fluctuate. That doesn't mean it is or isn't matched up at specific frequencies. This is why we test things at a static point, a speaker with no signal going through it.

Also, I haven't seen anyone say that running a 16Ω load into a 2Ω output transformer was a good idea. I said it was a bad idea.

[paolojm]

I think a very crucial piece of information is being ignored in this discussion. What the output wants to see is relative to what the speaker is getting. Neither is completely static. By trying to force the impedance value of the speaker at a specific value, is ignoring what the OT is doing at the same time. Yes, at different frequencies, the actual impedance load of a speaker will fluctuate. That doesn't mean it is or isn't matched up at specific frequencies. This is why we test things at a static point, a speaker with no signal going through it.

Also, I haven't seen anyone say that running a 16Ω load into a 2Ω output transformer was a good idea. I said it was a bad idea.

This I would agree with, the peaks of flyback voltage in this instance would be 8 times what would happen with a 2 ohm load, and I would also agree that a 4 ohm speaker may even be damaging to some output transformers.

What I was trying to say it that, assuming the UA is a purely resistive load, then these peaks aren't there with it, and so I don't see how it would be any more damaging for the amp than using a matched speaker.
Of course this is assuming the UA is purely resistive, and I don't know that. Does anyone else?
If it is reactive, and designed to simulate a real speaker load like other similar products, then it would have massive peaks vs nominal and I don't how they could get away with selling such a harmful and potentially dangerous product. Surely it couldn't achieve the relevent safety standards for an electrical product intended for domestic usage, if it is reactive?

[drgonzoguitar]

This I would agree with, the peaks of flyback voltage in this instance would be 8 times what would happen with a 2 ohm load, and I would also agree that a 4 ohm speaker may even be damaging to some output transformers.

What I was trying to say it that, assuming the UA is a purely resistive load, then these peaks aren't there with it, and so I don't see how it would be any more damaging for the amp than using a matched speaker.
Of course this is assuming the UA is purely resistive, and I don't know that. Does anyone else?
If it is reactive, and designed to simulate a real speaker load like other similar products, then it would have massive peaks vs nominal and I don't how they could get away with selling such a harmful and potentially dangerous product. Surely it couldn't achieve the relevent safety standards for an electrical product intended for domestic usage, if it is reactive?

As far as I have seen, the UA is a resistive load to line level with SS power amp. Rumor is the 30ohm selection of resistive load was selected due to its quality of sound for re-amping.

As I was explaining this to my wife, she said, "Huh. Sounds like heroin for guitar amps. Sounds good, a lot of fun, die a helluva lot faster." So yes, the UA is like heroin for your amp. Sure it'll sound good, but it's gonna die a lot faster than a normal resistive or speaker (reactive) load.

[jay42]

Is there any chance of Damage using my 2 ohm super reverb witha 8 or 16 ohm cab?Question -- when you talk of using a super reverb with a 16 ohm cabinet, do you mean connected to the EXT SPKR jack or in place of the normal 4 speaker (2 ohm) load? It makes all the difference.

[phsyconoodler]

Quote: "What I was trying to say it that, assuming the UA is a purely resistive load, then these peaks aren't there with it, and so I don't see how it would be any more damaging for the amp than using a matched speaker.
Of course this is assuming the UA is purely resistive, and I don't know that. Does anyone else? "

You are trying to quantify something here scientifically,when you need to use a little common sense.Whether it is a reactive or constant load is not the issue here.It's simply a load and it's too much for a tube and transformer.
I test amps at work using a load box.It is a constant load,not reactive like a speaer,and if I load an amp that wants to see 8 ohms with a 30 ohm load,it will likely find any weak spots and exploit them in short order.
I can run most amps for hours at the recommended load.However,increasing or decreasing that load can spell disaster for an amp.I have blown up a couple of amps using the wrong loading.And I'm not talking about 30 ohms.

Now here's the rub: if you use the Ultimate Attenuator to just bring the volume down a little,it works fine.It's when you use it in your bedroom that it starts eating parts.It's how much you drive the amp vs how low you want to turn down.

[jay42]

...if you use the Ultimate Attenuator to just bring the volume down a little,it works fine.I think you're forgetting that it's a reamp device, unlike an Airbrake which is shunting some load off.

[strat a various]

Anytime there is a mismatch, there is a possibility of damage. Bigger mismatches mean bigger chances of damage. It doesn't matter which way you go, there is still a possibility. Fender's USUALLY are ok with a one time mismatch, meaning running a 4Ω amp with an 8Ω or 2Ω load is ok. Some amps are much more sensitive to these mismatches.


I just got corrected about this. Fender used 32Ω speakers in some of their PA's.

Hey, don't forget the 12" speakers in the Rhodes suitcase piano.

[SatelliteAmps]

This I would agree with, the peaks of flyback voltage in this instance would be 8 times what would happen with a 2 ohm load, and I would also agree that a 4 ohm speaker may even be damaging to some output transformers.

What I was trying to say it that, assuming the UA is a purely resistive load, then these peaks aren't there with it, and so I don't see how it would be any more damaging for the amp than using a matched speaker.
Of course this is assuming the UA is purely resistive, and I don't know that. Does anyone else?
If it is reactive, and designed to simulate a real speaker load like other similar products, then it would have massive peaks vs nominal and I don't how they could get away with selling such a harmful and potentially dangerous product. Surely it couldn't achieve the relevent safety standards for an electrical product intended for domestic usage, if it is reactive?

The UA uses a 30Ω 100 watt fixed resistor. It is not reactive in any way. There are a number of very in depth threads on here that go into it's designs and potential flaws (that do cause serious harm to certain amplifiers). Again, this has to do more with matching to the output transformer than just the transformer or the load by itself. By matching the proper load to a transformer, an amplifier can work at it's optimum. It doesn't matter if the load is a speaker or a fixed resistor. The amplifier will still work correctly if properly matched. If improperly matched, then damage can and will happen.

Hey, don't forget the 12" speakers in the Rhodes suitcase piano.

I knew after I put that up there that I had forgotten something, and I was sure it was from you. Thanks for remembering.

[strat a various]

I knew after I put that up there that I had forgotten something, and I was sure it was from you. Thanks for remembering.

no problemo, it's my job.

[TubeStack]

Hey guys - I could really use some help here.

I have a Ho attenuator on the way. I just traded in my Carvin X100B half-stack for a Marshall JCM 800 (2204), after already ordering the Ho unit.

From what I've read, there's less danger with the Ho if you run 8 or 16 ohms out of the amp. With my old Carvin, this would've worked, without any mismatch on the amp/speaker relationship (amp had 4-8-16 ohm switch, cabinet was 8 ohms).

However, my new (used) Marshall only runs 4 or 8 ohms, there's no option for 16.

The 4x12 Marshall cabinet has a 4 ohm jack and a 16 ohm jack, but no 8ohm jack (except if running stereo, which I'm not).

My question is, using the Ho attenuator, can I run the head at 8ohms into the attenuator, and then use the 16ohm input on the speaker cab? Is there any danger in this setup?

From what I've read, it's okay to be off one level, either way, with no attenuator, but I'm not sure how Ho's unit factors into this?

[TubeStack]

Hey guys - I could really use some help here.

I have a Ho attenuator on the way. I just traded in my Carvin X100B half-stack for a Marshall JCM 800 (2204), after already ordering the Ho unit.

From what I've read, there's less danger with the Ho if you run 8 or 16 ohms out of the amp. With my old Carvin, this would've worked, without any mismatch on the amp/speaker relationship (amp had 4-8-16 ohm switch, cabinet was 8 ohms).

However, my new (used) Marshall only runs 4 or 8 ohms, there's no option for 16.

The 4x12 Marshall cabinet has a 4 ohm jack and a 16 ohm jack, but no 8ohm jack (except if running stereo, which I'm not).

My question is, using the Ho attenuator, can I run the head at 8ohms into the attenuator, and then use the 16ohm input on the speaker cab? Is there any danger in this setup?

From what I've read, it's okay to be off one level, either way, with no attenuator, but I'm not sure how Ho's unit factors into this?

I love quoting myself... :D

Okay, just got off the phone with the man himself, Mr. Ho.

He said if I run the head at 8 and use the 16 ohm speaker jack on the cab, there'll be no problems. Hope he's right....

[TTripp]

I love quoting myself... :D

Okay, just got off the phone with the man himself, Mr. Ho.

He said if I run the head at 8 and use the 16 ohm speaker jack on the cab, there'll be no problems. Hope he's right....

Doesn't matter what your cab rating is. Your amp never sees it. All it sees is the 30 ohm load of the Ho. What Ho is saying is don't worry about running an 8 ohm amp into a 30 ohm load. Ya pays your dime, ya takes your chances.

[SatelliteAmps]

A Marshall JCM 800 2204 should have a 16Ω tap. There is still a danger of running an amp into a 30Ω load. If your amp has no 16Ω load on it, it would be even more dangerous to run an 8Ω impedance into a 30Ω load. This is what causes flyback voltages that kill output transformers and arc tube sockets. If he hasn't sent it yet, call Mr. Ho back, and have him install a proper load of 8Ω or 16Ω into your attenuator. He will do it, and your amp will be safer.

[jay42]

A Marshall JCM 800 2204 should have a 16Ω tap. There is still a danger of running an amp into a 30Ω load. If your amp has no 16Ω load on it, it would be even more dangerous to run an 8Ω impedance into a 30Ω load. This is what causes flyback voltages that kill output transformers and arc tube sockets. If he hasn't sent it yet, call Mr. Ho back, and have him install a proper load of 8Ω or 16Ω into your attenuator. He will do it, and your amp will be safer.Canadian 2204? I once saw a thread suggesting that the 16 ohm tap is terminated in the bell end of the OT. No verification, however. Severe mismatch.

[TubeStack]

Thanks for the info, guys - I appreciate it.

And yes, mine is a Canadian version of the 2204 - they don't have a 16 ohm setting.

From LegendaryTones.com: Marshall Shopper's Guide: (http://www.legendarytones.com/marshallshoppers2.html)

"From the late ‘70s, the Canadian (CSA) models reverted to an interesting arrangement cosmetically and can be identified by their use of small metal toggles (different from those used in earlier standard-issue JMPs) as well as a similar square LED pilot lamp. Rather unusual in appearance on the bold-look (and later JCM 800 models), these amplifiers also retained the use of EL34s like the European versions. From circa '79, CSA models incorporated additional circuit fusing "under the hood" as well as an output transformer tap for 8 and 4 ohms only with the 16 ohm tap removed. It is unclear to me as to why these various changes were made only to the Canadian models, but one has to suspect that much of it is due in part to stricter Canadian electrical regulations."

[jay42]

The board in my USA 2204 has one or two un-stuffed fuse positions.

[SatelliteAmps]

I thought there was still a tap on the transformer, it just isn't hooked up. I could be wrong.

[ripoffriffs]

You can get one of these to ease the risk of amp/transformer damage:

https://taweber.powweb.com/store/attenmatch.htm

[SatelliteAmps]

Or for about $12, you can just replace the 30Ω 100w resistor in the UA/HO with a properly matched 16 or 8 ohm one. That's mainly what the Attenuator Matcher is doing.

[TubeStack]

You know, after all the excitement and fuss, I ended up selling my Ho. It sounded like complete ass, like a tinny radio broadcast of a recording of my tone. It also wasn't loud enough (ironically) went cranked. (Only wanted to take the edge off and still play be able to play in a loud rock band.)

Not sure if it's the Canadian JCM800, can-only-run-at-4ohms issue, but the Ho/UA definitely did not work for me (with all due respect to Mr. Ho, who's a cool dude.)

[tacorivers]

In the real world, there are not a whole lot of amps that have blown up using the UA. Everytime this discussion comes up, people argue that the UA should "theoretically" cause amp damage. The empirical evidence, however, just isn't there.

[hasserl]

We've seen numerous threads in this forum alone from users that posted here looking for help after using a UA. Items such as failed tubes and blown fuses and amps being sent back to the maker for repairs.

[tacorivers]

It is still not a significant percentage of all UA users.

[hasserl]

Significant compared to what? Compared to other attenuators? Yes it is.

[Keyser Soze]

A Marshall JCM 800 2204 should have a 16Ω tap. There is still a danger of running an amp into a 30Ω load. If your amp has no 16Ω load on it, it would be even more dangerous to run an 8Ω impedance into a 30Ω load. This is what causes flyback voltages that kill output transformers and arc tube sockets. If he hasn't sent it yet, call Mr. Ho back, and have him install a proper load of 8Ω or 16Ω into your attenuator. He will do it, and your amp will be safer.

This is the part where I get confused.

Reactive loads can create flyback voltages. Any time a magnetic field collapses the energy can be returned in the form of electric current. This is a normal operating aspect of every tube amp and a properly designed amp should handle them just fine so long as everything (including speaker load) is kept within design parameters.

The problem occurs when we deviate from proper operating parameters, in such cases the flyback voltages can exceed the handling capability of the amp and bad things may follow.

So a speaker mismatch can pose a real problem for the OT and the tubes.

But suppose you just hooked your output tap up to a honking big resistor and simply dissipated the output current in the form of heat. No magnetic fields means no flyback voltage right?

Now suppose you put said honking big resistor in line with your otherwise properly matched speaker load. Sure it's probably gonna sound like deep fried manure but it's should also be harmless to your amp because even though the total load is much higher than the 'proper load' that 'HB' resistor is going to dissipate so much of the current (both flowing from the OT and back to the OT) so as to render any actual flyback voltages well within the OTs safe handling parameters.

At least, in theory...

[tacorivers]

Significant compared to what? Compared to other attenuators? Yes it is.

Significant enough to justify the concerns on this thread. If the opinions stated herein were true, then one would expect to see a lot more blown amps. That just hasn't happened.

[hasserl]

It only has to happen to you once and it's significant enough.

[hasserl]

This is the part where I get confused.

Reactive loads can create flyback voltages. Any time a magnetic field collapses the energy can be returned in the form of electric current. This is a normal operating aspect of every tube amp and a properly designed amp should handle them just fine so long as everything (including speaker load) is kept within design parameters.

The problem occurs when we deviate from proper operating parameters, in such cases the flyback voltages can exceed the handling capability of the amp and bad things may follow.

So a speaker mismatch can pose a real problem for the OT and the tubes.

But suppose you just hooked your output tap up to a honking big resistor and simply dissipated the output current in the form of heat. No magnetic fields means no flyback voltage right?

Now suppose you put said honking big resistor in line with your otherwise properly matched speaker load. Sure it's probably gonna sound like deep fried manure but it's should also be harmless to your amp because even though the total load is much higher than the 'proper load' that 'HB' resistor is going to dissipate so much of the current (both flowing from the OT and back to the OT) so as to render any actual flyback voltages well within the OTs safe handling parameters.

At least, in theory...

Keyser, this isn't accurate. We've seen scope screen shots here in this forum posted by a well known and respected amp builder, known for his technical expertise and the tech info available on his website, showing the high flyback voltage at the power tubes from a 4:1 mismatch into a fixed resistive load. The voltages were well into the thousands of volts.

In addition to the high flyback voltage he also noted increased screen grid current caused by the mismatch that was sufficient to cause over heating of the screen grid resistors.

[SatelliteAmps]

Significant enough to justify the concerns on this thread. If the opinions stated herein were true, then one would expect to see a lot more blown amps. That just hasn't happened.

It has happened. This isn't just opinions. Just because you don't want to believe it does not make it not true.

[Keyser Soze]

Keyser, this isn't accurate. We've seen scope screen shots here in this forum posted by a well known and respected amp builder, known for his technical expertise and the tech info available on his website, showing the high flyback voltage at the power tubes from a 4:1 mismatch into a fixed resistive load. The voltages were well into the thousands of volts.

In addition to the high flyback voltage he also noted increased screen grid current caused by the mismatch that was sufficient to cause over heating of the screen grid resistors.

I hear ya, that's why I opened and closed with the big caveats.

I wasn't so much disagreeing with the reported problems with resistive load devices - those I accept - more laying out my limited understanding of what would be going on in inside the amp to cause the observed problems.

Which still leaves me wondering just where a flyback voltage would come from in the absence of a reactive load. For the life of me I just don't know, and that leads me to conclude there is a great big hole in my knowledge set, and that is very troubling when it involves something that seems like it should be fundamentally obvious.

So confused I remain.:huh

[pgissi]

I hear ya, that's why I opened and closed with the big caveats.

I wasn't so much disagreeing with the reported problems with resistive load devices - those I accept - more laying out my limited understanding of what would be going on in inside the amp to cause the observed problems.

Which still leaves me wondering just where a flyback voltage would come from in the absence of a reactive load. For the life of me I just don't know, and that leads me to conclude there is a great big hole in my knowledge set, and that is very troubling when it involves something that seems like it should be fundamentally obvious.

So confused I remain.:huh



Or it could mean that in that ONE example mentioned above, you know the one mentioning the respected builder conducting this experiment did not identify some condition beyond the attenuator and in this case a simple resistive load is/was the cause but possibly attributed to that specific amp or its design, faulty components or build etc. In the course of science, sometimes the real data is elusive, hence the rigorous standards of scientific analysis in place today and what do we see, we see even organizations, agencies and individuals known for doing the best science getting it wrong on occassion and shuttles, rockets or planes are lost, medicine gets it wrong and surely amp builders are included and are not infallible.

I am done suspending belief to the degree any conclusions are drawn 100% when the results are obtained with less than real scientific methodology. For me to believe any such experiment, it must be conducted like other research where parallel experiments known as controls are also conducted to then be able to derive better data, especially since in that example a resistive load was used not a reactive, WTF?

This should raise your awareness, there is something else happening here, its not so cut and dry, neatly explained away.

This is not a condemnation of that builder whomever it is and I would tend to want to believe but it seems more research would be required beyond conducting it on one amp and under one set of conditions if thats the case.

I say this based on the fact that these attenuators are out there and being used and no one has provided any real numbers as to the failure rates being seen in amps using them, conjecture, opinion, rumor?

I would think I would have heard of class action lawsuits and the makers of these devices themselves having built them would have done testing and came to similar conclusions.

Fact is, there may be some other underlying conditions with specific amps or designs that fail under that one particular condition.

You are as confused no more than those who claim to have the answer. Claim all day, the data is just not conclusive and tonight how many attenuators both resistive and reactive will be fired up and used and in some of those cases, mismatches are used be they intentional or not.

Consensus among posters in a forum does not science make

Personally, I hate attenuators, I prefer lower wattage amps coupled with lower wattage speakers versus starving the transducer thats 50% of an amps voice, the speaker. But I see the practical use for them.

I am not being a smart arse here, we need better scientific method and that means taking multiple amps from multiple manufacs and subjecting them to this under varying conditions and who is gonna do that$$$$$$

Until then we have the art of conversation which is less about science and more about validating our own beliefs by relating them and in that we are all the same.

[tacorivers]

It has happened. This isn't just opinions. Just because you don't want to believe it does not make it not true.

No it has not. There is anecdotal evidence of a few amp problems that may or may not have been caused by the UA. On the flip side, there are probably well over 1000 UA's in the market being used on a daily basis.

[pgissi]

I think the best way to sum this up is to identify the condition that presents the least risk and to quote hasserl-

"showing the high flyback voltage at the power tubes from a 4:1 mismatch into a fixed resistive load. The voltages were well into the thousands of volts"

Bottom line, a 4:1 mismatch is just wrong in any configuration and the flyback generated with a resistive load in this extreme mismatch must be caused by the load resistor especially a wirewound type which acts like and inductor, it has to be that, what else is it?

And John Phillips sums it up as so long ago-

http://www.thegearpage.net/board/showthread.php?t=64376&highlight=resistive+load&page=2 (http://www.thegearpage.net/board/showthread.php?t=64376&highlight=resistive+load&page=2)


It is safe to run a 16-ohm amp into a 30-ohm resistive load. In fact, that value is probably close to what the average of a real 16-ohm speaker is over the audio range (they rise with frequency). It might be safe with an 8-ohm amp, but if you have the option of running at 16, I'd definitely do it.

[brad347]

Something that wasn't mentioned in this thread with regards to the first poster and his misinformation:

Perhaps the books he was reading were not about TUBE amps?

The impedance matching rules for solid-state are a VERY different matter.

As for the UA/resistive v. reactive debate, I don't have anything to contribute but I find it interesting.

The only thing I have to add is that a guitar's frequency range is not the only relevant thing when determining the total reactance + resistance of a speaker in practice.

A fuzz pedal will produce significant content above 5k, even in an amp whose speaker rolls off significantly above that on paper. Once a friend did an experiment (to prove me wrong, actually) where he reproduced 15k sine tones on a 12" weber speaker in his studio. Yes they were down significantly compared to other frequencies, but the speaker did reproduce the 15k in limited amounts.

Additionally, intermodulation from distorted chords, etc. will produce content FAR below 82 Hz. Take a rectified fuzz pedal like a Foxx or Superfuzz and play some power chords that are very well in-tune and you will get lower notes than can be made on guitar due to the intermodulation.

Also, people use octave pedals, synths, alternate tunings like Drop C, etc. with guitar amps-- not just clean guitars in standard tuning-- and most of the time the speakers will die before the amp does.

The range of reactance + resistance an amp will tolerate seems to be pretty generous when the nominal impedance is pretty close to in-range.

[SatelliteAmps]

No it has not. There is anecdotal evidence of a few amp problems that may or may not have been caused by the UA. On the flip side, there are probably well over 1000 UA's in the market being used on a daily basis.

Your definition of "few" is much different than mine . Also, you may want to double check on that "1000 UAs" in the market figure as well.

For what it's worth, I like the concept of the UA, and I own one. I modified it to have a proper load (as anyone who gets an HO version can specify. Ho designed it, and he understands this issue. He feels that the impedance mismatch sounds better).

Any tube amp does not like to have an impedance mismatch. I don't know why this is so misunderstood here. It is nothing personal or specific to any product. Some output transformers are more sensitive than others, but the end result of mismatching is eventual flyback voltages. How much flyback voltage depends on a number of things, but there is enough potential to fry an OT or arc a tube socket.

This is NOT opinion.

To take the UA out of the equation, you might understand it better. Imagine you are playing an amp that is set for a 16Ω load, and you are playing through a 4x12 wired in series/parallel to get a 16Ω load. Now lets say you blow a single speaker. This will take out one half of the speakers (as they are wired in series). This will instantly create an impedance mismatch. There is now a large increase in the chance for a flyback voltage to occur. When this happens a voltage goes backwards through the OT, creating a high voltage condition. This can arc the inside of an OT and fry it, or it can hit a tube socket and arc the socket. Does it always happen? Nope. A lot depends on how long it is played like this, and at what volumes. Is it worth playing with a partially blown cab to see how long it takes for an OT to blow with it? Not in my opinion.

Please do yourself a favor and read the other thread where all of this is expressly laid out, before accusing all of us of any kind of hearsay, anecdotal-evidence or blind faith.

[tacorivers]

Your definition of "few" is much different than mine .

Well, how many confirmed cases are there where the UA damaged an amplifier? I did read the other thread, and there weren't that many mentioned.

Your are correct, I don't know the total number of UA's on the market. Only the manufacturer does. But, given the low number of reported failures, I bet that the failrue rate is < .5%.

[phsyconoodler]

I had two customers that the UA ate parts on.A Clone JTM 45 and a real Marshall 1987.That's just two from me alone.

[terminal]

a few reports of Attentuators causing failures.

even fewer reports of Attentuators making amps sound good.

anyone ever find them to improve sound?

[SatelliteAmps]

Well, how many confirmed cases are there where the UA damaged an amplifier? I did read the other thread, and there weren't that many mentioned.

Your are correct, I don't know the total number of UA's on the market. Only the manufacturer does. But, given the low number of reported failures, I bet that the failrue rate is < .5%.

I have had three. I've never had any other attenuator cause a problem that is so directly linkable to an impedance mismatch.

[reaiken]

A purely resistive load will not prevent "flyback" voltages - they are primarily a function of the leakage inductance of the transformer when the output switches on and off quickly, like when you drive the output stage into a hard clipping square wave. Different transformers have different amounts of leakage inductance, it is a function of the way the output transformer is wound. The magnitude of the spikes will, however, be a function of the reflected impedance, so higher load impedance mismatches will result in proportionately higher spikes.

I, too, used to believe a purely resistive load would be safe at most any reasonable mismatch - until I put one on the bench and took a hard look at the scope while driving the snot out of it!

While flyback voltages can cause arcing inside tubes an across sockets, the most likely cause of fuse blowing and amplifier damage is the massive increase in screen grid current that occurs into a 4:1 mismatch, or even a 2:1 mismatch when driving the output stage hard. I was really surprised by this one when I saw it on the bench, especially when there is a good bit of duty-cycle modulation occurring, such as you normally see in overdriven non-master Marshalls. The waveform asymmetry really drives one side much harder than the other, which can aggravate the problem.

Let me clarify - if the amp is not overdriven, there is no chance whatsoever of damage into a purely resistive load of even 4x the nominal impedance. You will see no flyback effects, and no excessive screen grid current, and it will be perfectly safe. The problems only occur under heavy clipping in the output stage. If you are not one to drive the amp hard, running a mismatched load will more than likely be entirely safe. But, if you are one to dime the amp and hit it hard with a boost for leads, watch out!!!

Randall Aiken

[brad347]

Well there you go. The aiken has spoken.

[reaiken]

Well there you go. The aiken has spoken.

Ha! Don't just take my word for it - if you have access to an oscilloscope and a few high power resistors, check it out for yourself, you may be surprised at the results, as I was.

[brad347]

I would have no reason to doubt you, so I'll save my time. :) Your information has never steered me wrong before. Thanks for weighing in... the topic really did make me curious even though I don't have a dog in the hunt (I don't use attenuators).

[Keyser Soze]

Ha! Don't just take my word for it - if you have access to an oscilloscope and a few high power resistors, check it out for yourself, you may be surprised at the results, as I was.

Unfortunately I don't have access to an O scope, but a little bit of googling on leakage inductance yielded this tidbit.

leakage inductance is that property of an electrical transformer that causes a winding to appear to have some inductance in series with the mutually-coupled transformer windings.

Which pretty much says that even though you may have a purely resistive load on the secondary of an OT it can still behave as though a reactive load is present.

Which tells my mere mortal brain that impedance matching is always a good idea.

Thanks for the schooling.:aok

[Quantum Cat]

Surely because of the reactive nature of a speaker, and it's increased impedance at certain frequencies, we must see corresponding flyback voltages, or am I missing something?


A speaker is not a purely reactive load. It has a resistive component that is constant at all frequencies.

[JerryP]

It has happened. This isn't just opinions. Just because you don't want to believe it does not make it not true.



Where have I heard this before? ;)
Jerry

[JerryP]

.........Let me clarify - if the amp is not overdriven, there is no chance whatsoever of damage into a purely resistive load of even 4x the nominal impedance. You will see no flyback effects, and no excessive screen grid current, and it will be perfectly safe. The problems only occur under heavy clipping in the output stage. If you are not one to drive the amp hard, running a mismatched load will more than likely be entirely safe. But, if you are one to dime the amp and hit it hard with a boost for leads, watch out!!!

Randall Aiken

Sounds like Smoke on the Water to me.
Jerry

[phsyconoodler]

Quote:"The problems only occur under heavy clipping in the output stage. If you are not one to drive the amp hard, running a mismatched load will more than likely be entirely safe. But, if you are one to dime the amp and hit it hard with a boost for leads, watch out!!!"

This is the whole key here.Clean playing can take pretty much any load within reason.It's when you dime the amp and add too high a load,that's when the problems occur.
There are some guys who use the Ultimate attenuator with no issues,but I bet they don't use it to turn down to bedroom whisper-quiet levels.
The larger the output wattage would have the most risk for damage.
The attenuator will exploit ANY weak spots.
This argument could go on for a long time,it simply comes down to this:use at your own risk.
I have not heard ANY attenuator sound good yet.I would rather just use a smaller amp or power scaling any day.

[brad347]

use a smaller amp

With so many good-sounding choices out there nowadays, it seems like an incredibly obvious solution, doesn't it?

Especially if you have to move/carry the damn thing. I'm trying to find ways to make smaller amps louder, not the other way around! :D

[Tone_Terrific]

There are some guys who use the Ultimate attenuator with no issues,but I bet they don't use it to turn down to bedroom whisper-quiet levels.


I don't believe this is accurate, at all, as the UA is a full load all the time.


The larger the output wattage would have the most risk for damage.
The attenuator will exploit ANY weak spots.

Full stress on the whole amp, attenuated or not, brings out the worst.

[jzucker]

thanks for the great advice with a minimum of chest beating.

[hasserl]

thanks for the great advice with a minimum of chest beating.

You're welcome! :D

[phsyconoodler]

quote:"I don't believe this is accurate, at all, as the UA is a full load all the time."

Yes it's full load,but if you don't crank the amp it doesn't have the same effect.And if you don't turn it down too far it won't harm the amp either.Turning down a little to get the sound with a band is not particularly dangerous.It's when you dime the amp and turn the attenuator to whisper quiet levels that causes the problems.I tested this theory with the UA on a JTM45 clone.If you are stingy with the amount you turn it down,the amp would be fine.As you turned it down further to get a living-room volume level,it blew the fuse every time.
I also did this with a Fender Bassman head and a home made L-Pad attenuator.At bedroom levels it exploited every weakness the amp had.

[hasserl]

Pyscho, with the UA it really shouldn't matter how far you turn it down, because it is always putting a full load on the amp. It is not a real attenuator, it is a load/re-amp device. All the amp sees is the load of the UA, a portion of the signal is taken off of it and sent to a solid state amp and re-amped back up to whatever volume you want. Turning it down will only effect how much power the ss amp puts out, it does nothing to the load that the amp sees, which is at full load at all times. Whether the volume of the UA is at full blast or full silence, the load on the amp is the same.

[GBStratman]

Sorry to be late to the party, but I find these attenuator threads fascinating (really!). A while back someone was asking why a 30 ohm fixed load is worse than the spike in the impedance curve of a speaker. I've thought about this too, and here is my best guess (amp gurus please correct me if I'm wrong). What is ultimately important for the safety of the amp is the power being dissipated in the tubes and the transformer. Power is dissipated as heat. We all know that amps get hot, but they don't get hot simply by turning up to 10. You need to dime it and hit with a hot guitar signal. The power carried by the amplified signal is spread across a wide range of frequencies, only a small portion of which will coincide with the spike in the impedance curve of the speaker, so in terms of the total power being dissipated, that little spike does not account for a large portion. A further problem with cranking the amp is that the signal begins to distort. When this happens, high frequencies are introduced, which also happen to coincide with the rising portion of the speaker impedance curve. So a fuzz will cause more power to be dissipated at the same volume. A 30 ohm load will raise the impedance across all frequencies, so the overall power dissipation will go up.

That said, I do use a Weber 150 with a Germino Club. However, I measured the DC resistance with the cab attached as I rotated the dials. The resistance seems to run from about 8 to 22 ohms, which I believe is still close enough to run off the 16 ohm tap on the head. Just to be safe, I marked on the dial where the resistance hits 12 ohms, which would be comparable to a 16 ohm speaker, and stay below that.

Does that sound right?

[drbob1]

You know, I'd be excited about getting Ho to mod a UA with a safer 8 or 16 ohm resistor except that I finally did get a chance to try one at American Music in MN. Bypassed it was OK ;), with the UA engaged but no attenuation the EQ and quality of distortion had changed significantly for the worse. At least with Weber or THD the first few clicks don't suck any tone at all....

[Tone_Terrific]

Bypassed it was OK ;), with the UA engaged but no attenuation the EQ and quality of distortion had changed significantly for the worse. At least with Weber or THD the first few clicks don't suck any tone at all....

I don't have a UA but I believe I understand how it works and have tried similar schemes.
Based on ongoing comments many do not recognize its basic functionality, hence, the limits, hazards or potential.:horse

[phsyconoodler]

Hasserl,
It may indeed be a reamp device,but why did these two amps only have issues when the Dr Ho was turned down a lot?
Neither amp has any issues when dimed without the Dr Ho or with a Dr Z airbrake or a Marshall power brake.
And the fuses only blew when the Dr Ho was turned down low,not half-way.
,

[jzucker]

You know, I'd be excited about getting Ho to mod a UA with a safer 8 or 16 ohm resistor except that I finally did get a chance to try one at American Music in MN. Bypassed it was OK ;), with the UA engaged but no attenuation the EQ and quality of distortion had changed significantly for the worse. At least with Weber or THD the first few clicks don't suck any tone at all....

i don't agree with that and I did a demo which disproves that IMO. The UA device doesn't significantly change the tone. I no longer use one due to amp safety concerns though. Ho will make an 8 or 16 ohm version but he indicated that the attenuation will not be as effective at the lower impedance ratings.

[hasserl]

Hasserl,
It may indeed be a reamp device,but why did these two amps only have issues when the Dr Ho was turned down a lot?
Neither amp has any issues when dimed without the Dr Ho or with a Dr Z airbrake or a Marshall power brake.
And the fuses only blew when the Dr Ho was turned down low,not half-way.
,

Without the amp on hand to observe what was actually happening it's impossible to say. But turning the HO down to low volume settings shouldn't effect the load on the amp.

[drbob1]

i don't agree with that and I did a demo which disproves that IMO. The UA device doesn't significantly change the tone. I no longer use one due to amp safety concerns though. Ho will make an 8 or 16 ohm version but he indicated that the attenuation will not be as effective at the lower impedance ratings.

Strange! This was American Band, a JTM45 clone IIRC, plugged into a 412 Marshall cab with greenbacks. And there was a significant, audible difference even playing clean between the UA active and bypassed, even with no attenuation.

[jzucker]

Strange! This was American Band, a JTM45 clone IIRC, plugged into a 412 Marshall cab with greenbacks. And there was a significant, audible difference even playing clean between the UA active and bypassed, even with no attenuation.

again, I demo'd the thing in a video.

<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/kJmUMshxE2o&hl=en_US&fs=1&"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/kJmUMshxE2o&hl=en_US&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>

[phsyconoodler]

Nice video demo!

Just one problem: Your amp volume control was not mentioned.By what I was hearing,I'd bet it wasn't over '4' or '5'.
Do the demo again with the amp volume dimed.

That's where issues crop up.There are no issues even with a 30 ohm load when the amp is not getting a full signal to the PI. Most generic attunuators that reflect an 8 ohm load to the amp have no issues either,unless the amp has issues.
With a 30 ohm load,all bets are off.I would be willing to bet the UA would have no issues with the right reflected impedance.

Dime the amp! Then turn on the UA and do it again.Thats' where people have problems.It's all about that phase inverter signal.

[jzucker]

My demo wasn't for that purpose. I wasn't claiming you could dime the amp and get the same tone. I don't even like the sound of a plexi with the amp dimed. I just wanted to get the amp into the volume range where it "sang". You're right. It wasn't past 1/2 way up.
Nice video demo!

Just one problem: Your amp volume control was not mentioned.By what I was hearing,I'd bet it wasn't over '4' or '5'.
Do the demo again with the amp volume dimed.

That's where issues crop up.There are no issues even with a 30 ohm load when the amp is not getting a full signal to the PI. Most generic attunuators that reflect an 8 ohm load to the amp have no issues either,unless the amp has issues.
With a 30 ohm load,all bets are off.I would be willing to bet the UA would have no issues with the right reflected impedance.

Dime the amp! Then turn on the UA and do it again.Thats' where people have problems.It's all about that phase inverter signal.