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16 Ohms into 8Ohm cab?

slegros

Member
Messages
499
Ive read that for JTM45s and Bluesbreakers one mod is to set the impedance selector switch to 16 Ohms to run into an 8 Ohm cab? I also read that Cesar Diaz -SRV's late amp tech- used to do similarly with his Fenders. What's the theory behind this? What is the effect? Safe to do on a late '90s Bluesbreaker reissue?

Thanks in advance!
 

jay42

Member
Messages
7,044
hmm...a mismatch creates inefficiency. Clean power will drop by a few watts. If you have a 35W Fender, you'd end up seeing about 28W clean. With a Marshall, a downward mismatch should be safe and should sound a bit different. I recall Andy Marshall explaining a two position Z-switch on one of his THD combos, as something to set for tone.

Just don't go up with the mismatch, and don't go to a 4 ohm cab on the 16 ohm setting.
 

VaughnC

Silver Supporting Member
Messages
17,914
It will also stress the output tubes but shouldn't really hurt anything.

However, I have to disagree with jay42 on the 4 ohm cab with a 16 ohm amp setting scenario. While this will stress the tubes, this is much safer than a 16 ohm cab on a 4 ohm amp setting. Flyback voltage is usually what kills output transformers & tube sockets...and flyback only occurs when the cab impedance is higher than the amp impedance setting and the larger the impedance gap the higher the flyback voltage generated.
 

VaughnC

Silver Supporting Member
Messages
17,914
Seems like current in the tubes should go up by about one third.
Yup, the tubes are stressed but they are self current limiting so they can't deliver enough current to damage the output transformers primary winding. Sometimes short lived tubes might be worth the tone ;).

But, in the 4 ohm amp/16 ohm cab scenario, flyback voltage can hit upwards of 3000 volts in a typical guitar amp, which is potentially enough to break down an output transformers insulation and/or arc across a tube socket.
 

ohmslaw

Member
Messages
208
Yup, the tubes are stressed but they are self current limiting so they can't deliver enough current to damage the output transformers primary winding. Sometimes short lived tubes might be worth the tone ;).

But, in the 4 ohm amp/16 ohm cab scenario, flyback voltage can hit upwards of 3000 volts in a typical guitar amp, which is potentially enough to break down an output transformers insulation and/or arc across a tube socket.
I'm confused about this...I had understood it was acceptable to run a higher impedance speaker than the recommended output impedance, e.g., using a 16 Ohm cab with an 8 Ohm output, but never the inverse. I spoke to THD about this regarding the Hot Plate, as I had some 16 Ohm speakers and wanted to run them out an 8 Ohm Hot Plate. The docs with the HP said this was fine, but never run less impedance than the rating of the HP (which must also be the same as the amplifier). Thus, according to what THD is saying, you can run an 8 ohm speaker connection from the amp into an 8 ohm Hot Plate and then into a 16 ohm cab, or just eliminate the HP and you can run the 8 ohm amp into a 16 ohm cab.

My understanding was that less impedance that recommended would cause the amp damage, but a greater load would reduce efficiency but not cause any harm to the output transformer. Any explanations would be appreciated!
 

hasserl

Member
Messages
4,709
My understanding was that less impedance that recommended would cause the amp damage, but a greater load would reduce efficiency but not cause any harm to the output transformer. Any explanations would be appreciated!
This is basically backwards, as VaughnC posted above. A lower impedance load will result in increased current draw, making the tubes work harder. But the increased current should not stress the transformer, as tubes as basically self limiting devices and can only flow a certain amount of current no matter how low the impedance. A higher impedance load however can result in flyback voltage spikes that can damage the power tubes/sockets and/or output transformer. This is more dangerous for an amp than increased current draw from a reduced impedance load.

The confusion often comes about because the situation is opposite with solid state amps, where a reduced impedance load can cause the transistors to work themselves to death trying to meet the current demand. Transistors are not self limiting in the way that tubes are. Tubes can hit their current flow limit and continue working at maximum flow, they might wear faster, but they won't burn up, transistors will. And transistor amps do not typically use an output tranformer, so high flyback voltage spikes are not an issue. So it is perfectly fine to run a higher impednace load on a SS amp, but NOT on a tube amp. Why this seems to confuse so many tube amp techs is beyond me, but it really causes confusion, because the same bad advise keeps going around & around.
 

ohmslaw

Member
Messages
208
This is basically backwards, as VaughnC posted above. A lower impedance load will result in increased current draw, making the tubes work harder. But the increased current should not stress the transformer, as tubes as basically self limiting devices and can only flow a certain amount of current no matter how low the impedance. A higher impedance load however can result in flyback voltage spikes that can damage the power tubes/sockets and/or output transformer. This is more dangerous for an amp than increased current draw from a reduced impedance load.

The confusion often comes about because the situation is opposite with solid state amps, where a reduced impedance load can cause the transistors to work themselves to death trying to meet the current demand. Transistors are not self limiting in the way that tubes are. Tubes can hit their current flow limit and continue working at maximum flow, they might wear faster, but they won't burn up, transistors will. And transistor amps do not typically use an output tranformer, so high flyback voltage spikes are not an issue. So it is perfectly fine to run a higher impednace load on a SS amp, but NOT on a tube amp. Why this seems to confuse so many tube amp techs is beyond me, but it really causes confusion, because the same bad advise keeps going around & around.
I don't know enough to disagree but I wonder why THD's manual and their guy at the "factory" say it's OK to run a higher impedance out of the Hot Plate, and why does the back of the amp always state a minimum impedance (such as 8 or 4 ohms) if running a lower impedance is OK? Just curious. I'm not trying it!
 

VaughnC

Silver Supporting Member
Messages
17,914
Most tube amps will tolerate a one step impedance mismatch upwards with no problems (Ex: 2 ohm amp/4 ohm cab, 4 ohm amp/8 ohm cab, or 8 ohm amp/16 ohm cab). But, as you increase the amp to cab upward impedance gap beyond one step, flyback voltage also proportionally increases....so, at what point will your output transformers insulation and/or tube sockets break down? Difficult to say as the affected components in every amp aren't made exactly the same way...so it's better to be safe than sorry and not go beyond one, amp to cab, upwards impedance step.

As to why an amp manufacturer would state a minimum load impedance for a tube amp, I'd guess they are just trying to keep the user from stressing the output tubes so they don't have to replace them under warranty. I can't see how too low of an impedance load would hurt anything in a properly designed tube amp beyond stressing the output tubes. Besides, every modern tube amp I've repaired had shorting type speaker jacks to protect the amp from flyback in case you forget to plug in a cab...and a 0 ohm shorting jack is the lowest possible load impedance.
 

hasserl

Member
Messages
4,709
why does the back of the amp always state a minimum impedance (such as 8 or 4 ohms)
Can you give an example? The only amps I can think of that I have seen this on are solid state amps.

Let me ask you one, why do most all Fender amps have no selectable impedance, but they include two speaker jacks on the back of the amp; one for the main speaker and one for an extension cabinet? Would they do this if running a reduced impedance was harmful to the amp?
 

GP_Hawk

Member
Messages
570
Same here...as per example on the back of a LabSeries L9 I got here...it says 8 ohm min. But I have never seen this on a tube amp.
 

jchan

Member
Messages
1,687
I wouldn't do it! Just match the impedance or get the right cab/speaker for what you want to achieve.
 

ohmslaw

Member
Messages
208
Can you give an example? The only amps I can think of that I have seen this on are solid state amps.

Let me ask you one, why do most all Fender amps have no selectable impedance, but they include two speaker jacks on the back of the amp; one for the main speaker and one for an extension cabinet? Would they do this if running a reduced impedance was harmful to the amp?
The second jack is switched and impedance is automatically selected. They are not wired in parallel as you would think. Never plug the combo's speaker connection into the "extension" output, always use the main speaker output. Fender assumes two speakers of the same impedance so if you are running an 8 ohm, it assumes when you plug an extension in, it's also 8 ohms and the switching on the extra jack then sets the amp to a 4 ohm configuration, so it's safe. Some Fenders do have selectable impedance, such as the SuperSonic head. The combo has the former setup. This is my understanding but read your manual first!!!
 

hasserl

Member
Messages
4,709
The second jack is switched and impedance is automatically selected. They are not wired in parallel as you would think. Never plug the combo's speaker connection into the "extension" output, always use the main speaker output. Fender assumes two speakers of the same impedance so if you are running an 8 ohm, it assumes when you plug an extension in, it's also 8 ohms and the switching on the extra jack then sets the amp to a 4 ohm configuration, so it's safe. Some Fenders do have selectable impedance, such as the SuperSonic head. The combo has the former setup. This is my understanding but read your manual first!!!
I was referring to the classic Fender amps. No tweed, black face or silver face Fender amp had a switched second jack w/ auto selected impedance. The speaker jacks were wired in parallel.
 

ohmslaw

Member
Messages
208
Same here...as per example on the back of a LabSeries L9 I got here...it says 8 ohm min. But I have never seen this on a tube amp.
Every Fender amp I own has this. The SuperSonic combo states "4 ohm min/60 watts" when using both spkr outputs (8 ohms into just the combo spkr), the Princeton Recording-Amp states "8 ohm min/15 watts", the Superchamp XD states "8 ohms min/15 watts" and the Vibrochamp XD and Champion 600 state "4 ohms min/5 watts".

The idea that you can run less impedance than the minimum stated is very new to me. I started playing in 1970 and have assembled a lot of large sound reinforcement systems using 1970's technology which means a lot of tube amps. The rule was always that you can go UP in impedance which will reduce power but provide a load for the amp, but never below the amps minimum rating. The worst thing is for an amp to have no load at all; better to have a higher impedance than minimum than lower. But then again, this is the field engineering rule of thumb here. Not saying that you can't do the opposite if you know what you are hooking up but this is new to me. Ohm's law tells us this is a bad idea, though...you cut the impedance in half and you double the power. Power = heat and if the amp cannot handle the heat, it'll burn. Peavey had some power amps back in the day that could put 800 watts RMS into 2 ohms f'r example. Cut that to 1 ohm and now you've got 1600 watts the amp is trying to make. The power transistors can't take it and will burn. If you take your tube amp and run it into 4 ohm and it's designed to put 50 watts into 8 ohms, you are now asking that amp to put 100 watts into the 4 ohm cabinet. That is just simple Ohm's law electronics math for us field engineers but I never lost an amp or a cabinet going by that.
 

ohmslaw

Member
Messages
208
I was referring to the classic Fender amps. No tweed, black face or silver face Fender amp had a switched second jack w/ auto selected impedance. The speaker jacks were wired in parallel.
I believe my Fender Deluxe Reverb Reissue had this feature but I no longer have that amp. However I know when I mistakenly plugged the combo spkr into the "extension" jack, I got no sound! If they were in parallel either jack woulda worked. Probably a new feature on the reissues.
 

ohmslaw

Member
Messages
208
I'm going to bump this thread again because I've had this nagging thought in the back of my mind that some very bad advice was given which could cause someone to burn up an amp.

The rule about speakers and amps is not negotiable, it's Electronics 101 and can be found in Aspen Pittman's "bible" for tube amp owners, "The Tube Amp Book" on page 260 of the 4.1th edition. As I said earlier, you can go UP with cabinet impedance without endangering the amp but you will get less power. You can NEVER go down with speaker impedance with respect to amp output impedance without putting your amp in danger.

To quote Pittman:

"Some people have the misguided notion that putting an 8 ohm load on an amp with the impedance selector set to 16 ohms somehow increases the output power (by some sort of "magic", I suppose) or that running a 16 ohm load with the selector at 4 ohms gives you a "good" sound at a lower volume. Regardless of the myths, one thing that doing these things will do is to cause the power tubes to work too hard (due to the impedance mismatch) resulting in premature tube failure and possibly output transformer damage as well."

I explained in an earlier post why Ohm's Law shows this to be true. If you cut the resistance in half, the amp will TRY to double its power output, unsuccessfully, by working too hard. The results are increased heat and fried components.

As I had suspected, Ohm's Law has not changed since I started playing music. Don't mismatch cabinets or you'll be buying a new amp.

Aspen Pittman is the founder of Groove Tubes and leading authority for many years on tubes and tube amplifiers. His book ain't cheap but it could save your equipment and maybe your life. I highly recommend it.
 
Last edited:

hasserl

Member
Messages
4,709
No, Ohm's Law has not changed, but you do not understand what you are talking about, and the advice you are giving is wrong. Aspen Pittman is NOT an amp tech and carries little respect among techs (kind of like Gerald Weber and Dan Torres in that regard, but less knowledgeable than either of them), he is a slick salesman who did a great job with his GT Tubes business, buying and selling tubes, but he's not a tech; taking your advice from him is not a good thing. Pittman's book is an interesting collection of amp pictures with some histrical info and some schematics, but it's filled with technical inaccuracies, be careful using it as a tech manual, not a good thing to do. Calling him a leading authority exemplifies your ignorance here. In any case, what you posted above, from Pittman, does not support your position. Note that he says that mismatching the load in either direction can be harmful. He does not say that mismatching it in only one way can be.



Just to clarify this, concerning a TUBE amp (not SS) it's best to match the impedance between amp/speaker load. Most amps are tolerant of slight mismatch, usually 50% less or 100% greater than rated is tolerable. When mismatching beyond these points a load lower than the amp rating is LESS harmful to the amp than a load that is greater. That is correct, the lower load is LESS harmful, or dangerous, to the amp than a load that is greater than the amp rating.

And just to repeat what I posted earlier, a load that is lower than the amp rating will result in increased current thru the tubes. This will cause the tubes to work harder, but it should not cause damage to the transformer; unless the transformer is so undersized that the added current will cause excessive heat buildup, breaking down the insulation, leading to a short circuit. On the other hand, a load that is higher in impedance than the amp is rated for can cause high voltage spikes to occur on the primary side of the output transformer and at the power tube pins & sockets. The high voltage can arc across the pins, laying down a carbon trail that is inductive, that will lead to further arcing, possibly even after the mismatched load is corrected; neccesitating a socket replacement. Also, the high voltage spikes can damage the insulation on the transformer windings, leading to arcing across the windings, and short circuit (OT failure). The damage tends to be accumulative, the spike of voltage puts pressure on the insulation, weakening it in spots, repeat occurences causes the the weakened areas to grow increasingly weaker until the the pressure of the voltage exceeds the strength of the insulation and arcing occurs. The insulation does tend to be somewhat self healing, but repeat occurences can lead to failure of the insulation.

note: the reason your Fender amp put out very low voltage when you plugged the internal speaker into the external speaker jack, is because the jack for the internal speaker containes a switch connecting the circuit to ground if no plug is inserted. Fender did this because a direct short to ground is safer for the amp than an open circuit, or rather zero impedance is safer than infinity. And that supports what I posted above.
 

pgissi

Member
Messages
2,481
Been using an imp mismatch for years with a 30 watt Rivera (stock 8ohm output with a 16ohm cab) and in fact the Rivera manual states this is acceptable while he makes no mention of using a 4 ohm cab.

My rules for mismatch are simple

-only amps rated 40 to 50 watts or less and even then not if the amp is
required to run full out
-some amps more tolerable and tonally pleasing than others with a mis
-never more than a 100% mismatch

I prefer using a mismatch using a cab with a higher impedance than the amps output having tried both examples, for its sonic effect on the tone using a higher imp cab yields a slightly flattened freq response in lows and highs with a slight mid emphasis, the opposite of a hifi loudness circuit, for me it smoothes things out in the highs and with a slight compression. This permits me to get a hotter tone at lower volume since I am driving the amp slightly harder due to the reduced efficiency of the mismatch.

Amps I have mismatched, my SF Twin, 67 BF Super, 30 watt Rivera, several single ended amps over the years

I found the theory on why using a lower imp cab is sound but it leaves out the fact that the increased current draw this causes stresses other components to their detriment and also wears out tubes faster. I personally have repaired burnt pcb traces and fried Bridge Rectifiers and power trannys after using an lower Imp cab with a mismatch.

Its reasonable to assume that on most quality built amps, the OT can handle any additional stress of higher flyback more so than amps in this day of PCBs, cheaper components and inferior tubes when using an lower than rated imp cab.

Just my experience
 

drgonzoguitar

Member
Messages
4,710
The rule about speakers and amps is not negotiable, it's Electronics 101 and can be found in Aspen Pittman's "bible" for tube amp owners, "The Tube Amp Book" on page 260 of the 4.1th edition. As I said earlier, you can go UP with cabinet impedance without endangering the amp but you will get less power. You can NEVER go down with speaker impedance with respect to amp output impedance without putting your amp in danger.
Aspen Pittman is not a tech. If you want the true bible, look at Jack Darr's book on Electric Guitar amps.

http://www.pacificrecone.com/JackDarrBook.html


Or you can look for some info from Duncan Munro
http://www.duncanamps.com/

Why is impedance mismatching so dangerous?
The crux of the problem lies in the inductive nature of the output tranny. Inductive loads are pretty special things, since they STORE energy in a magnetic field. A property of this effect, as has been pointed out, is that the voltage can soar to levels above the supply voltage in the amplifier-- sometimes WAY above. You can't do that with any other kind of load other than inductive.

Now the transformer doesn't have an impedance of its own; it only reflects an impedance from one winding to another in proportion to the turns (or voltage-- they are the same) ratio squared.

So imagine that you've got an open secondary. This impedance is for all intents and purposes infinite. Thus, regardless of the turns ratio, the primary impedance is infinite as well. (leakage inductance and parasitic capacitance-- two unavoidable nasties of real-world trannies-- will limit this to some finite number less than infinity, but suffice it to say its really high.) This means that the primary will act like a constant current source, attempting to keep changes in currents through its windings to a minimum. This will be an important point later.

Operating into such a humungous load impedance will cause the plate to swing HUGE voltages, according to V=IR. Especially with tetrodes/pentodes, which are much better at cranking out current, the delta Ip will stay the same regardless of the load. Consider what happens when the R goes sky high.

Now, if the load were NOT inductive, the maximum possible voltage generated would be equal to the rail voltage. No problem. This is how it is in SS amps. But with tube amps, that's not the case.

The primary danger here is in the development of these extraordinarily high voltages, which can punch through winding insulation, arc over tube sockets, even arc inside the tubes themselves. Once an arc has struck you can be pretty sure it will happen again. And again.

This is not good. Probably the worst scenario is that the OPT primary arcs to the core, which is grounded, and that will cause mega current to flow. The OPT is toast, and the power supply will be too unless something stops that current in a big hurry.

So that's what can happen with too high a load. Admittedly, this is an extreme case scenario here, where you've got an OPEN secondary, and thus a very very high primary impedance to work into.

Notice above how I pointed out that tetrodes/pentodes will have a worse time of this than triodes. This is because of their much higher dynamic plate impedance, which can also be described as their being an approximation of current sources. The pentode will just keep cranking out plate current-- regardless of what potential the plate is. The electrostatic shielding effect of the screen grid continues to "pull" electrons from the cathode with the same force. Thus the plate current is largely independent of the plate voltage, a mark of high plate impedance. That current is what "builds up" (so to speak) when working into a high load impedance and generates the excessive voltages. It's almost as if you've got a constant current load (the unloaded OPT) on a constant current generator (the plate of the pentode)-- obviously if these two devices are "concerned" with currents, not giving a flip about the voltages involved, you can get some pretty crazy effects.

With a triode, the much lower plate impedance limits the extent to which the plate voltage will swing about uncontrolled. As the plate swings high, for example, the attraction of electrons from cathode to plate will increase due to the higher voltage. More electrons will be pulled to the plate, regardless of what the control grid is doing. More negatively charged electrons means less positive voltage, so the voltage is "automatically" decreased. This is a direct measure of plate impedance.

In fact, running a triode into a very high impedance is done all the time with interstage transformers, which generally are very lightly loaded. The inherent degeneration in the plate circuit keeps the peak voltages from becoming a problem. Actually, triodes "love" current loads of very high impedance-- the tube is operated in its most linear fashion and is free to do what it does best-- generate an output VOLTAGE.

You can think of the dynamic plate impedance of the tube as forming a voltage divider, with the inductive tranny between plate and B+, and the tube itself between plate and ground. Obviously, with the low plate impedance of a triode, the voltage cannot swing madly about. Now consider the very high plate impedance of a pentode, and how much higher those plate voltages can swing.

OK, that's the situation of too HIGH a load impedance. So what about too LOW of an impedance? Let's consider a dead shorted secondary on the OPT.

Now the primary presents a very low load to the tube, a low impedance, a vertical load-line. We will notice that the tables have exactly turned.

Since the triode's plate is like a voltage source, it will attempt to pass incredible amounts of current in a heroic attempt to make the plate voltage swing. Operating into a dead short, it cannot do this, so something eventually will give. The cathode will attempt to emit way more electrons than it can, and it will have a short, hot life.

The pentode, however, is more of a current source, so it will continue to pass the total plate current in accordance with the screen voltage and the control grid voltage. These have not changed with the alteration of the load, so the pentode will continue to merrily pump its current swings into a dead shorted load.

Take a look at some plate curves, if you need to. Find some for pentodes and for triodes. Better yet, find some for the same power pentode connected as a triode (g2 connected to anode).

First the pentode case: look at the way the curves lie on the page. Imagine a horizontal load line (infinite load, open secondary) drawn on the graph. Notice how the pentode doesn't look like it would work this way-- an infinitesimal control grid voltage change would produce a gargantuan change in plate voltage. The tube is NOT happy. Now imagine a vertical load line (zero load, shorted secondary). The pentode's peak current for a given control grid voltage doesn't change much at all-- the vg1=0 plate curve is nearly horizontal for most power pentodes, cutting right across all of the various plate voltage points. It doesn't matter what Vp is at all-- no matter where you draw that vertical line, the peak plate current is pretty much the same. The tube is happy.

Now the triode case: imagine the horizontal load line now. Notice how the plate voltage is almost PERFECTLY proportional with respect to control grid voltage. No matter which tube you try, or what current you draw that horizontal line at, it will be VERY linear. The tube is happy. Then consider the shorted output tranny case, with a vertical load line. Notice how the vg1=0 curve will produce a humungous plate current since the plate curves are so much "steeper" than the pentode's case. The tube is NOT happy.

What the heck does all this mean? Well, hopefully you aren't running ANY tube amp into a shorted or open load... Since no pentode is a perfect current source, and no triode is a perfect voltage source, the actual characteristics are somewhere between the two idealized cases. As LV pointed out, you're much better off running a pentode amp into a lower load impedance than it expects. For those of you with triode output tubes, or a triode switch, you're better off running into a HIGHER load impedance. If you don't see why by now, reread this essay. It's also a good idea to take a high value power resistor, say 470R, and hard wire it right from the OPT secondary's 16R tap to ground. This will dissipate a very small amount of power under normal conditions, but will limit the extent to which the primary impedance will tend towards infinity in the case of a disconnected load.

For what it's worth, I've been deliberately "mismatching" load impedances by one tap for years. In other words, either a 4R or a 16R load on an 8R tap, and so on. This small mismatch will limit output power and will change the clipping points of the output tubes, but will not damage anything in a properly designed amplifier. Keep in mind that a higher load impedance in a pentode amp will put additional stress on the screens, so you may want to have at least 1k stoppers installed. A lower load impedance will cause more plate current to flow, and if you're running the tubes at the edge of acceptable quiescent plate dissipation that may push them over the edge into the red zone. If you've got an old vintage amp you'd hate to see get damaged, by all means, play it safe and don't mismatch at all. But if you're wondering about how it sounds, and the amp's got good trannies in it, then mismatch away. Just keep it within ONE TAP please, for safety's sake.
 




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