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I thought Amp>Cabinet impedance match was important, until this..

Sherwood

Member
Messages
490
Bought an Ampete 222 amp and cabinet switcher for my new rig and decided to learn some more about impedance matching.

I always though i couldn't run a cabinet with lower impedance than my amp, until i read this..

(skip to page 18 of this manual)
http://ampete.com/wp-content/uploads/2014/03/Ampete-222-Manual-Jan15.pdf

To sum it up... Sounds like i won't blow my tube amps nor degrade my tubes any faster by mismatching impedances, even if i run a 16 Ohm amp out to na 8 Ohm cabinet.

Your thoughs/knowledge?
 

cap47

Member
Messages
2,277
It is worse to run higher than rated because it generates higher heat in the O.T. than to run lower OHMs which can induce higher tube heat.
 

Laurence

Gold Supporting Member
Messages
9,285
Folks will disagree about this (is lower or higher safer?), but it really depends on the amp and the nature/quality of its transformers. I run vintage Fenders at higher (one range, say, 4 to 8 ohm) than indicated impedence all the time with no issues. I do the same one range down, but only to 4 ohm, no lower, with no issues. I wouldn't do that with old Marshall's or Voxes, just to be safe.
 

rumbletone

Silver Supporting Member
Messages
7,379
Depends on the amp. Some MESA amps don't have 16ohm outputs and say to use 8ohm output for 16ohm cabs. Stephenson says any mismatch into 4->16 ohm cabs is fine for his amps - use the impedance selector as an additional tone control(!).
 

darkfenriz

Member
Messages
206
The problem was solved decades ago using solid state technology ;) ;)

P.S. In vacuum state the mismatching problem is often exaggerated I believe and it is a real issue only with extremes such as open/short circuit. In practice a factor-of-2 mismatch should not be any issue at all.
 

pdf64

Member
Messages
8,238
There are a number of technical points made on pages 18 and 19 which are at odds with my understanding / experience.
eg my understanding is that transformer windings don't themselves have an impedance, at least not in the sense used in the manual; rather it may be more helpful to think of them as acting to reflect a ratio of the impedance present at the secondary back to the primary. Hence if the nominal value load impedance is present at the secondary, then the intended impedance will be reflected back to the primary and the tubes will be able to work at their best, and be able to put out the maximum possible signal power, whilst minimising the power wasted in plate/screen grid dissipation.
The chart of power dissipated in a load against load impedance at various amp output settings confuses me, as it's unclear whether the measurements made are the max clean power output, or just the power delivered to the load resulting from a fixed input signal to the amp.
If the latter (as I suspect), the chart seems pointless technobabble perhaps intended to impress the lay reader; if the former, it makes no sense at all (the amp used would have very unusual characteristics).
The point is that the manual is not a statement of technical certainty, rather just one guy's carefully worded opinion; obviously you're invested in their competence as you've bought the product.
However, even given that, I suggest that you don't read more into it than what it says.


To sum it up... Sounds like i won't blow my tube amps nor degrade my tubes any faster by mismatching impedances, even if i run a 16 Ohm amp out to na 8 Ohm cabinet.
Note that the manual doesn't say that tubes won't wear out faster.
Often no damage will be caused by 1 step mismatch; if the amp isn't being pushed hard, then generally it's not an issue.
However, at a given output power, a tube's plate or screen grid dissipation will increase if the load is mismatched.
That increased dissipation will tend to reduce the tube's operational life.

The power tubes in regular tube guitar amps generally aren't run conservatively, rather they are pushed up to and beyond the tube manufacturer's limiting values.
If an amp is being heavily overdriven then the increased dissipation caused by a mismatched load will tend to be exacerbated, when even with a matched load, the tube's limits may already be exceeded.
Hence the likelihood of tubes failing catastrophically is increased.

Generally, my finding is that load mismatches are less of a problem for beam tetrode amps (eg 6V6, 6L6, KT66), but more likely to cause a problem with pentode amps (EL34, EL84). I suggest to avoid running running eg a Marshall SLP set to 16 ohms into an 8 ohm load.
 
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swiveltung

Senior Member
Messages
14,485
Folks will disagree about this (is lower or higher safer?), but it really depends on the amp and the nature/quality of its transformers. I run vintage Fenders at higher (one range, say, 4 to 8 ohm) than indicated impedence all the time with no issues. I do the same one range down, but only to 4 ohm, no lower, with no issues. I wouldn't do that with old Marshall's or Voxes, just to be safe.
I agree with this. Also, the lower wattage the amp is, the worse it will sound with a mismatch IME. But tone is in the ears of the beholder, so any given person may like it better that a good match! I liked my 50 watt Bassman head with an 8 ohm speaker better than with a 4 ohm speaker set. OTOH, a DR with a 50% mismatch doesnt sound good at gigging levels at all to me.
 

Mayhem13

Member
Messages
1,247
Nominal resistance values don't tell the story,.....it's the T/S value Re, Le and a full spectrum impedance sweep that matters.

Take the Celestion TF1220, a similar 12" PA speaker since Celestion doesn't bother with TS Params for guitar speakers. From 150-500 hz, this nominal 8ohm driver is actually at 5.5 ohms and then rises higher in frequency to the breakup mode. This is commonplace impedance. The more variable the load over frequency, the more difficult the load for tubes to drive. The guitar amp and speaker companies really like to remind us how these non linear performance features are preferred......really?

I'll take predictable, and controlled even order harmonic distortion any day over a non linear speaker/amp system laden with odd order products.
 

zenas

Member
Messages
8,873
This discussion keeps coming up. I've even seen "gurus" talk about using huge impedance mismatches as a poor man's attenuator.
At the end of the day it's my amp and I gotta fix it. So I run mine matched.
 

Sherwood

Member
Messages
490
Any advice regarding what can i do to check if my amps are being put to stress with this mismatch? I'm not a tech
 

supergenius365

Silver Supporting Member
Messages
11,404
I remember the manual to my Mesa IIC+ saying something like "go ahead and experiment with different impedences. You won't hurt anything."
 

pdf64

Member
Messages
8,238
Any advice regarding what can i do to check if my amps are being put to stress with this mismatch? I'm not a tech
As described, all else being equal, a load impedance mismatch will tend to cause more stress than a matched load.
I can't think how a non-tech could assess the degree of additional stress.
Do you overdrive the power amp sections of your amps? If not, just be aware you'll likely need to replace the power tubes more often.
If yes, then I suggest to avoid mismatches on amps that use power pentodes / particularly high HT voltage eg >500V, and check that your power tubes aren't red plating when they're pushing out high power levels, eg
 

Sherwood

Member
Messages
490
Ok! Thanks!
I don't play loud enough nor do i use heavy attenuation to overdrive my power tubes. But my tubes usually run around 480v on the plates..
 

pdf64

Member
Messages
8,238
To my understanding / experience, that article contains errors and gross oversimplifications, eg
For example, if you connect an 8 ohm speaker to the 8 ohm output on a tube amp, the internal resistance of the source (that is, the tube output stage) is exactly that of the load, or speaker
is incorrect.
The output impedance of a regular tube amp is a complex variable that is affected by many factors; my finding is that at high signal levels, operating open loop, for an AB amp it tends to be about twice the nominal load.
It tends to be higher at lower signal levels.
If negative feedback is applied, its open loop value will be reduced according to the degree of feedback.

The point is that the nominal rated load of an amp is a different thing to its output impedance.

Running a 4 ohm load on eg an SLP set to 16 ohms seems a really bad idea to me; when pushed, its EL34 may well redplate.
 
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robrob

Member
Messages
408
This is what I have on my website:
Impedance Mismatch Between the Power Tube and Speaker
As mentioned earlier to get the most power out of the amplifier the power tube and speaker should match their impedance. But what happens when they don't match?

Low Mismatch

If your output transformer is designed to match your power tubes to an 8 ohm speaker and you connect a 4 ohm speaker (4 ohm load and 50% decrease) then the impedance as seen by the power tube plate decreases by 50% too. Less impedance will cause plate current to increase. The power tubes are stressed by this increased plate current so the power tube lifespan can be shortened. This is especially true of Class A amps because they idle near max current flow. Since the plate current idles near maximum, the entire power supply also runs at maximum output so the rectifier tube and power transformer will run hotter. Power filtering effectiveness is reduced as current demand increases so hum and noise may increase, especially in Class A amps. Increased hum can cause ghost notes which are caused by hum interacting with musical notes to create false harmonic tones.

The increase in primary current will cause the output transformer to run hotter.

Many vintage Fender tube combo amps have an aux speaker jack that's wired in parallel with the built in speaker. Plugging in a speaker rated at the same ohms as the built in speaker will cut the speaker load in half for a "one step" low mismatch. Fender designed their output transformers to handle this and is considered safe.

But there are two possible sonic improvements with a power tube and speaker low mismatch. Sweet sounding second harmonic distortion in the power tube increases and not-so-great sounding third harmonic distortion decreases dramatically. This is why it's worth experimenting with different speaker loads, you may like the tone you get.

High Mismatch

If you connect a 16 ohm speaker to your 8 ohm output transformer the impedance as seen by the power tube plate increases and plate current decreases which can lengthen the lifespan of your power tubes, especially in Class A amps. This decrease in plate current will decrease demands on the power transformer and it will run cooler. Power filtering effectiveness is increased as current demand decreases so hum and noise should decrease, especially in Class A amps. Decreased hum can help prevent ghost notes.

Since the power tube and transformer are not coupled at maximum efficiency the amp's power output is reduced.

We get the opposite of the sonic improvements of a low impedance mismatch: Sweet sounding second harmonic distortion in the power tube decreases and nasty sounding third harmonic distortion increases.

The main problem with a high impedance mismatch is flyback voltage generated in the output transformer which can damage the power tubes and the output transformer itself. The flyback voltage spikes can cause the power tube to arc between pins or burn through the thin lacquer wire insulation used in the transformer windings. This is normally not a concern when going "one step" away from a match such as running a 4 ohm output transformer with an 8 ohm speaker unless the output transformer is cheaply made or really old. But running the 4 ohm transformer with a 16 ohm speaker can generate very high flyback voltages when running the amp hard near max volume.

If your power and/or output transformers run hot with a matched output transformer and speaker load then mismatching them is more of a risk. The bottom line is the greater the low impedance mismatch the harder your amp works, and the greater the high impedance mismatch the more likely you are to burn out your output transformer and/or power tubes. For tube amps a low mismatch is typically safer than a high mismatch. The opposite is generally true for solid state amps.
 

Tomm Williams

Member
Messages
964
I had this same conversation with Ben Fargen awhile back. He stated that a 50% mismatch going either up or down is not an issue.
 

382

Member
Messages
89
Wow. What a website. Rob you have quite a collection of information there.
I will be perusing there for awhile.
I am not a builder but I've always believed that a quality amp can handle the one step mis-match.
It may not be recommended, but an amp that has tighter tolerances or uses lower quality parts would be more susceptible to failure.
I might have a different opinion 3 months from now when i finish Mr. Robinette's website...

D.
 




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