Amplifier and cab impedance - mismatch tales?

[hunter]

According to this source (and I guess Mike Soldano knows a bit something about amps) it's all the other way round. I mean common sense is that mismatching is OK id the cab load is higher than the amplifier expects it to be. But Mike says the other way:

"Well, Dave, I don't know who you've been listening to or what you've been reading, because they're just plain wrong. Here's how it works: If the load is lower than what the amp is set for, like using a 4 ohm cabinet with the amp set at 16 ohms, the power tubes will be worked harder and will run hotter. This, of course, will shorten the life of the power tubes. However, if the load is higher than the amp's setting, like using a 16 ohm cabinet with the amp set at 4 ohms, the voltages in the output transformer will be higher than normal. These excessively higher voltages increase the risk of arcing, which can destroy the output transformer and/or tube sockets. That's why running an amp with no load at all invariably ends up blowing the output transformer. I'd say tubes are easier to replace, wouldn't you?"


This is the source:
http://www.musicianshotline.com/archive/monthly/qa_soldano/07_03.htm

[hamfist]

That's been my understanding too. Running into a too high impedance, caused "flyback voltages" (something like a charge bouncing back to where it came from), which are dangerous to an OT's health.
  But running into a too low impedance will stress the power tubes, increasing their chances of failing.
   There has a been a huge amount of misinformation about this on the internet. Even many amp techs seem to have wildly differing views.

  As always, the safest way is to match impedances, wherever possible !

[martinw]

One additional point, which invariably doesn't get mentioned, and complicates the matter even further, is that there is no "correct" impedance for any valve or set of valves.
There are instead a range of values where the valve(s) will function perfectly happily, although with different operating characteristics and power output.
The ones normally quoted on data sheets are for most efficient operation, and in any case vary with class of operation, quiescent anode voltage etc.

There are many examples of amps that operate perfectly safely, with good valve life, at fairly large mismatches from the "correct" primary impedance. The Trainwreck Express for example operates either EL34's or 6V6's with no issues, as do many Matamps.

[jpfamps]

With respect to impedance matching, the advice that too low is worse than two high is correct for solid state amplifier but not valve amplifiers. I have posted on this elsewhere. Generally, the output valves in an amp are matched to an impedance that gives maximum efficiency, although as the data sheets for most valves don't really cover the conditions used in many guitar amplifiers there is often some guesswork required here.

A speaker's impedance varies considerably with frequency. A 16 ohm speaker may present an impedance of 100 ohms or more at resonance down to 12 ohms or so in the midband, and rising again at high frequency. Thus presenting a load around about the correct area is going to be fine. Furthermore, the manufacturer, if they know what they are doing, will have designed the amp to function correctly into certain set of loads, so I would always advise following the manufacturers instructions.

Incidentally, the rise in speaker impedance at resonance can be a real problem for guitarists who detune. When chatting to our speaker guys, they said that they had to design some speakers with especially low resonance for a Matamp customer who was blowing transformers by playing loud detuned guitar. At the speaker resonant frequency the load reflected into the valve was way to high and caused voltage spikes that destroyed the transformer.

The Trainwreck Express was designed to run EL34s and 6V6s. However 6V6s work better into twice the load of EL34s so Ken Fisher its designer recommended that the impedance setting was switched to half the load when 6V6s were used, eg a 16 ohm load would be connected to the 8 ohm tap.

[shaman]

Thus presenting a load around about the correct area is going to be fine. Furthermore, the manufacturer, if they know what they are doing, will have designed the amp to function correctly into certain set of loads, so I would always advise following the manufacturers instructions.

...this applies to Mesa Boogies-every one I have owned says a mismatch is ok(...4 ohm into 8 ohm speaker,for example...)

[martinw]

Incidentally, the rise in speaker impedance at resonance can be a real problem for guitarists who detune. When chatting to our speaker guys, they said that they had to design some speakers with especially low resonance for a Matamp customer who was blowing transformers by playing loud detuned guitar. At the speaker resonant frequency the load reflected into the valve was way to high and caused voltage spikes that destroyed the transformer.

There's substantially more to that story than that, but this forum probably isn't the place to go into it.

[MDV]

Incidentally, the rise in speaker impedance at resonance can be a real problem for guitarists who detune. When chatting to our speaker guys, they said that they had to design some speakers with especially low resonance for a Matamp customer who was blowing transformers by playing loud detuned guitar. At the speaker resonant frequency the load reflected into the valve was way to high and caused voltage spikes that destroyed the transformer.

There's substantially more to that story than that, but this forum probably isn't the place to go into it.

Yes, it is.

Please do.

[jpfamps]

I expect there could well have been an element of pilot error in this as well.....

[viking]

Thus presenting a load around about the correct area is going to be fine. Furthermore, the manufacturer, if they know what they are doing, will have designed the amp to function correctly into certain set of loads, so I would always advise following the manufacturers instructions.

...this applies to Mesa Boogies-every one I have owned says a mismatch is ok(...4 ohm into 8 ohm speaker,for example...)

Yes.It's the same with my M&B Nomad.There is no 16 ohm on the amp,for ex. and,in the manual,they describe in great detail all the possibilities;the" good mismatches" and the" bad" ones...

[HairyChris]

One personal experience of mis-matching...

JCM900 @ 8 ohms +
Stereo cab @ 8 ohms a side (so 4 ohm parallel load on amp) +
several months =
crispy fried output transformer and a large repair bill....

 :oops:

I was an ohm-n00b at the time but learned quickly after that!