Question about simul vs. class A switching and impedance

[Whoopysnorp]

OK, I know that what Mesa calls 'class A' in their Simulclass amps isn't really class A but is really just triode wiring, but to make things easy I'm just gonna call it class A for the purposes of this question...

As far as I know, when Mesa allows you to switch between Simulclass and class A, what is really going on is that the pentode-wired pair of tubes is cut out of the signal. Does this affect impedance in any way? If you pull a tube pair in a class A/B amp, you've doubled your output impedance, so an 8 ohm cab would need to be connected to a 4 ohm jack (whose OT tap is now actually 8 ohms). Does the same not hold true when only the triode-wired pair is running?

The reason I'm asking is that today I had the idea of changing the way I run my Simul 395...normally I just use one side and leave the other side on standby with old tubes in it. Today I thought it might be cool to pull a pair of tubes on both sides, leaving the pentode pair in one side and the triode pair in the other, and run each side of the amp into one of the sides of my 4x12. Trouble is, the minimum impedance available on each side of the amp is 4 ohms, and that's the rating of the individual halves of the cabinet as well (8 ohm speakers wired series-parallel). I don't really feel like rewiring the cab.

 

[trem]

OK, I know that what Mesa calls 'class A' in their Simulclass amps isn't really class A but is really just triode wiring, but to make things easy I'm just gonna call it class A for the purposes of this question...

You mind elaborating this further? As far as I know, the outer pair is biased hotter. In Mark IV the outer pair can be selected either triode or pentode. I'm not sure it's really class A either, but I think calling it "really just triode wiring" is inaccurate.

The impedance question is interesting. I'd like to know the answer as well.

 

[GIG4FUN]

My Mark III green striper it's wired pentode. It breaks from the rest of the herd in this regard. I too did not know what you say to be true about the class "A" in our amps. Please do tell more.

 

[Whoopysnorp]

I'm not positive that's the exact explanation of what the class A really is...I just know that in Mesa's amps (with the exception of the Lonestar) the 'class A' tubes are actually working in a push-pull pair, making them distinct from a true single-ended class A design. However, what I'm really curious about is the impedance question...I know the manual for my 395 specifically says that the class A switch cuts off one pair of output tubes.

 

[trem]

According to Aiken, class A can be made either push-pull or single ended.

Q: Is it true that the only true class A amplifier is a single-ended amplifier?
A: No. You can design a true class A single-ended or push-pull amplifier. The presence of a phase inverter tube does not automatically mean the amplifier is class AB, just as the presence of a cathode resistor doesn't automatically mean the amplifier is class A. It is all a function of where the output tubes are biased, and under what voltage/impedance conditions they are operating. In fact, unless you know the plate voltage, plate bias current, and output transformer reflected impedance, you can't tell the class of an amplifier just by looking at the schematic. A push-pull class A amplifier differs from a single-ended class A amplifier tonally in that it cancels even-order harmonics generated in the output stage (but passes through even harmonics generated in the preamp stages, of course). It also has inherent power supply rejection for lower hum and noise levels than a single-ended class A amplifier. Typically, a push-pull class A amplifier will clip rather symmetrically, while a single-ended class A amplifier usually clips asymmetrically, most often rounded on one side while hard-clipped on the other. While both amps are indeed true class A amplifiers, their tones are dramatically different. This further illustrates the fact that there is really no such thing as a "class A" tone.

http://www.aikenamps.com/TI_Aiken_Q&A.html

In any case, I'm also interested in knowing whether the impedance changes when "class A" is selected. In regular 100 watts amp, taking off either pair would require impedance change in speaker hook up, while in the simul-class case, there's no such instructions in the manual.

 

[Whoopysnorp]

Interesting explanation...I never understood why a class A design had to be single-ended anyway.

RE: the impedance question, perhaps Mesa uses sufficiently beefy transformers in their amps that they feel the impedance mismatch caused by this configuration is not going to hurt anything, if indeed there isn't something clever going on in the circuit to get around this?

 

[trem]

Whatever the explanation may be, since it's not mentioned in the manual, I think you shouldn't be worried about impedance. Don't pull the tubes though, even if you're in class A setting.

 

[Whoopysnorp]

How come? Not even if I were to rewire my cab and correct for impedance?

 

[trem]

About the impedance, I'm not sure why myself, but I would like to believe Randall Smith for this one :lol: . Hopefully Boogiebabies can provide some answer.

About pulling tubes, it should be okay to do. But you'll never know when you (or anyone) might get the urge to switch back to simul without looking at the tubes, then it's BBQ OT.

 

[Whoopysnorp]

It'd be the other way around, right? Meaning if somebody switched to class A on whichever side only had tubes in the class A/B sockets? Yeah, that I could see being a problem.

 

[Whoopysnorp]

Or is it something to do with the fact that the class A sockets are biased hotter than the others, and this prevents you from doing the typical 'pull one pair' thing safely?

 

[trem]

Hey sorry, I wasn't watching. Anyway, no, I'm right. I'm not sure how the tube arrangement is on a 395, but on the simul equipped Marks, the outer sockets are class A, while the inner are class A/B. If the class A switch is selected then the inner pair is turned off. Theoretically it should be okay to pull this inner pair in class A selection, cos there's only heater current flowing through. But should you flick the switch back to simul without the inner tubes in their place, then you'll fry the OT.

Anyone wanna chime in on the impedance question?

 

[Yetti]

Hey Fellas

Im learning more and more on here when i read this stuff. This might not have much to do with your discussion, but i wanted to throw this out there and see if anyone else has different ways to wire up to their cabs.
I run my 395 and split my cab up i usually run Channel A on Class A and that splits my 2 left side speakers and On the B side i set it for SimulClass for mad lead, and those power out on the right side of the cab. Put them together and you get some cool stereo. And Go between track 1 and 2 for different volumes. Still getting to know the 395 but love it so far.

 

[trem]

If that's what they say then I stand corrected. Read through the patent paper. There's definitely going to be an impedance mismatch if either pair of tubes is pulled. I suppose Whoopysnorp's statement that their OT is beefy is correct. In any case I won't pull any either.

Did a bit more researching and found this interesting FAQ:
http://www.londonpower.com/faq.htm

 

[Whoopysnorp]

Mesa just called me back...I explained my idea to the guy (forgot his name) and he went to go ask a tech. The tech thought it should be OK. I also asked him specifically about impedance changes when using the class A sockets only, and he said the amp takes care of that. Either there's something switching in the circuit, or the output transformers are just too big to care about that degree of mismatch.

 

[MrMarkIII]

If switching to "Class A" in a Simul-Class amp changed the output speaker impedance, it would say so in the manual. I'm sure it does not.
Note that running the two outer tubes on a Simul-Class amp is NOT the same as pulling two tubes from, say, a 100 watt Recto so you can get it down to 50 watts. In this example, you ARE advised to use the 4 ohm speaker tap instead of the 8 ohm.
Completely different animals.