What is REALLY going on in the class A sockets of a Mark?

[Whoopysnorp]

I know enough to know that most of the time, in the world of amps, something claiming to be running class A is probably not doing so in the strictest sense of the definition, and my understanding is that this holds true for the outer sockets in the Mark series amps. So can anybody explain what is actually going on in those sockets, and what that means with regard to how much idle current I should be able to measure? I am keen to take my bias probe to my Mark III and see how cold or hot my power tubes are running.

 

[scottywompas]

Yes, Randall Smith can explain.

http://www.mesaboogie.com/US/Smith/ClassA.htm

 

[Whoopysnorp]

I had read that before, but I did need a refresher. So, are the outer sockets actually biased at 50% of maximum? If I stick my bias probe in there, will I see some huge value? If I put my bias probe on it and don't see a large value, is something wrong?

 

[Whoopysnorp]

Well I went ahead and measured the current on the outer and inner sockets...I'm getting about 45 mA on the inner sockets, which is pretty much exactly 60% of maximum at the plate voltage that the amp runs. On the outer sockets, I'm getting like 46-47 mA. I thought that if these are really running in class A, they should be pulling a lot more current at idle...what am I not understanding here?

 

[Whoopysnorp]

Bump.

 

[mr_fender]

I've wondered the same thing. My understanding of Class A is the same as yours... tubes biased to approximately 50% and each tube amplifies the entire waveform. Based on this, and looking at the schematic, it seems that simul-class amps don't run in Class A at all. Both pairs appear to be getting the same level of bias and would have roughly the same amount of idle current. Switching between Simul-class and Class A simply lifts the ground connection at the cathode of the two inner tubes, effectively turning them off. It seems to me that this is more of a simple half power switch and the whole simul-class thing is creative marketing. After all, any class A/B amp is running in a combination of class A and Class B at the same time (i.e. class B but biased hotter more like class A to minimize crossover distortion).

The only thing I can think of is by turning off the two inner tubes, the input impedence seen by the bias circuit is increased (voltage divider effect?), therefore increasing the bias voltage and current flowing through the remaining two tubes. If that's the case, I still don't see how having all 4 tubes going makes the "simul-class" mode any different than other regular class a/b push-pull amp. Again seems like creative marketing.

I might be terribly wrong here (very possible since I'm no electrical engineer), but I sure would like to know for sure.

 

[mr_fender]

Out of curiosity, did you measure the idle current with the amp in class A or simul-class mode. I'd like to know the reading for all 4 tubes in each mode.

 

[mr_fender]

Bingo! It just occurred to me that the output transformer is different. Normal class A/B push pull amps like a Marshall plexi or Fender Twin Reverb have only three OT leads on the primary side (center tap supply and one lead for each of the positive and negative plate pairs). The Simul-class OT has 5 leads (center tap supply and two different pairs of positive and negative plate connections). This probably accounts for the difference. The idle current might be the same for all 4 tubes, but the outer tubes are referenced to a different plate voltage. In reference to this lower plate voltage, the idle current would appear higher by comparison and the outer tubes would be closer to Class A operation.

Again, I might be way off here, but it seems to make sense in my twisted little mind.

 

[Whoopysnorp]

The first measurement I took was one of the outer sockets, where I got the 46 mA figure. I know I had it in class A at the time because when I moved the probe to the adjacent socket, I got something like 6 mA before I remembered to switch it to simul-class. The thing is, I am pretty sure I also took the plate voltage measurement off the class A socket. It was 438 volts, which still puts 46 mA in class A/B zone. I suppose it's possible I didn't remember to measure plate voltage until I was on one of the inner sockets. I don't feel like pulling the chassis again at the moment, but I have some parts on the way to do the C30 cap mod and the R2 master mod, so I'll check again at that point. I am about 90 percent sure, though, that I took the plate volts measurement off one of the outer sockets.

 

[stokes]

Bingo! It just occurred to me that the output transformer is different. Normal class A/B push pull amps like a Marshall plexi or Fender Twin Reverb have only three OT leads on the primary side (center tap supply and one lead for each of the positive and negative plate pairs). The Simul-class OT has 5 leads (center tap supply and two different pairs of positive and negative plate connections). This probably accounts for the difference. The idle current might be the same for all 4 tubes, but the outer tubes are referenced to a different plate voltage. In reference to this lower plate voltage, the idle current would appear higher by comparison and the outer tubes would be closer to Class A operation.

Again, I might be way off here, but it seems to make sense in my twisted little mind.

The output tranny on the simul-class amps is whats known as an "ultralinear" transformer.It was originally intended to use the inner pair of leads of the primary as screen taps,so instead of a seperate screen supply the screens were supplied a slightly lower voltage than the plates,from the main supply.Just take a look at the 135 watt Fender Twin schematic.That is basically a simul-class transformer.Insted of connecting the ultralinear tap to the screens,he just connected them to a pair of plates for slightly less voltag on that pair and called it "simul-class".The class A sockets on the Mark amp is not even close to class A.Like many amp manufacturers he's just using class A as a marketing ploy.

 

[Whoopysnorp]

Bingo! It just occurred to me that the output transformer is different. Normal class A/B push pull amps like a Marshall plexi or Fender Twin Reverb have only three OT leads on the primary side (center tap supply and one lead for each of the positive and negative plate pairs). The Simul-class OT has 5 leads (center tap supply and two different pairs of positive and negative plate connections). This probably accounts for the difference. The idle current might be the same for all 4 tubes, but the outer tubes are referenced to a different plate voltage. In reference to this lower plate voltage, the idle current would appear higher by comparison and the outer tubes would be closer to Class A operation.

Again, I might be way off here, but it seems to make sense in my twisted little mind.

The output tranny on the simul-class amps is whats known as an "ultralinear" transformer.It was originally intended to use the inner pair of leads of the primary as screen taps,so instead of a seperate screen supply the screens were supplied a slightly lower voltage than the plates,from the main supply.Just take a look at the 135 watt Fender Twin schematic.That is basically a simul-class transformer.Insted of connecting the ultralinear tap to the screens,he just connected them to a pair of plates for slightly less voltag on that pair and called it "simul-class".The class A sockets on the Mark amp is not even close to class A.Like many amp manufacturers he's just using class A as a marketing ploy.

So I guess any tonal difference is primarily due to triode vs. pentode, except in the green stripes. I had a feeling something was up.

 

[mr_fender]

OK cool. So I'm not totally crazy. I had a feeling it was a bit of creative marketing. "Think of it as two different power amps working simul-taneously. One extracts the juice of Class A sweetness while the other delivers the high power punch of Pentode Class AB" They do have a way with words though (Doug West?). This is kind of amusing since they have that booklet on how class A works. Their "class A" in simul-class amps is nothing of the sort. Well, anyway you slice it, class A or not, the simul-class power sections do sound really great. It always really shocked me to hear the difference between the ugly harshness of the direct output of my old Triaxis vs the super sweet tones that poured out of the 2:90 it fed. That sweet tube amp magic clearly happens in the power section. My MkIV makes me grin for sure.

 

[rabies]

But what about the Mark V 10 watt mode (pseudo-single-ended?) So is that true class A or not?

When I had my Mark IV, I could definitely tell a distinct tonal and feel difference b/n class A and simul modes. The Mesa Class A is way more compressed and thicker sounding to my ears...

I've read that a true class A circuit must have only one output tube (not a pair of push/pull which is the typical scenario in most tube power amp sections).

So what are some modern examples of true class A amps?

 

[rabies]

BTW, I measured the outer sockets @ 8ma with EL34s in my Mark IV. The non-adjustable bias is a dual-edged sword (one reason I sold my IV).

Who (other than Mesa) needs or wants frozen bias like that???

 

[mr_fender]

But what about the Mark V 10 watt mode (pseudo-single-ended?) So is that true class A or not?

Possibly, it depends on how it's biased. If DC current is continuously flowing through the tube and never stops regardless of input, then it would be class A. In theory you could take any class A/B power section and bias the tubes super hot to the point that the amp would be running in true class A. I say in theory, because while it might work for a short while, the output transformer would most likely suffer a terrible death due to all the excess idle current. BTW, output tube life is also shorter in class A amps, also due to the high idle current.

I've read that a true class A circuit must have only one output tube (not a pair of push/pull which is the typical scenario in most tube power amp sections).

Class A amps can be push-pull configuration. Class A simply refers to the fact that the tubes are always in an state of conduction. They don't "turn off" for any portion of the wave but rather amplify it in its entirety (not counting clipping). This makes for a very responsive circuit, but comes at the cost of drastic inefficiency. Class A amps waste a lot of power due to the high constant idle current. Preamp tube sections are almost always class A single ended config. They are just as inefficient, but since there is A LOT less current flowing through preamp tubes it's not as big of a deal.

Single-Ended amps use only one tube. For audio purposes, single-ended stages are always class A, since class B would result in DRASTIC distortion (rectification practically) of the signal. There are no class a/b single ended amps, as class a/b implies a push-pull configuration requiring 2 or more tubes.

So what are some modern examples of true class A amps?

True class A guitar amps are not super common in the mass marketplace. Due to their inefficiency, you almost never see any class A amps that are more than 20 or 30 watts. The Epiphone valve junior is a single ended class A amp. The Matchless Chieftain is a push-pull class A amp. While commonly mistaken for one, the Vox AC30 is NOT a class A amp, but rather class A/B that is biased hot (closer to class A operation, but still A/B).

 

[dmcguitar]

thd univalve.. its only got one tube.... so id say its true class a. and there bivalve prob is also

 

[rabies]

anybody can cite a true class A popular song?

 

[straitouttahell]

So can anybody explain what is actually going on in those sockets

Something magic :lol: