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previous: Steve Ahola Doc.. Could you elaborate a bit... -- 10/24/1997 4:59 AM View Thread

Re: Half Power Mod

10/24/1997 1:04 PM
Jim S.
Re: Half Power Mod
I'll try to answer this one...  
The idea of SimulClass is to combine a push-pull pair of power tubes running in fixed-bias class AB with another push-pull pair of tubes running in class A triode. It turns out that there's more hype than reality when you look at the actual circuit.  
It turns out that BOTH pairs of tubes are run using fixed bias (negative bias voltage applied to the control grids). Now the "class A" pair does run hotter than the class AB pair - this is accomplished by a voltage divider that both reduces the negative bias voltage AND the drive signal going to the control grids. However, this pair of tubes is running much cooler than maximum plate dissipation. Typically, the class AB pair is run at around 25ma per tube idle current and the "class A" pair run around 35ma per tube, all tubes having a plate voltage between 440 and 480 volts (depending on what amp model). What supposedly limits crossover distortion in the "class A" pair of tubes is the fact that the drive signal is attenuated to such an extent that it will clip (in the driver stage) before it can cause either tube to be driven into cutoff. Thus, the claim of "class A" operation.  
The pseudo triode connection (I say "pseudo" because tying the screen to the plates is not really the same as using true triode power tubes, like certain audiofile amps do) does help to soften the sound and aid in the illusion of class A tone.  
It's interesting to note that the classic class A guitar amp output circuit - the AC30 - is completely different than the Simulclass "class A" circuit. In the AC30 (and many other new amps that use 2 or 4 EL84's in class A) the tubes are run in full pentode cathode bias mode. This creates a great, classic tone that many guitar players feel is unequalled. IMO, the SimulClass circuit sounds nothing like this, even when switched to class A only operation. (I remember reading a Ken Fischer article on this subject - his opinion is that when the SimulClass circuit is driven into distortion, the sound "turns to poo poo". I'm not sure if I agree it's THAT bad...)  
It would be interesting to see if anybody ever comes out with an amp that uses 2 or 4 6L6GC tubes in a class A cathode biased pentode configuration. I wonder what it would sound like? (Come to think of it, I did once work on an old Supro which was basically a copy of a Tweed Deluxe, except it used 6L6s and had a 15" speaker. It probably ran at class A or at least pretty near. Once it was repaired the amp sounded great!).

John Greene OK, let me test my understanding of... -- 10/24/1997 1:57 PM
Steve Ahola Jim S. said ...It would be inte... -- 10/25/1997 1:36 AM