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previous: J.D Re: Mesa simul class -- 4/22/2000 11:43 AM View Thread

Simul-Class rant

4/22/2000 2:55 PM
Ray Ivers
Simul-Class rant
JD,  
 
The 'Class A / Simul-Class' switch in a Boogie is, in my opinion, actually a uniquely-implemented pentode/triode switch. Here's my take on this circuit (I'm using the schematics from pages 714 and 715 of the Groove Tubes 4.1 edition):  
 
With the switch in 'Simul-Class' position, the pentodes will dominate the transfer function (doesn't that sound just so.. TECHNICAL?), i.e., you won't hear the 'triodes' too much, directly. They WILL try to load down the pentodes' grids and plates, but the 220K grid stoppers will limit this (triodes have greatly increased input capacitance, which accounts for most of the treble drop people notice with triode/pentode switches, I believe), and the output transformer primary windings will also isolate the pentodes' plates a bit from the triode loading on the output (I don't have the bird's-eye low-down on this transformer, so I don't know how much isolation is available here) so the sound will be what I call 'mellowed pentode' - kind of cool, and at the time it was well accepted.  
 
Flick the switch, though, and hype takes over. Now you've actually got triodes operating into a proper load (roughly), instead of the way-higher-than-max-power hi-fi type loading you will typically get on a guitar amp with P/T switching. On a four-tube amp, though, if you can switch to half-power AND triode mode, you're in the ballpark, at least as far as the output goes. The lack of screen resistors will ensure short V8/V9 tube life, but at least we've got healthy Class AB bias levels on the grids ("Class A", indeed!). Perhaps if you put some KT90's into the V8/V9 sockets and replaced the 980K grid resistors with a 100K pot wired as a variable resistance... have the fire extinguisher handy.  
 
This output stage has potential for some experimentation, though, and unlike the preamp section, this part of these Boogies' circuitry is not hard to get to or work on.  
 
Did I answer anything you asked? Or did I just ramble? The 220K grid stopper resistors form a voltage divider with the 980K's to drop the bias from -56 to -45 volts, but I suspect they may be there for stability and/or driver-tube loading reasons as well. Sticking in a cathode resistor, even with a big bypass cap, will give you what I call "bouncy Class AB" operation in this circuit. The voltage is just too high in this amp to achieve true Class A operation (symmetrical CURRENT swing - it's transformer-coupling, remember) unless possibly with KT90's running REAL toasty and a fairly large value of cathode resistance, and then you get the operating-point shift inherent with a large cathode resistance... oh man, there I went and stepped into the Class A thing, which has got to be a close third to politics and religion on the amp tech 'no-no' list.  
 
If you want to make a couple of simple mods, I would add 1K, 5 watt screen resistors to the V8/V9 (outside?) sockets, pin 3 to pin 4, and see if you like the sound (those tubes will definitely like YOU a lot better, and may sing more sweetly in return). Whatever you do, I would keep the 980K grid resistors - these are a great addition to any output stage - I would use the 1 meg standard value - and seem to give the amp a more 'solid' sound (it's a vague description, I know - sorry), possibly through giving the bias voltage a fixed reference rather than the constantly changing internal resistance of the tube. Bias voltage will drop to a varying degree depending on grid circuit resistances, and must be compensated for. Another added benefit is making any bias-balance scheme work much better (replacing the 220K grid resistors with a 500K pot, preferably with a 1M resistor connected from each end of the pot to the wiper for reliability).  
 
That's about all I can think of for now. Let me know what you try!  
 
Ray

 
Replies:
J.D Wow!!Thanks Ray!!That was e... -- 4/22/2000 3:21 PM
Steve A. Ray:  &nb... -- 4/22/2000 6:14 PM
Gil Ayan Simulclass Update Re: Simul-Class rant -- 4/22/2000 9:29 PM