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Questions on the Vox AC-30TB... and the Peavey Classic 20

8/4/1999 3:58 AM
Steve A.
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Questions on the Vox AC-30TB... and the Peavey Classic 20
    I was comparing the schematic of the humble Peavey Classic 20 with a AC-30TB and was wondering about switching over the preamp section to something like the Vox. The AC-30 does use a long-tailed driver for the PI while the Classic 20 uses only a single stage cathodyne driver. So I see where an AC-30 conversion might lose some gain in the signal to the output tubes.  
 
 
 
    Speaking of output tubes, the Classic 20 uses a pair of EL84's which are cathode biased with a pair of 220R 1W resistors in parallel (along with a 22uF/25v cap). I don't know what the B+ voltage going to the plates are (my guess would be ~330vdc), but would that be Class A or Class AB? The AC30 uses 100R resistors on each of the EL84's which I assume would bias them into Class A. If I were to try replacing the 220R resistors with 100R resistors, it might overheat the PT- right? BTW over at the Hoffman site they have a layout drawing for an AC30TB amp (one of their board kits?) that runs the cathodes from the 4 EL84's to a single 50R resistor (which would be the equivalent of 200R per tube- right?) Thinking that this kit would be used in a Marshall or Bassman head, I figured that they decided to keep the bias current lower than a real AC30 so that it might not overheat the stock PT. But if the Hoffman AC30 kit sounds pretty good with 200R Ck per EL84 then maybe the Classic 20 would, too? (That's assuming that I rewire the preamp section per AC30TB specs, of course.)  
 
 
 
    As for the lowered gain from the cathodyne PI, maybe I could use an IRF820 MOSFET immediately ahead of it as a pre-driver (similar to what Peavey does with their Classic 30 and 50 amps)? BTW has anybody tried adding a IRF820 MOSFET pre-driver to a BF/SFPR amp? Or another amp using a cathodyne PI?  
 
 
 
Thanks!  
 
 
 
Steve Ahola  
 
 
 
P.S. I had gotten e-mail from a guitarist who hated his Classic 20 a few years ago and was going to gut it and rebuild it as a Vox AC15. I didn't really think it was "doable" back then but after looking at the schematics I'd have to agree with his idea- only forget the AC15 and build an "AC30/2TB"... (identical to an AC30TB but with only 2 EL84's). Hmmm... instead of the IRF820 MOSFET what if I was to add in a third preamp tube socket and wire it up as the PI per AC30 specs? (MOSFET's are totally new to me and I'd feel more comfortable working with tubes.) Question: can you energize just the heater of the tube stage you are using (with DC) or should you always energize the heaters of both stages? (I'm asking because while the power supply might be able to handle the added current of a single heater it might choke if I expected it to handle both heaters of a third 12AX7...)  
 
 
 
    Thanks! (After doing the "Tweed Bassman" mod to my Classic 50 it would be interesting if I was to do an "AC30TB" mod to a Classic 20...)

 
Replies:
Benjamin Fargen Steve, <... -- 8/4/1999 5:17 PM
SpeedRacer I'd go ahead and build it and then ... -- 8/4/1999 5:56 PM
Brian Steve;An AC15 is *a... -- 8/4/1999 7:41 PM
John Stokes This "class A" thing has been beate... -- 8/5/1999 12:51 AM