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Weird question!PP with single valve?


 
1/11/2000 10:08 PM
Max
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Weird question!PP with single valve?
Hi again.  
 
I was wondering if it was possible to make a push pull with a single output valve?  
 
with a transformer with primary windings seeing different reflected load?  
Anyway best regards.  
Max.
 
1/11/2000 10:29 PM
jp
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Well, if you only need a little power, say to power headphones, you can set up a dual triode (eg 12ax7) in push pull. I believe this is what on-board Fender reverbs do before the reverb drive transformer (no schems on hand to confirm this). A reverb transformer would make a fairly good OT for this application, and quite a few are 8 ohm rated.  
 
You might have seen the Gerlitz amp in GP a while back that uses a similar scheme with industrial grade 12AU7's to simulate powerstages for Marshall and Fenderish preamp stages in front of a solidstate power amp. Their verdict was that this didn't offer the same sort of output stage coloration as an EL-34 or 6L6, not surprisingly.  
 
I realize it's not quite what you were asking for, but maybe of some use.  
 
j.p.
 
 
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1/11/2000 11:33 PM
Randall Aiken
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The Svetlana 6N1P dual triode is ideal for this purpose because it has a 2.2W dissipation per triode and a 4.4K Rp. It should be happy with a 17K or lower plate-to-plate load in push-pull.  
 
BTW, standard reverb trannies can't be used because they are single-ended, and have no center-tap. The Hammond 125D or 125E "universal" output transformers would be an excellent choice for this application.  
 
Randall Aiken
 
1/12/2000 3:19 PM
jp
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Naturally, I go home and look at a schematic for a fender reverb amp and it's a single ended design. Gotta get my head out of tweeds some times. So reverb transformers are ought. But the 3 watt Hammond 125 series ought to be sufficient for using a twin triode, and they're fairly cheap for the quality.  
 
j.p.  
 
1/13/2000 8:49 PM
Brian
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jp;  
 
onboard Fender Reverbs use a 12AT7 in parallel, not push pull. The plates, grids and cathodes are all connected together. This allows for more oomph thought the reverb trannie.  
 
Brian
 
1/11/2000 10:58 PM
Marc
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Max:  
Stephen Delft's moonlight amp is a good example of the single tube PP amp....I forget where the schematic is. Here or Steve Ahola's site...  
 
Marc
 
1/11/2000 11:13 PM
Don Symes
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Check the AX84 forum for Delft's moonlight project. The fender reverb circuits I've seen use a single-ended OT, sometimes with two triodes in parallel ... so that xfmr won't work as a PP output.
 
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