[JN] Looking for a low cost (stereo) P/P 2a3 amp design.

Phil Sieg triodelover at comcast.net
Thu Feb 26 14:10:55 CST 2009

bfranchuk at jetnet.ab.ca wrote:
> Now that I have it working, the SET sounds great. I just need that extra
> bit of headroom however for classical music, now that I have some.
> Once I get the basic amp up, later I upgrade the system if it works well.
Coming out of a long-time lurk...here's an idea if you want a little 
headroom but like the SET sound.

A few years back I built a PP/bridged SE 45 amp, and it's going to get a 
rebuild this year.  Driver is a 45 but you can pick your flavor 
(although a 2A3/6A3/6B4G could be nice).  IT as a phase-splitter, but 
again, you can go another route if you want.  Output stage is a pair of 
choke-loaded 45s in a parafeed configuration.  B plus delivered though 
the choke instead of the CT of the OPT primary.   Reference the CT to 
ground through a SPST witch.  Bias the tubes to run in Class A1.  When 
you lift the switch and float the ground you're running in bridged SE. 
Close the switch and you are in Calls A1 PP.  And there's no need of a 
parafeed blocking cap.

For your 2A3s, you'll get about 6W either way.  Scavange a pair of 
output transformers from an old ST-70 and you're in business.  If you 
find an ST-70,with a working power transformer, that should get you what 
you need on the plates. BTW, Triode Electronics has replacement ST-70 
iron if you want to go that way.


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