I'm in the process of building a shop amp from parts I have laying around .The ST 70 chassis was rusty and had huge holes cut into the to and front , a friend ( Blair ) machined a plate to go on the top , and a wood front panel .
the power transformer is from another ST 70 and the output transformers are from early Fisher console that used four 6V6GT's in PPP , the impeadance is 4.2K , and has a tertiary winding just like the Fisher 80AZ amps .The Fisher output are made by Todd and are about 30% larger than the original ST 70's .
Building it to use 6B4G and 5881's interchangeably , can do this because of the pin out of the 6B4G and the 5881 , are the same except for the cathode .
The 5881 uses pin one for the cathode and the 6B4G has a directly heated cathode , with use cathode bias on both tubes using 820 ohm cathode resistors on the 6B4G's and 250 ohm cathode resistors for the 5881's .
Using a choke input power supply , the B+ will be around 350 volts which will be good for both output tube types , will be using SS rectification for the B+ and will use the 5 volt winding for DC filaments for the driver tubes .
The driver board is a DIY-TUBE ST 70 driver board that I traded for from Shannon for some tubes he needed , the driver tubes will be 12BH7's and the input tube will be a 7025
http://www.diytube.com/phpBB2/viewtopic.php?f=6&t=813 .
Will post some schematics later .
It should be around 10 watts with the 6B4G installed and 20 watts with the 5881's , I'm going for sound quality over power out . No soldering has started , so the transformer will be easy to clean up