Frankenhouse 6V6 PP Amp

What an odd name, you might ask! Indeed. I came up with it because me and my oldest brother shoe-horned in a Philco power transformer and filter capacitor into a Westinghouse power amp chassis. So.. frankensteined Westinghouse.. Frankenhouse.. :-) And yes, in case you were wondering, this new transformer puts out more voltage, making for more output power... and a cool red glow on the 6V6 plates. ;)

Schematic

The circuit is pretty basic, but has some twists of course. I wanted something compact so that I don't have to cut a new hole in the chassis. But I need enough gain to go from about .5V input to the 30V or so grid drive required for the 6V6s, plus another 10x or so for NFB. That's a good 600 gain, and rules out triodes. Not many pentodes will do that either, so I'll lower my input figure to 1 or 2V; now I need 150 to 300 gain (about 47dB).

Photo

The thought had crossed my mind to use a pentode-based LTP (Long-Tailed Pair, a differential amplifier), which would produce the gain and split the phase for the PP output as well. But I don't know of any dual pentodes that'll fit...(well... now that I think of it, 6HS8 might work here). In any case, I need a third pentode for CCS (that makes the tail "long"), though now that I know some solid state I could use a transistor CCS. (It's perfectly okay to use SS, so long as it's in submission to the tubes. :> )

Underside

I chose one of those triode-pentode TV tubes: 6U8. The pentode gives a good 150V or so output for 1V grid drive, while the triode is used as a cathodyne (a.k.a. concertina, a.k.a. split load) phase splitter. For the output, class A clearly won't work because that's how it was originally - running past limits. So I chose class AB1, and added a small negative voltage supply for grid bias. Small 10 ohm resistors in the cathodes allow me to measure bias current.

Update, Fall 2007:

Summer over, Frankenhouse and Hept'AU7 were up for more amplifying duty. Evidently, the 5Y3 didn't take it too well! It appears this old GE-made, stem base 5Y3GT was just leaky enough that its getter had ran out over the summer idle period. Turning it on found me a big suprise: sparking and blue glow discharge! I even took a video of it...


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