What a difference a transistor can make!

[Mark Hammer]

Thanks, gentlemen.  Much to chew over here.

[R.G.]

Well don't chew too much on that schemo I posted. I had time to drop in into the simulator. Paul, you're right, it's too touchy to be used.

The N-FET/PNP compound gives really good results, response from DC to daylight and steady DC levels over many devices. A simple JFET source follower in front of the existing circuit works too, although the existing circuit will drift unacceptably in the real world. 

An N-channel JFET like my favorite the 2N5485 gives about a 1V peak range on the input signal, so it'll distort with humbuckers, but that may be nice. The J201 is too small a Vgsoff to give good results with a simple 1M-to-ground bias on the JFET.

[alparent]

I changed a bunch of the larger value electrolytics this evening - some with larger values, like 220uf instead of 100uf - and that did a nice job of improving the hum specs, though the 4000uf/50v unit will need to wait a bit until I can score a suitable replacement.

Electronics goldmine, Panasonic 18kuFs for $2, last I looked. I bought 20 for some old amps I'm reworking.

Hey Mark! I looked in my treasure box and these are some of the big caps I have.

4000u-40v Callins
7500uf-15v Sprague Powerlytic
3600uf-40v Sprague Powerlytic
13000uf-15v Sprague Powerlytic
and last but not least........16000uf-50v Sprague Powerlytic

all are NOS.

If any of those interest you ...... just let me know and I'll bring them to work.

[PRR]

> drop it into the simulator. ...it's too touchy

Counting on thumbs, I thought Heath's biasing was too touchy. But someone did the homework. Simulating Heath's plan, it is quite stable against hFE variation, though at different op-point than Heath's notes show. (May be Vbe difference.) It varies with temperature, but not bad over the range of temps that plucking-fingers can operate over.

That's not even counting the non-negligible DC feedback from the power supply dropping resistor to four near-identical stages. (Which may be less-effective if one stage is converted to some other plan.)

> N-FET/PNP compound gives really good results, response from DC to daylight and steady DC levels over many devices.

Yes, and we can improve a 1961 Cadillac by cutting off 4 feet of nose and 8 feet of tail-fin, replacing the QuadraJet gas-leak with CPU control.... until we have a 2011 Lexus. Not that this is a Cadillac, nor that Mark is looking for a Lexus.

If he just wants a clean fault-free transistor amp, the pawn-shops are full of such stuff. I assume there is some attraction to this hunk of history. Adding transistors is anachronistic. OTOH, my neighbor has a 1937 Chevy Six with a 2003 V-8 engine....