[sdiy] tube preamp and fx chain

[jhaible at debitel.net]

> But which 
> only are present when the power stage is nearing its rated output. With 
> lower volume, most of the sound does come from the preamp.

You can get remarkable sounds from preamps which are connected like
output amps. In the process of building a variable mu compressor, I
had tested just the variable gain input stage. That's a push/pull
circuit, similar to the differential pair of an OTA input. Two halves
of a 6BC8 tube work into two 5kOhm resistors @ 108V supply voltage,
and an interstage transformer (UTC A-19, 80kOhm : 15kOhm) is used
to couple the next stage. This stage produces the best overdrive sound
I've ever heard. I have no idea what's the main part in it, the quite
nonlinear tubes which weren't intended for audio, or the UTC transformer,
or both. In the final compressor circuit, there is never so much overdrive
(the following stage, also tube, but with high feedback, would be
clipping), but one day I'll build just this input stage as a stndalone
distortion box.

Just wanted to mention it, because the topology is so similar to a
pp power amp.

JH.

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