SFT 337 Germanium Equivalent

[sarakisof]

I have an old 2x12 Farfisa TR 60 amp here (two channel germanium preamps - 2N3055 power stage - spring rvrb & tremolo germanium).



Channel 1 signal fades out while distorting until it finally goes away after 5 mins of amp function. Common germanium temp related issue.
Verified & sorted down using signal tracing & freeze spray.
Culprit found. SFT 337.



I have on hand dozens of NOS:
SFT 323, AC 122, AC 131 , AC 132
I had found at a flea back then
and four only matched AC 125 I had specially bought years ago for a fuzz project I never used (had ended up with Russian MΠ's).
So I wouldn't spend those four AC125's, only in case it's the most close choice than others.

I know all of them should work fine, but if you were about to choose, which one would you end up with in terms of the most identical choice?

I'd like to go with SFT 323's.

[antonis]

Σοφοκλή, I think a fan blower as close as possible to SFT337 should be more convenient..
(it should also cool down that 50μF Emitter resistor bypass capacitor..)

More seriously, TR2 configuration is just a stiffly biased humble CE amp so, IMHO, proceed as you like..

[amptramp]

If you replace the germanium transistors with silicon, you will eliminate problems due to leakage current, which is often on the same order of magnitude as the desired collector current with germanium.  This is not a Fuzz Face where the characteristics of the transistor are part of the "mojo" of the effect.  This is designed as a linear stage, so conversion to silicon does not affect performance except to stabilize base current.

The 33K resistor that pulls the base high may have to be replaced by a higher value and maybe the pulldown resistor would have to be decreased in value to get the right bias point, but that should not be difficult to do.

The base to emitter voltage on a germanium transistor would be about 0.3 volts whereas for silicon it would be about 0.6 volts.  If you want to retain the "mojo" of germanium, be prepared to have circuit performance that varies all over the place.  This appears to be the case as the channel 2 preamp stage does not seem to be doing the same thing as the channel 1 preamp even though they are identical.

It would be fun to go through this amp and change germanium to silicon wherever it is used and change out some of the lower-value electrolytics to film capacitors and see just how much better it behaves.