ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

PLexi gain

:

10/15/1999 8:00 AM
Jeff	PLexi gain Is it possible to tweak a ('67) Plexi's preamp circuitry to get a bit more gain? Thanks, Jeff

10/15/1999 8:47 AM
A.S	Yes Jeff. Did you look at the mod over at Hoffman´s? It looked pretty interesting to me, fully reversable. One thing about the plexi is that it lacks a master volume. Go to the "schematics" page here on Ampage and check out the schematic of the Marshall master volume pre amp. This design is a good start. Use "volume I" as gain control and "volume II" as master. Good luck A.S

10/15/1999 2:18 PM
dr	Jeff, Does you plexi already have a .68uf bypass cap across the second gain stage cathode? If it doesn't, put one in there and you'll get a tad more gain. dr

10/15/1999 2:51 PM
Mook	What's your FB resistor? Up it to 100K for a bit more gain, too. Mook

10/15/1999 7:24 PM
Jeff	Yes, it does have to .68uf. I think the fb is 47 on the 4 ohm tap. Thanks Jeff

10/16/1999 10:21 PM
Trace	Try switching the feedback wire. Lift it off from the 4-ohm tap and try it on the 8ohm tap. Speedracer had soem great comments in a thread (the longer feedback thread) about the over all effects. Trace

10/18/1999 2:24 PM

Trace:

Did you mean that the other way around? (Like "lift it from the 8 ohm and connect it to the 4 ohm"?) There's less voltage available for feedback at the 4 ohm tap. Moving up to the 8 ohm tap increases the available voltage by 1.4 (square root of the impedance multiplier, 2x), so with feedback applied the overall gain will drop a little on the power amp. If the loop's already connected to the 16 tap and you move it to the 8, then gain will go up a little.

Doc

Page 1 of 3