Solid State Guitar Amp Forum | DIY Guitar Amplifiers

How I went down this rabbit hole is that for years I took it for granted that there is a diminished high end when using maximum output volume from the preamp and lowering the input volume on the aforementioned power amp. I chalked it up to some kind of "gain staging" issue and didn't look into it. But recently, I started digging into it: what if it is just the input circuit of the power amp?

I took a look into the schematic and indeed: there is a shunt capacitor of 470 pF (actually installed: 330 pF, and listed as 330 in the repair manual's BOM also). It is CX14 in the schmatic: the X is interpreted as 2 for the left channel, 1 for right. This is preceded by a 5.6Kresistor, before which is the input volume, a 10K pot. So indeed, the 10K pot and 5.6K resistor form a variable low-pass filter with the cap; turning down the volume adds more of the pot's resistance into the signal path, rolling off the high end more.

What I didn't expect is how that little ceramic disc was degrading the signal in other ways!

First I removed it completely. Bam that was it: the frequency response issue went away. Different level-matched combinations of input volume and output volume on the preamp now sounded the same to my ears.

I was surprised at different improvement; like some kind of veil of a harsh diffusion was removed from the tone. That ceramic cap was adding distortion!

Next, I put in a 100 pF silver mica. And, wow, the sound opened up to a better definition. The biggest surprise is the low end: I had to cut bass a little bit in the preamp on the heavily distorted tone to dial it to where I like it.

If you search about this amp, there is a forum thread on a certain audio site about someone improving the output side of the amp, with good reasons: removing the ferrite core from the output inductor, and dropping the values of the compensating caps. I might try those changes also. That person missed the input side ceramic though, or at least didn't write about it in that thread.

Just with this input-side fix, the amp sounds way better now when you point the speaker at your face, and also sounds way better cranked up.