Ampeg B12XT Channel 1 Tone Issues

[tdlunsfo]

Got a Fliptop B12XT on the bench. Amp has been recapped and channel 2 sounds bassy/lush but channel 1 has the ice pick highs and lacks bass with the controls set the same. Coupling caps and resistors have tested in spec up this point. Looking at the schematic, I noticed that Channel 1 only goes through the "vibrato" circuit and this is where I start to get some differences in tone between the two channels. Anyone familiar with these amps and can set me straight if this is a circuit issue or just how they are designed?

[tdlunsfo]

schematic

[g1]

Ch.1 from treble wiper goes through .005 cap, ch.2 has 470K resistor but no cap there.

[tdlunsfo]

I noticed that and temp added a .047 but didn't make much a difference.

[Helmholtz]

The corner frequency with the 5nF cap is 27Hz.

[tdlunsfo]

Digging into this a little further and the tone controls utilize couplets, and the tone variation appears to be coming from these. Bummer.

[g1]

I like Haiku myself , but do you mean those little P.E.C. RC networks? If not too complex they can always be replaced by discrete components.

[Helmholtz]

Digging into this a little further and the tone controls utilize couplets, and the tone variation appears to be coming from these. Bummer.

I can't find a useful German translation for couplet. What does it mean?

[g1]

I think he is referring to these: https://www.fliptops.net/catalog/p-1...ge-ampegs-used

[tdlunsfo]

That's them. Sorry I've seen this term used on numerous occasions and I assumed it was a part of the amp tech vernacular. Customer didn't want to fool with them so I guess I can call it good. Really cool amp but really heavy.

https://retrovoltage.com/2014/10/09/...tric-circuits/