Plexi treble pot does nothing

[audiosalvage]

I have a 1987 20watt clone that I built last year. I put in a Merlin tube effects loop and a lar mar master volume that stays full all the time. I get a nice mid heavy, brown tone, which I like, but the treble pot seems to have no effect at all. Presence and bright caps help but no treble bite from the control. Any ideas?

[Stevem]

Yes I have a idea, post a schematic please!

[bepone]

check pot wirings and value, but is funny that someone need to mention that

[Littlewyan]

Is it just the treble control that has no effect? Do mid and bass work?

[audiosalvage]

Bass and mid pots work

[syscokid]

FWIW, I recently came across something very similar with a friend’s handwired Deluxe Reverb. I found that the amp’s 250pF tonestack treble cap lost connection to the 100K slope resistor/plate terminal. In your amp’s circuit, that would be the 500pF tonestack treble cap to 33K slope resistor/cathode terminal. Or, the treble cap’s connection to the first lug of the treble pot.

[audiosalvage]

Ill check those, thanks

[audiosalvage]

Loop circuit is completely removed from the amp and still no treble control. I checked the pot and all the tonestack wiring. Stumpted

[wpaulvogel]

Loop circuit is completely removed from the amp and still no treble control. I checked the pot and all the tonestack wiring. Stumpted

Post pictures please and lift the capacitor leg and check that it has the proper capacitance.

[audiosalvage]

I replaced the cap and it has some effect, but I guess I never realized how little the tone controls have on the shaping.

[syscokid]

I replaced the cap and it has some effect, but I guess I never realized how little the tone controls have on the shaping.

Glad you posted a pic. Your tonestack treble cap looks like a 3.3K instead of a 33K.

20240406_183843 (1).jpg

[audiosalvage]

Well garsh, color me blind. That must have happened as I was trying out different slope resistors. You are a life saver, I was going to button it up and live with it. Thank you

[Stevem]

Golden rule number 1.

Whenever working on a amp and especially a customer amp where time is money, alway double check the for the needed resistor value with your ohm meter to confirm what you think your seeing with your eyes.

Over the years I have had more then a few that had the wrong color bands in regards to what the checked out at.