Issue with losing signal as treble pot turned up. Badcat ToneCat

[hesamadman]

I have a badcat tone cat. No schematic but these are two channel amps. Clean side is suppose to be similar to DC30 and the gain side is suppose to be that of a HotCat (which I do have schematic for).


The issue is in the gain channel. I have pulled all tubes to channel one and the reverb so that these parts of the circuit can be ruled out. When the treble pot is turned up, there’s a great loss of signal. I have changed the pot and treble cap. I’ve rolled tubes in gain channel.  Other parts of the tone stack I’ve ruled out buy unhooking and clipping new temporary components in. It’s quite frustrating because it seems so simple and the area of issue is not complex. I’m not sure what I’m missing, but it’s definitely easy to miss things in these amps the way they are cram pack built and no real schematic to go off of.


Any thoughts or ideas?


https://el34world.com/charts/Schematics/files/_miscellaneous/Badcat_hotcat_30.pdf

[shooter]

does the gain/clean feed the PI the same way, one feed "G" the other "F"?
IF so;
swap "F" n "G" n see if you hear a change


got a scope?

[hesamadman]

does the gain/clean feed the PI the same way, one feed "G" the other "F"?
IF so;
swap "F" n "G" n see if you hear a change


got a scope?

GREAT idea. Yes I have all the necessary tools for troubleshooting. What do you suggest? I’d love to hear your thoughts.

[shooter]

"losing signal as treble pot is turned up"


that should easy to verify with a scope, once verified, simply "walk it back" along the signal path til it doesn't get lost.  replace what's "in-between" lost n found, problem fixed, to much though doesn't really help. 

[AlNewman]

Is it a new problem, or has it always been like that?

[stratomaster]

This screams parasitic oscillation.

[pdf64]

Does the issue affect the level for both channels?

[stratomaster]

Can you temp in a largish, say 0.001μF, cap across the cathode resistor on the follower driving the tonestack and see if that eliminates the effect you're observing?

Looks like there a snubber cap across the grid leak for the 3rd stage, so I'd imagine they know this amp is prone to high frequency oscillation.  Also, I'm not a fan of "voltage divider as gridstopper/grid leak" designs. In my mind only the 2nd stage has a real gridstopper.  If this is an amp with PCB mounted sockets, then things can get complicated adding in gridstoppers. 

If you find the cap above helps I'd recommend trying to add in real gridstoppers or add in small value grid to cathode caps at the socket. 

If the cap above doesn't help, but putting the same cap across one of the PI plate resistors does, then I'd be suspect of the tonestack.

[tubeswell]

Can you reverse-engineer a schematic of the Hot Cat channel for us?

[stratomaster]

Can you reverse-engineer a schematic of the Hot Cat channel for us?

Check the end of the first post

[tubeswell]

The circuit’s simple enough, so if it’s just caused by turning the treble pot, it’s either a wiring error, or the pot’s gone bad, or the 50pF is leaking DC (which will affect the bias of the following stage as you turn the pot up). I’d start with measuring for DC on the cap. Do you know how to do that?