Kustom '36 Coupe distortion problems

[timeamps]

I am working on a Kustom '36 Coupe Guitar Amp. It has distortion in both channels and in the effects return. Volume is also low. I have tested and replaced all tubes as necessary. I have switched out guitars, cords and speakers - all with no chaange to the volume or distortion problems. Any ideas or schematics available? Thanks

 

[Llamarama]

Have you replaced the capacitors and/or resistors? The old can style electrolytics can dry out and are often worth replacing straight off. I have a friend who restores vintage valve equipment and this is his first port of call after damaged leads.

 

[tunedwolf]

Ideally a sine wave source and a silly scope would be the tools to have here, but failing that, try splitting the signal path at the effects return and inject a signal directly into the main amp, that should rule out the 6L6's and the phase inverter stages around the 12ax7 and give you some direction to head in. Keep splitting it down stage by stage until you can identify which is causing your problem, then go to component level fault finding within the stage that's giving you trouble. Watch your fingers on the HT

 

[timeamps]

This amp is only a few years old and the caps / power supply looks good. It has a IC voltage reg circuit and it appears good as well. Not sure on the volt value, schematic would be nice! Thanks anyway - I am still trying.

 

[Nigel Goodwin]

You really need to use a scope to locate where the distortion is occurring, however, assuming this is a fully valved amplifier (and with out a circuit I've idea) then by FAR the most common source of trouble is high value anode loads in the preamp triode stages.

 

[timeamps]

First, thanks to all for your help. It took a while, but I found the problem. I traced it with a scope using the RCA receiving tube manual as a guide. I narrowed it down to the inverter tube (12AX7). When I took voltage readings Pin 1 (plate) it read 0 vdc. I checked the resistors feeding it and they read ok, but I noticed the 82K R24 was @ 100K. But when you applied B+ it opened up. Replacing it resolved all of the problems. Thanks again.

 

[Nigel Goodwin]

First, thanks to all for your help. It took a while, but I found the problem. I traced it with a scope using the RCA receiving tube manual as a guide. I narrowed it down to the inverter tube (12AX7). When I took voltage readings Pin 1 (plate) it read 0 vdc. I checked the resistors feeding it and they read ok, but I noticed the 82K R24 was @ 100K. But when you applied B+ it opened up. Replacing it resolved all of the problems. Thanks again.

Like I said above, it's by FAR the most common failure in valve amps - always check the voltages on pins 1 and 6 (anodes) of the double triodes.

As the other triodes probably use the exact same resistors, it's worthwhile changing all the anode loads for better quality resistors.