Random Noise generated in power output stage of Fender Super 112...new guy here

[fredcapo]

Hello to all. This is my first post here in a few years. I am retired now at 59, and setting up shop at a local music store doing some amp repairs... for fun and profit...

I have pretty solid background in electronics and troubleshooting having worked 30 some years at AT&T/Lucent/Bell Labs as an Associate Engineer, and working on my own guitar and amps for probably 45 years.

I am working on a Fender Super 112. There is a random swelling/swooshing/rumbling low level of distortion that appears to be generated in the output stage. It is there when the volume is fully off and also when I patch a signal into the output stage via the Jack. I replaced the inverter and the 2 6l6's and it is essentially the same. The amp is kind of a dog to work on - not sure that I can have it powered up and probe the top of the board where the components are at the same time. Flexing and probing has not affected it, and seems to occur after the amp is on for a while though when powered off for 30 minutes and powering on it is immediate.

I have not started to debug this but dont think this type of noise would be visible on a scope, though it may show up on a sine wave i am injecting. Didnt look yet.

It is very low level though seems to ramp up after a while.

I am thinking this may be a common type of problem with amps of this type. I know this is somewhat abstract, but any tips would be appreciated. Thanks.

[Enzo]

If it comes out the speaker, it will come out the scope.

Schematic:
http://www.webphix.com/schematic%20h..._112_schem.pdf

Noise like that in any tube amp is usually from a resistor, once you have eliminated the tubes and semiconductors. The power amp in jack does route the signal through that TL072 before it hits the phase inverter. after the IC, I'd look to the three plate resistors for the phase inverter tube.

[Jazz P Bass]

Do not overlook bad solder joints.
I had one of these Supers that three techs tried to fix.
The phase inverter tube had a 'nonexistent' solder joint on one of the pins.
The wire was there, just no solder.

[fredcapo]

Thanks for pointing that out. I see that the signal goes thru the op amp when fed thru the jack only. That explains the level. since the noise is the same with the normal signal route the chip may be unlikely source.

I'll try to heat or cool a resistor at a time or just replace a few. Thanks.