Background oscillation

[dezmoduo]

I took one of my first builds and blew out the cat fur to assess it's status. The project was from 2013. https://ampgarage.com/forum/viewtopic.php?f=4&t=17084 and was basically a D'lite 2x 6L6 in a Bassman chassis. At that time it was very enjoyable to play, since then other builds wiggled into the spotlight.
So everything works, but even casual listening of single notes, 'specially in the lower registers, has a background oscillation underneath the guitar note. It's as if there was a vibrato set to a medium fast speed and the faintest intensity.
If I play chords it gets buried but there's still an underlying interference that sounds crappy, and single notes don't mask it at all.
It happens on both channels, PAB in or out. There are no tube rattles or harmonics at volume. I've swapped out the 6L6's with another set and the problem remains.
SO:
1. I will make a little YouTube vid to share.
2. I don't own an oscilloscope. I would appreciate suggestions on that purchase ($200-$400 range), and then track a
generated signal source.
3. I'm going to check the grounds. Is the vibrato just a really, really low hum?
4 On the chance that someone has heard this type of distortion, I welcome all input.

This was my first full build, with all sourced parts. It gets only the best cat fur.

Dez
NYC

[xtian]

3. If it was power supply ripple, you'd hear the hum.

You have an FX loop, right? This is a handy way to split the circuit in half. Can you see if you hear this weird anomaly in the preamp, power amp, or both?

[thetragichero]

man i was chasing down (mostly) high frequency oscillation and scratchy treble pot in a vibro champ with James tone stack, added plate bypass caps and the whole nine yards and it turned out i just needed a grid resistor going from time stack to the next preamp stage. chased all sorts of red herrings until the "a ha" moment came

[dezmoduo]

Thanks Tragic & xtain,
Yes there's a passive loop, I will give a listen to both sides. I have a preamp effects unit to feed the 6L6 power section, and an outboard power amp to listen to the preamp section.
I'll investigate a grid resistor after that.

cheers,
Dez

[dezmoduo]

The oscillation is present on the output side driven by an external guitar preamp.
Using the amplifiers preamp send into an external amp the oscillation is all but eliminated.

On an internet inspired whim, I piggybacked a new, matching cap (22/450) to each of the power supply caps starting with E & D. There was no change. When I got to C there seemed to be modest to significant improvement and moving to B, the oscillation seemed to have disappeared. I actually changed guitars to get a different tonal output and give my ears a "reset", but it really seems gone.

I'm going to buy new caps for the four 22uf caps and the two 100uf. Break open my 401K piggy bank and spurge on some good stuff. WAY less expensive than a scope, but since I've stopped buying guitars maybe I'll pick one up for future work.

I'll call it solved for now and post back if it all goes to crap again.

Dez
NYC