Ampeg B22X with multiple problems

[Helmholtz]

This refers to filter grounds, output tube cathodes, ground side of PT center tap standby switch, input jacks, speaker jack, and ground to main board.

Thanks, just wanted to make sure.
You reported that one end of R59 is sensitive to probing. Both ends of R59 connect to PS filter nodes which are supposed to be signal grounds. Signal grounds must not be sensitive to probing, provided that the attached filter cap has sufficient capacitance, low ESR and good ground contact.

[Randall]

Signal grounds? I don't understand, R59 is supposed to have 320v on one side and 470v on the other.

[Helmholtz]

Signal grounds? I don't understand, R59 is supposed to have 320v on one side and 470v on the other.

Signal is AC and high capacitance means an AC short. So PS nodes are AC signal ground. Amplifier tubes produce AC currents and these need a ground path. Otherwise the circuits won't work properly. As the ESR adds to the low capacitive impedance, high ESR will prevent a good signal ground. (DCV doesn't bother AC currents.)

[Randall]

OK, fair enough. What would your advice be at this point? Should I disconnect that 20/450v and sub a fresh one?

[Helmholtz]

OK, fair enough. What would your advice be at this point? Should I disconnect that 20/450v and sub a fresh one?

A good, low ESR cap in parallel should do for a test. But soldering makes better contacts than clipping in. And cap leads should be as short as possible to avoid unwanted inductance.

[Randall]

I soldered in a fresh F&T 33uF/500v cap across the can section serving point Y art R59, and it made no difference. Both channels make a loud low frequency oscillation when turned up past 5, with or without the added new cap.

[The Dude]

I haven't read the whole thread, so apologies if this has already been mentioned or tested. I quite often see burnt up hum balance controls on these amps. Have you checked the hum balance pot? If it's bad, I'll usually just unsolder it and add a couple 100 ohm resistors like the typical Fender circuit.

Edit: Oh hell, never mind. After a quick skim read, I see we are chasing a motorboating problem and not a hum problem. My apologies. It's still worth checking the hum balance pot if you have hum issues. It's a pretty common problem in these amps.

[Randall]

The hum control seems OK. It's a little flaky at the extremes, both fine in the middle area. There is no hum.

The motor boating seems to have ceased. Presently the problem is a loud trumpet oscillation at approx. 250Hz when either channel is turned up past about 5. Channel 1 does it with nothing plugged into input, Channel 2 only does it when a guitar is plugged in and played. If get through this with my shop speaker intact I will be amazed..

[SoulFetish]

Randall, have you eliminated the 7199 as being the source of the noise?
Ive found these to often become unstable, you can get socket adapters or rewire it for a 6U8A, if you can’t find any reasonable 7199s.

[Randall]

I have not ruled out the 7199. It tests good, but we know that doesn't tell the whole story. It sounds like a reasonable theory since it is common to both channels.

[Randall]

I have ordered a 6U8A and adapter socket, and we shall see. While I wait I can at least get this evil thing off my bench for a week.

[SoulFetish]

I have ordered a 6U8A and adapter socket, and we shall see. While I wait I can at least get this evil thing off my bench for a week.

Good luck! I hope it works out. Either way, I'm a fan of the 6U8A, and having one around with the adapter will come in handy either way.

As an aside, I read that you used acetone to wash out the inside of the chassis. I would be nervous about breaking down of some of the wire insulation, and other composites. I really only use it if I need to break down silicone, or as a degreaser/cleaner for machine parts.
You haven't run into any issues with it?

[Helmholtz]

[QUOTE=SoulFetish;n912618]
As an aside, I read that you used acetone to wash out the inside of the chassis. I would be nervous about breaking down of some of the wire insulation, and other composites. [QUOTE]

Absolutely agree!
I forgot to mention this. Acetone attacks (partly dissolves) most plastics and coatings. And it's highly flammable/explosive (vapor) as well as toxic.
Most board cleaning can be done with concentrated alcohol. In tough cases a little naphta (or white spirits) on a Q-Tip can be used, both are safe on components. And residues of spilled beer or coke are best removed with warm water on a cloth. Allow for drying.

BTW, resin flux residues are non-conductive.

[Randall]

Yes, I used acetone with a Q-tip selectively to remove flux, and whatever else was one there, because it was nearly impossible to see where traces connected or did not connect, keeping in mind components are not identified on these boards. I followed this with a good soapy toothbrushing and garden hose washing and a long drying time in my driveway in the hot SW Florida sun, and then a few days in my air conditioned shop, before I started any work. It was probably two weeks later before I even powered it up.

I'm not really too concerned about damage from that.

[Randall]

The amp sat untouched for the time it took to receive the 6U8 tube and adapter. I installed it, and with nothing plugged in, both volumes down, the machine gun sound happened as soon as the tubes warmed up. I put the 7199 back in, same result. I am including a video link to see if someone may have any insight of what this is, or how to approach it, because I really just don't know what else to do about this.

https://drive.google.com/file/d/1lAn...ew?usp=sharing