Oscillation/Squeal when touching power tube while plugged into amp

[lespaulnmarshall]

Hi All,

I'm currently working on a JTM45 build. When I up crank the bright channel volume, presence and treble and touch or get close to one of the power tubes with my hand while a guitar is plugged in, I'm getting very high pitched oscillations/squealing. This does not happen if I don't plug in a cord at the input. I don't think I know quite enough to be able to explain why this happens, but I can't find anything about it anywhere online. The amp also does the same thing when you plug in a guitar and let go of the strings at high volumes (without putting my hand near the power tubes), when standing close to the amp. At say 1,5 or 2 meters distance from the amp the problem goes away.
I've tried different sets of tubes, and that didn't solve the issue. It also seems to be only one of the two tubes that does it when you get close with your hand. Does anyone have any clues as to why this happens and if it could be/should be resolved? Has anyone else ran into this? I have encountered this problem once before with another amp now that I've come to think of it, but since the amp behaved well under normal playing conditions I didn't go through too much trouble to resolve the problem.


Thanks!

[Stevem]

Is the amp bolted down on a steel plate sitting in its cabinet to shield out what sounds to me like 120 HZ buzz that your discribining?

Also are the power tubes wired pin 2 to pin 2 and 7 to pin 7?

Is the wire coming off each pair of your input jacks sheiled?

[pdf64]

There’s dozens of permutations of the 5F6A circuit used in JTM45. Exactly what schematic have you used, including PT, OT and choke spec?

[lespaulnmarshall]

Is the amp bolted down on a steel plate sitting in its cabinet to shield out what sounds to me like 120 HZ buzz that your discribining?

Also are the power tubes wired pin 2 to pin 2 and 7 to pin 7?

Is the wire coming off each pair of your input jacks sheiled?

Thanks for the reply, it's definitely a lot higher pitched than 120Hz (or 100Hz in Europe). I've had the amp attached to my scope as well, but didn't pay attention to the specific frequency. It's probably in the 8k Hz range or zo.
The amp is securely bolted down to an aluminum sheet in the cab, and while the sheet does make things better, it does not eliminate the problem. The filaments on the power tubes are wired correctly, however I have not used shielded wire coming off of the input jacks or input grid stoppers, and I've always experienced that these lower gain designs traditional designs don't need it. That said I have not built a bassman or jtm45 before so maybe this is an exception?

[lespaulnmarshall]

There’s dozens of permutations of the 5F6A circuit used in JTM45. Exactly what schematic have you used, including PT, OT and choke spec?

The OT is an MM Radiospares 045RS-L, the PT is an MM P4550JT-G2M and the Choke is an MAR100C, also by MM. As for the schematic, I've gone with the vintage JTM45 schematic on the Marstran website (not the 1966 one but the earlier version), with a 500pf cap across the 270k mixer resistors and a 100pf bright cap on the volume control. I have put the choke between the first two filter caps however, instead of having it before the first filter cap, and I have added 5k6 grid stoppers for the power tubes.

[pompeiisneaks]

Okay so from my experience, even with the amplifier in the chassis, the closer I get the guitar to the amplifier, the higher the chance to induce hum/noise due to the guitar. This only seems to be a serious problem with single coils though, humbuckers seem to avoid it a ton better (thus the name humbucker)

Are you sure this is a problem with the amp and not the fact that you're bringing a sensitive device near a high output magnetic field induced by the amplifier?

I've got amps that sound amazing out of hte chassis if I'm 10' away and twisted at a slight angle with the pickups away from the chassis, and the second I rotate towards it horrid noises start happening, some nice sounding feedback, others not so much. of course this is at higher volume settings. Once in the cabinet, I can get a lot closer but still when I'm standing right next to them with the pickups facing the tubes right by the transformers, it can do some pretty awful sounding things.

I just think this is expected behavior for a pickup next to a highly magnetic producing thing like a tube amp power and/or output transformer, with all the other components in an amplifier working together to do more to that same tone.

Maybe I'm off though.

~Phil

[lespaulnmarshall]

Okay so from my experience, even with the amplifier in the chassis, the closer I get the guitar to the amplifier, the higher the chance to induce hum/noise due to the guitar. This only seems to be a serious problem with single coils though, humbuckers seem to avoid it a ton better (thus the name humbucker)

Are you sure this is a problem with the amp and not the fact that you're bringing a sensitive device near a high output magnetic field induced by the amplifier?

I've got amps that sound amazing out of hte chassis if I'm 10' away and twisted at a slight angle with the pickups away from the chassis, and the second I rotate towards it horrid noises start happening, some nice sounding feedback, others not so much. of course this is at higher volume settings. Once in the cabinet, I can get a lot closer but still when I'm standing right next to them with the pickups facing the tubes right by the transformers, it can do some pretty awful sounding things.

I just think this is expected behavior for a pickup next to a highly magnetic producing thing like a tube amp power and/or output transformer, with all the other components in an amplifier working together to do more to that same tone.

Maybe I'm off though.

~Phil

Hi Phil,

Much appreciated! My thinking was kind of similar last time I encountered this issue. The odd thing is that I can also get the effect from the amp if I have a cable plugged into it going to my signal generator with the output turned off on the generator. I have done this with an aluminum shield over the chassis attached with aligator clips to create a fraday cage, as it it would be when installed in the head cab. So in that scenario there was no pickup in the chain at all and it was still sensitive when I touched the tube.

I might have left the wires to the pots and tube sockets a little long inside the amp though. I've routed them away from the connection and then bent them down and back to where they need to go so that they would lay nice and flat against the bottom of the chassis. I am thinking of shortening them now to see what happens then. And then there are some unused taps on the OT primary that I've tucked under the board, maybe those have something to do with it....

[pompeiisneaks]

Lead arrangement/alignment can be very important with this kind of thing. SImply pushing them around while the problem is ocurring with a wooden chopstick can often show you exactly where the problem is. have you also checked that it isn't the instrument cable to be sure?

~Phil

[lespaulnmarshall]

Lead arrangement/alignment can be very important with this kind of thing. SImply pushing them around while the problem is ocurring with a wooden chopstick can often show you exactly where the problem is. have you also checked that it isn't the instrument cable to be sure?

~Phil

I have been chopsticking the leads, might have helped slightly, but I've had it happen before where just shortening a wire did the trick and chopsticking didn't do all that much much. I've tried two or 3 cables, so the cable can be ruled out.

[wpaulvogel]

The bright channel of many amplifiers is very sensitive to oscillation without any interaction provided by the user getting too close to the tubes. We are antennae. If you grab the lead of an oscilloscope you’ll see this. Best advice is to stay away from the tubes. I know this is a terrible answer but I became aware of the phenomenon a long time ago and it’s just a no go situation for me.