Amp Ringing as Note Decays

[EVMode]

I own a custom built single channel blackface circuit amp, built on turret board. I have this issue where there is a high pitched ringing which follows the reverb trails, using amp reverb or reverb pedal same results. At first I immediately suspected a microphonic tube but at this point I have tried all of the following and they all have a "ting" when I tap on V1.

GT: ECC83-S
GT: 12AX7-R
JJ: ECC83 x2
Sovtek: 12AX7 WB x 2
Telefunken (reissue): ECC83-TK
Jan GE: 12AX7WA

I realize quite a few of these are just re-brands of the same tube but the point remains that every single one of these tubes "tings" in V1. Is it possible that my tube socket is microphonic? Any suggestion on how to resolve the ringing reverb trails issue? Is it possible I'm barking up the wrong tree and the issue is something else?

[pompeiisneaks]

It would probably be a lot easier if we had photos of the amp.

amp problems can be caused by many things, Poorly soldered connections, caps gone bad, the board material used absorbing moisture etc. you should use a chopstick (wooden) to tap around all components connected to it, the tube may just be coupling with another antenna like device near it. It could also be the socket, it could be many, many things.

without a schematic or a picture of the impacted V1 area, it's pretty hard to say for sure.

~Phil

[EVMode]

It would probably be a lot easier if we had photos of the amp.

amp problems can be caused by many things, Poorly soldered connections, caps gone bad, the board material used absorbing moisture etc. you should use a chopstick (wooden) to tap around all components connected to it, the tube may just be coupling with another antenna like device near it. It could also be the socket, it could be many, many things.

without a schematic or a picture of the impacted V1 area, it's pretty hard to say for sure.

~Phil

https://drive.google.com/file/d/1kkM-qx ... sp=sharing

20200516_111114 (1).jpg

[pompeiisneaks]

Yeah looks pretty decently built from my limited experience. I do know I've had noise coming from those smaller Mica caps but there's only one somewhat close to V1 there after that resistor. Also using shielded wire was a good idea to avoid noise pickup. The board looks like FR4/Garolite which is quality stuff. So the only thing I could guess at would again be to chopstick the areas around V1 and see where the noise 'lives' it is possible the tube socket has had some solder drip down and that can cause all kinds of odd behavior. I know because while I was still learning to solder I did all kinds of bad things to tube sockets

~Phil

[EVMode]

I spoke with the amp builder and we both have come to the conclusion it is probably an internal problem and I'll need to do some poking/prodding with a chopstick while a loop is playing into the amp.

I have ruled out the reverb tank as I have a spare of the same model and swapping makes no difference. Also to note the amp still makes this noise even when I disconnect the reverb RCAs and use an external digital reverb into the front of the amp.

I have swapped every single tube in this amp to no avail.

The strange thing is reverb (internal or effect pedal) seem to make the sound much worse. Here is a short demo of the issue. Any advice is appreciated.

https://drive.google.com/file/d/19UQnrT ... p=drivesdk

[Phil_S]

This may not be helpful or too obvious. If the problem is amplified by the reverb, then the problem is being created in front of the reverb. So it is something confined to the preamp section. You might see if you can jumper over the 2nd triode and see if it goes away. Depending on whether the problem goes away or not, then you will know with reasonable certainty if the problem is with the 1st or 2nd triode section. From there, you might find a way to narrow the search further.

[EVMode]

This may not be helpful or too obvious. If the problem is amplified by the reverb, then the problem is being created in front of the reverb. So it is something confined to the preamp section. You might see if you can jumper over the 2nd triode and see if it goes away. Depending on whether the problem goes away or not, then you will know with reasonable certainty if the problem is with the 1st or 2nd triode section. From there, you might find a way to narrow the search further.

Definitely helps as I am new to tube amp repair/troubleshooting. I am comfortable with things like discharging the filter caps and checking/adjusting bias but I don't know enough to start jumpering inside of the amp. I tend to agree with you that it is probably before the reverb circuit. This amp does not have the normal channel or vibrato circuit so it is 12ax7 (pre-amp) -> 12at7(reverb driver) ->12ax7 (reverb recovery) -> 12at7 (PI).

[Phil_S]

Assuming you are working on the normal channel of an AB763 circuit, use a bit of insulated wire soldered to 'gator clips at both ends. Clip at the grid input to the triode and the input to the PI. The grid input is the top of the 10 pf cap on the grid side, not the dry signal input from channel 2 if you are looking at the schematic. The input to the PI is between the .1 uf cap and the 220K resistor. This will bypass the triode. If the ring goes away, you know that triode is the problem area. If the ring remains, look at the triode section between the input jack and the wiper of the volume pot.

BE CAREFUL. Don't allow the clips to touch something unintended. I like to use clips that have an insulating cover.