Strange high pitch noise on Ampeg V-4.

[chikov]

Hello again. I met a guy who asked me to take a look at his Ampeg V4 amp head . He just wanted me to make sure it’s in a good working order. I tested tubes and found one output and one preamp tube that needed replacement – I replace them. I also cleaned potentiometers And checked ESR on the filtering caps - seems to be all right. But when I turn my treble knob up The amp generates high pitch squeal. I also hear increase in noise and squeal when I touch the preamp tubes with my fingers. When I unplug V1 tube The noise stops. I tried replacing V1tube but it did not help. Here is the video but please turn your volume down because the high-pitched noise is not loud but nasty :
https://youtu.be/BHyuwT3VUws
Any adeas what might cause it?

[pompeiisneaks]

Could be caused by dirty power, how old is this V4? Even if the ESR tests within range, they could still be underperforming due to age. If they're over 15 years old it's always a very good idea to replace electrolytics. Some debate that idea, but the data sheets of every electrolytic shows they have a shelf life of X hours, on top of that the hotter they get, the lower that time becomes. Tube amps, obviously, run hot.

Have you tried chopsticking the different components around V1 to see if something there has gone microphonic?

~Phil

[chikov]

Thank you for your help. I had a chance work on it again today for a little bit. I did a lot of pocking with a bamboo stick and could not find a bad ground or a bad connection. I did find the grid wire coming from an input being grounded on both ends, but fixing this problem did not fix 10k oscillating. I even disconnected the input plug wire on the left plug, but hi pitch is still there when I turn my treble knob up. I do not want to change all the filter caps because the owner of the amp did not ask me to do that big of a job... i might have to.. if anyone has any more ideas, please share it with me. Thanks.

[Stevem]

Don't assume that just because the filters test good for ESR that they are still uncoupling the circuit they are filtering good from the last gain stage!
Take a good new filter of at least 20 UF and clip lead it across every filter section in the amp and see if that kills the oscillation.

The amp may also have a dirty or bad connecting preamp tube socket that can make for this issue.
You should be able to rock each preamp tube around in its socket with no noises being heard, or signal cut out.

Along the lines of what I just posted in these amps is the tube socket solder connections on the circuit board, I would reflow any of them that look the least bit iffy!

[chikov]

I have an update. After trying to add 20uf 500V across every filter cap I did not get any improvement. I also replaced very cantankerous 6AN8A tube and removed one ground lead off the dual ones on the guitar plug in jacks ( that go to the grids). I also resoldered the PI tube’s soket and V1 socket. Finally, I think I got some improvement when I decided to use instead of 12AX7 in V1 not so hot 12AU7 ( the schematic calls for 12DW7 but I read that it is ok to use more common ones above). It is getting late, so I will do one more test tomorrow- see if it is going to be ok.

[martin manning]

Why would you not just get the correct replacement 12DW7?

[jeanfrançois]

Good evening, messieur, to ceramic resistors, if too close to audio signals, can raiming this problem/ HT = oscillation
keep it away from the top of the pcb

[chikov]

Good evening, messieur, to ceramic resistors, if too close to audio signals, can raiming this problem/ HT = oscillation
keep it away from the top of the pcb

I will look for those ceramic resistors and see how I can isolate them. BTW, what is the location of the resistors that you are talking about?
In the meantime I replaced all the filter capacitors , but the problem is still there...
There are these four .47 caps in the screen voltage circuit that appear to be cracked ( see the photo). I better replace them too...

[chikov]

One more question. Does anyone know what is the purpose of this part on the photo :

[chikov]

I have a somewhat good news. The V1 tube has a metal shielding cap that is supposed to be grounded. However it had a bad ground connection (grey wire) to the metal peg ( see photo). I restored the good ground and the hi pitch ocellation went away!
Now, i am starting to believe that 12AX7 is too powerful for this schematic. When i use it, whem my volume approaches 9 or 10 it makes screeching noise, but my 12au7 does not. I might try 12at7 or the original 12DW7... let me see whAt happens

[pompeiisneaks]

One more question. Does anyone know what is the purpose of this part on the photo :

Looks like a bodged on inductor to replace/addon to L2 maybe? It's connected to that linkage to C16 by the traces. See here:

v4-inductor.PNG

~Phil

[chikov]

I have a small update. After I replaced all the capacitors and installed proper tubes in the preamp my high pitch isolation’s are gone. However it has a new problem. On the first channel, when I turn the volume all the way up it clips. I made a video that shows the problem. Does anyone know what can be causing it?
https://youtu.be/uNLd785LAs8

[TUBEDUDE]

I don't hear any clipping, but your pots need to be cleaned, or replaced if necessary.

[chikov]

The problem is that this is already a pot #2 on the imput one. I might have to get a more expensive one. But all 3 of the pots I used had this “cut off” thing when it is turned up to 8 thru 10.

[pompeiisneaks]

If you've replaced the pots then maybe you're getting DC on the pot, meaning a coupling cap somewhere is failing, passing DC.

~Phil