Strange high pitch noise on Ampeg V-4.

[chikov]

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

That is an interesting idea. I will look into it. Here is an interesting update, after I left this amp plugged in for a couple of hours, the problem went away and now the volume is smooth, and much less hum...

[Phil_S]

That black thing with the silly cone on it looks like a trim pot to me. I am willing to be, if you lift the board, either there is a resistor across those two attachment points or the resistor was clipped out. In the first case, it would make the pot parallel to the resistor, allowing you to reduce the overall R value. It was likely dialed in and then fixed in place with a dab of the goo. Where that is in the schematic I don't know, so I'm of no value on telling you its purpose.

[chikov]

Quick update. I have replaced coupling capacitors around V1 and V2, I also replaced them passed fase inverter but it still makes this noise ( see the video below, just make sure your volume is down). Does anyone can help me and guide me to what I can check in order to pin piont the problem ( i do have a scope). I will attach the schematic too
https://youtu.be/BcJCl1ffstM
PS, on the pin 6 of V1 instead of a normal 165v i am reading 262v ??

[chikov]

Another update. After checking voltage is on the V1 I discovered that when I removed the ground loop from the shielded cable from input 2, I actually removed the ground that both imputs were using. I restore the ground and voltages went back to normal. However, The second input was still making a squealing noise. I checked the voltages on strange V5 phase inverter And found that plate voltages were too high. I had an extra tube of that kind so I replaced it. The voltage on pin six is about 160 V instead of 139V. It did help and there is no more squealing noise on the input two, But when I turn volume one to max I still lose volume ( it almost clips, like on video). Can anyone tell me what should be the value of that potentiometer , I don’t think schematic is very clear . The old one I think was reading about 850kOhm. Right now I installed 500 kOhn... It may not be the main reason but I’m just wondering
BYW, The second channel still squeals but only when guitar is not plugged in in it, weird

[chikov]

Another update. I replaced all the capacitors in this circuit and yes it made the world of a difference. The Amp has gotten more silent , it works perfectly well through the second input. The only issue is left happens when you plug into the first channel. When I crank travel all the way up to 10 and master volume of the first channel all the way up I get high pitch Ocillation sound. I am happy with the results but I wish I could get rid of this last nuisance. I’m going to try to change the value of the treble potentiometer.

[chikov]

Final update. I reduced the value of treble pot from 1M to 500k and the vslue of the first channel vol pot frim 500k to 100k. It got rid of ocellations and made the output a little louder. Thank you for some of your help - I do realize it is hard to diagnose a difficult problem from a distance. I leave ned that sometimes you just have to replace all old caps in order to bring your amp back to the good working order

[chikov]

Nope, the ovellation is back when my 1st vol and treb all the way at 10. I just realized that i did replace ALL the capacitors, so I am back at Square 1...,

[tubeswell]

Might be a bad/cold solder joint or PCB trace/pad in that channel or loose tube socket pin clamp etc. (I take it you swapped that channel's pre-amp tube?)

[chikov]

I think I finally have the good news. I decided to enclose the amp with the top and the bottom covers. It successfully Created the shield that killed all the oscillations. I could not believe that extra screen or lack of thereof Can’t kill all these pesky noises. Wow, lesson learned but it’s so hard to discover it because you have to have your chassis open to work on it. Wow. I’ve done a lot of work on this amp and that sounds very good, loud and has a very good distortion. Thanks again for all your help.

[Stevem]

Just for future reference on oscillation issues you can most times pin down what preamp stage it's coming from by taking a length of wire, soldering to the plate and then rap it around the grid wire 3 or 4 times.
This will either make things better as in you found the issue stage, or make the problem worse!