Fender Red Knob Twin 2nd channel high pitch squeal/ whistle another cause?

[jalexquijano]

'CHannel 2 - Gain at 7 and volume at 2 causes loud squealing'
OK, turn the vol down from 2 (to the very lowest level of audibility) - does it stop squealing?
Pete

As i mentioned before the problem is only in channel 2 (overdrive channel). I plug the guitar to the amp, switch to channel 2 and raise the gain to 6 with volume on 2 and there is no whistling. However, if i do increase the volume to 4 the whistling starts. Of course if i lower it to 1 there is no squeal. Could the problem be caused by a bad 22uf 500v cap? I have swapped as much as 10 tubes in V1, V7 and V2 and the whistle continues even with the guitar volume at 0. So feedback is not the problem.

We need to find out which of the caps or bad solder is causing the annoying whisling or squealing? Maybe i should have bought better quality electrolytic caps and not Illinois brand.

[g1]

Good to know that the squeal continues when the guitar volume is turned down.
Before the problem started, how high could you turn up the volume with the gain at 6?
Did the problem start at the time the jacks and caps were replaced?

[52 Bill]

Could the problem be caused by a bad 22uf 500v cap?

We need to find out which of the caps or bad solder is causing the annoying whisling or squealing?

Yes, it could be a bad cap. Test them to find out. Do you still have the original caps that were removed from the amp? If you do, take one of the original 22uF caps and temporarily connect it across the new cap at C112, (in parallel). Does this stop the squeal? Try across C111 and C110 as well. If the squeal stops then you know you have a bad cap, if it doesn't stop, then the problem is somewhere else.

If you do this test, be very careful when connecting and removing the test caps. Be certain that the power supply has been drained of any voltage before adding or removing the additional caps. And be certain to match the polarity of the caps as well.

[jalexquijano]

Yes, it could be a bad cap. Test them to find out. Do you still have the original caps that were removed from the amp? If you do, take one of the original 22uF caps and temporarily connect it across the new cap at C112, (in parallel). Does this stop the squeal? Try across C111 and C110 as well. If the squeal stops then you know you have a bad cap, if it doesn't stop, then the problem is somewhere else.

If you do this test, be very careful when connecting and removing the test caps. Be certain that the power supply has been drained of any voltage before adding or removing the additional caps. And be certain to match the polarity of the caps as well.

I dont have the original caps anymore! Unfortunately my former tech stayed with them to build an experiment. I got pissed at him because he could not manage to fix the squeal nor stick to his promise of exchanging a marshall shredmaster pedal. Anyway, i am heading to singapore for three days on Sunday and heard i could purchase them over there. Do you recommend to buy all the other caps and preamp tube sockets? Or this are the only caps related to the whistling/ squealing problem in channel 2.

[dbdoc]

I had a MB Subway Rocket with very similar symptoms. I narrowed the problem to the preamp then found a damaged mid tone control preventing a good ground in the tone control circuit. Your tech did check this type stuff, but maybe he missed something.

[52 Bill]

If the problem is caused by a decoupling problem in the preamp power supply, it will be one of the three 22uF caps C110, C111 or C112. If you want to buy a cap to try the test, then buy one 22uf 500volt cap and try the test that I suggested.

But remember that the problem could also be caused by a number of other things, like a bad ground connection. You must test things to find the problem. Random replacement of parts is not good troubleshooting methodology.

[jalexquijano]

I finally got a hold of my former tech. He will be bringing the 22uf 500 volt caps he replaced and branded TAD (Tube Amp Doctor) to my house. I hope the issue will be solved with such action. If not where do you recommend to look for? I did not found any bad solders in any of the PCB boards or tube sockets terminals. Should i also consider on replacing the preamp tube sockets?

[jalexquijano]

No success! Whistiling continues!

I finally got a hold of my former tech. He will be bringing the 22uf 500 volt caps he replaced and branded TAD (Tube Amp Doctor) to my house. I hope the issue will be solved with such action. If not where do you recommend to look for? I did not found any bad solders in any of the PCB boards or tube sockets terminals. Should i also consider on replacing the preamp tube sockets?

Okay, last night me and my tech stayed from 6 pm to 11 pm trying to troubleshoot the whisling/ squealing trying the following:

-Measured the voltages on all Filter caps including the 22uf 500v of the preamp section. All them measure correctly!

-Replaced 10k 100w resistor R131

-Replaced both power resistors (R128 and 129) 2.7 k 10 w and 30k 10 watts

-Checked for bad solders. Everything fine.

Anyhow, the whistling keeps showing up when dialing after 6 on the gain pot and raising the volume over 2.

Should i consider replacing the caps C11, c12, c13, c14, c15, c10, c18, c19, c16, c17, c22, whose values are 750pf, 250pf, 0.1pf, .047pf, 50pf, .0068uf, 250pf, .68uf, etc. and the following potentiometers of channel 2:

Gain control, 1 meg aud dual Part Nš 26046

Treble control, 250k au with switch part Nš 25946

Mid control, 25k aud with switch part Nš 26049

Bass Control, 250k aud with switch part Nš 25946

Presence Control, 1 meg "s" with switch Part Nš 26048

Volume control, 250k linear Part Nš 25946

[Jazz P Bass]

It could be any or all of those items.
Did the repairman show up with a signal generator & a scope?
You have to find what circuit the oscillation is originating in.
What happens to the squeal when you start pulling tubes? (V1 & V2 specifically)
Which tube stops it?
You are not going in the parallel input jack are you?

[52 Bill]

Did you at least try to test the three filter caps?

[jalexquijano]

Did you at least try to test the three filter caps?

We did not test the former ones because they were really beaten up physically. I have ordered a new set for replacement and a brand new gain and volume pot.

[jalexquijano]

Did you at least try to test the three filter caps?

Im here in Singapore and went to this TUbe amp store. The chinese guy sold me a brand new set of 22uf 450v electrolytic axial caps branded EPCOS. He also sold me some of the orange drop capacitors mylar that go under the pcb where the pots are soldered. Do you recommend replacing these orange drop caps besides of the gain and treble pot of the amp? I talked to him about the tension of the 9 pin tube socket and he advised me to replace them as every socket must be tight to avoid noises.

ANy opinion on this?