High pitched whistle continues on Fender Red Knob Twin, I am getting desperate

[jalexquijano]

Having replaced the following caps with Jensen HI FI and EPCOS filter caps and i am still getting a high pitch oscillating whistle or squeal whenever i turn the Gain pot after 6 and dial the volume after 2:


C110, C111, C112 -----------------22uf 500v (3)

C303,304,305----------------------100uf 100v (3)

C301, 302--------------------------220uf 300v (2)-------------------THIS HAVE NOT BEEN REPLACED

C306, 307-------------------------47uf 350v (2)

C202------------------------------100uf 100v (1)

I have not installed the former input jacks it had and have kept installed a brand new set. I would like to know if this could be the cause of the whistling as i can reinstall the former ones or it could be attributable to the gain and treble pot of the overdrive channel - Channel 2 of the amp.

I have spent quite a lot of money and time in stupid techs and all they do is either replace parts, move cables or say its a microphonic tube. This is non sense. Can someone give me a more accurate or precise symptom and solution to fix my Fender Red Knob Twin?

[Enzo]

I am assuming you mean "The Twin"?

What do you mean about input jacks? Have you tried to replace the original ones with some other type? All those extra contacts on the original jacks are part of muting circuits, and without them, the amp is running without that protection. Also, they are insulated from the chassis, so if you install a metal bushing jack, it can compromise the grounding scheme.


I tend to think all those electrloytic caps are not involved, as you found out.

Does this happen in both channels?


Sorry, but my first thought was to move grid wires away from other signal carying leads.

[jalexquijano]

I am assuming you mean "The Twin"?

What do you mean about input jacks? Have you tried to replace the original ones with some other type? All those extra contacts on the original jacks are part of muting circuits, and without them, the amp is running without that protection. Also, they are insulated from the chassis, so if you install a metal bushing jack, it can compromise the grounding scheme.


I tend to think all those electrloytic caps are not involved, as you found out.

Does this happen in both channels?


Sorry, but my first thought was to move grid wires away from other signal carying leads.

Enzo,

I replaced the original 4 stereo input jacks with a new set i bought in amplified parts online:

Jack - Stereo, 9 Pin, PC Mount, Original Fender, Amps '88-'99 | Amplified Parts

As i mentioned before i have already changed all the filter caps and have spent around US$80 dollars on the Jensen Hi FI 100uf 150V and some other in the other caps i have mentioned. I also changed both power resistors (2.7k 10 watts and 30k 10 watts), 1.2 k 3 watts cement resistor. I could have changed the 3 full pcb but for that i would have bought a new amp. I am really concerned that in my country we do not have any competent techs and i have to troubleshoot my amp myself.

Where do i go now? Should i replace the original 4 stereo input jacks? or should i also buy the gain and volume pot of channel 2.

This is the schematics of my amp:

http://www.blueguitar.org/new/schem/fender/the_twin.gif


As i said i dont want to replace more parts if they are okay.

[Enzo]

OK, new jacks of the same type will not be the problem, unless you made a bad solder connection. And all those caps were probably not needed.

Can you answer if both channels do this or just one?

Does it do this with NOTHING plugged into the input jacks? or only with a guitar plugged in?

Replacing circuit boards would not be a good idea, what if the problem is interaction in the wiring?

The approach I would use would be to set the amp chassis on my bench, connect a speaker, turn the controls so it starts to make the noise, then use an insulated probe (like a wooden chopstick) to move each tube socket wire, and push against each component on the boards. If we can mve something and change the noise, we have found something involved.

[jalexquijano]

The high pitch oscillating squeal only shows up in channel 2 which is the overdrive channel. I already tested the amp outside of the cabinet and there is no squeal present. The squeal comes out when I place it inside the cabinet. It should be something related to vibration.

[jalexquijano]

The high pitch oscillating squeal only shows up in channel 2 which is the overdrive channel. I already tested the amp outside of the cabinet and there is no squeal present. The squeal comes out when I place it inside the cabinet. It should be something related to vibration.

It does this with the guitar cable plugged in even with the volume at 0 on the guitar. However, no squeal with the chassis out of the cabinet.

[Chuck H]

It's a microphonic tube, capacitor or an intermittent solder joint. If isolating the chassis from the resonant vibrations of the cabinet and speakers eliminates the problem then it MUST be the physical contribution of the cabinet and speakers since there is no electrical difference.

[Enzo]

There is one electrical difference. I usually consider it chasing hum, but this case could be appropriate too.

The cabinet has shielding, which normally would make the amp more stable in the cab, but conceivably it could be involved. But I doubt it, and think you are probably correct.

[jalexquijano]

Thanks to bot of you guys for your assistance. The problem was solved by replacing the former original plastic stereo input jacks (9 pins) which are made in japan. The new ones I purchased in amplified parts were causing the problem. Maybe cheap quality???

[Chuck H]

Ah ha! That would explain why it worked when out of the cabinet then.

???

Glad you fixed it. Perhaps it's as Enzo said then. The shield, in combination with an open jack switch could have caused it.

[jalexquijano]

Do you think that even if the input jacks are brand new they could have come shorted? Or was it because I install the old ones and resoldered everything again? The ones they are selling now are not the original made in japan jacks.

[jalexquijano]

Ah ha! That would explain why it worked when out of the cabinet then.

???

Glad you fixed it. Perhaps it's as Enzo said then. The shield, in combination with an open jack switch could have caused it.

Fixed it! I wish i had! The squealing started again and now after 10 mins of warming up you can start listening to a dripping sound like water drops from a faucet in the background in channel 2 (overdrive channel). Could this be attributable to the 22uf 450v preamp capacitors or to the 220uf 300v Caps in the power section? You can only hear it once the amp has warmed up after 5 mins in the overdrive channel but not in the channel 1 (clean channel).

I am getting fed up with this squealing. I plug the guitar to the amp, start raising the gain in the overdrive channel ater 7 and volume after 3 and the squealing begins. Please bear in mind i put the guitar volume to 0 and the squealing is there, so its not pickup feedback.