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

[jalexquijano]

I own a Fender Red Knob Twin amp which has been producing a high pitch whistling on channel 2 (overdrive channel) whenever you turn the gain past 6. Channel 1 is perfect, no squeals even with volume at 10. I tried to troubleshoot the problem myself but finally decided to take my amp (Fender Red Knob Twin) to the best tech in Panama.

At the end he determined that there were no bad solders, damaged caps or resistors. Please bear in mind i had just replaced all the filter caps and front 4 input jacks last month.

He said the problem was in V1 an V7 preamp tubes (first gain stage for 1st and 2nd channels). He mentioned that those two tubes were microphonic after swapping many preamp tubes. THose positions were very critical in the amp and recommended to get some 7025 preamp tubes. I already retensioned the pins on all tube sockets and sprayed some contact cleaner on each of them. Should i replace all the preamp tubesockets which are not grabbing the tubes firmly as opposed to the preampsockets on my other amps?

Right now i have 2 12ax7 Electroharmonix tubes in V1 and V7 and have swapped them with as much as 10 different 12ax7 and ecc83. Squealing is still there even with the guitar a zero. What else could be the cause? i have checked all solders and everything is perfect. Both power resistors and all filter caps were changed 2 months ago.


Your opinions and assistance are highly appreciated. Im at the point of selling the amp if the squeal continues.

[g1]

Well, V7 is for ch.1 and you have no ch.1 problem so you can forget about V7.
Ch.2 involves V1 and V2. The problem is, where are V1 and V2? This is not like an old Fender where we just count from right to left straight across.
This should help, I'm pretty sure it is correct:

See if holding V1 or V2 with a rag affects the squealing. If so you should be able to improve it by using a less microphonic tube.
Be aware that you may not be able to get rid of it completely at higher master volume settings, these amps have quite a bit of gain in ch.2.

[jalexquijano]

I have already swapped almost 6 different brands of 12AX7 and the whisling continues whenever you turn the gain in channel 2 past 6. I even swapped 2 12AT7 preamp tubes and the same. Can there possibly be a faulty cap or resistor causing this whistling? I would appreciate if you can check that area between V1 and V2 in the schematics.

[pdf64]

Does it still squeal even if the master is turned really low, so you can barely hear it?
If so, suspect C112 the C node B+ cap; maybe the new one is bad, maybe it got overlooked for replacement.
http://www.webphix.com/schematic%20h...s/the_twin.pdf
Pete

[Jazz P Bass]

It is not a "standing to close to the amp' feedback is it?

[jalexquijano]

No. I suspect it should be a capacitor or resistor in that area. You can even hear the whistling with a cable attached and no guitar plugged.

[52 Bill]

I don't think that this is an acoustic feedback problem, it sounds more like a high frequency oscillation. I'd check for problems with lead dress, broken solder connections and bad filter cap in the preamp circuit.

[jalexquijano]

Does it still squeal even if the master is turned really low, so you can barely hear it?
If so, suspect C112 the C node B+ cap; maybe the new one is bad, maybe it got overlooked for replacement.
http://www.webphix.com/schematic%20h...s/the_twin.pdf
Pete

I guess that can be probably be the problem. Those illinois filter caps were recently replaced on January. But as you said one of them could be faulty. The sound is an oscillating whistle and is just being produce in channel 2 when you turn the gain past 6.

Can you point out which filter caps are involved in this area, so i can order them right away?

[jalexquijano]

I guess that can be probably be the problem. Those illinois filter caps were recently replaced on January. But as you said one of them could be faulty. The sound is an oscillating whistle and is just being produce in channel 2 when you turn the gain past 6.

Can you point out which filter caps are involved in this area, so i can order them right away?

[oc disorder]

Did it squeal before you replaced the jacks ?
Just thinking there's a lot of complex switching within them so I would be going over those with a fine tooth comb!
Secondly I'm not sure if it's a micro-phonic squeal i.e. acoustic feedback from loose construction within a pre-amp tube or oscillation due to electrical induced feedback within the amplifier.
By separating the amp and speaker combination you should be able to tell if its just microphonic - the tubes misbehaving will have a certain ring or ping to them when tapped with a pencil.
I assume the internal foil shielding is still intact in the top of the cabinet.

I recall having to re-terminate a number of the wires from the tubes to the front board after having to remove the board for a pot replacement.

Its a typical power supply as the pre tubes mainly connect to point "C" C112,C111 and C110 are the main supply filter caps there. Probably wouldn't hurt to change C303 & C304 if you really want.

I agree with Bill "lead dress, broken solder connections" most likely cause.

[jalexquijano]

Did it squeal before you replaced the jacks ?
Just thinking there's a lot of complex switching within them so I would be going over those with a fine tooth comb!
Secondly I'm not sure if it's a micro-phonic squeal i.e. acoustic feedback from loose construction within a pre-amp tube or oscillation due to electrical induced feedback within the amplifier.
By separating the amp and speaker combination you should be able to tell if its just microphonic - the tubes misbehaving will have a certain ring or ping to them when tapped with a pencil.
I assume the internal foil shielding is still intact in the top of the cabinet.

I recall having to re-terminate a number of the wires from the tubes to the front board after having to remove the board for a pot replacement.

Its a typical power supply as the pre tubes mainly connect to point "C" C112,C111 and C110 are the main supply filter caps there. Probably wouldn't hurt to change C303 & C304 if you really want.

I agree with Bill "lead dress, broken solder connections" most likely cause.

Please bear in mind that on January i replaced the following filter caps which i bought online and branded ILLINOIS:

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

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

C301, 302--------------------------220uf 300v (2)

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

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

I also replaced the 4 input jacks with original fender stereo jacks.

How come brand new filter caps are faulty?? Just dont get it!

[52 Bill]

You may or may not have a bad cap, this is just a theory. Just because they are new, that doesn't mean that they are good, and just because we have theorized that they could be bad, that they are bad. A cold solder joint or a broken pc trace on one of the new caps can cause problem just as much as a bad cap can.

Someone told you that a microphonic tube was the cause or this, so you tried multiple tubes and the problem remains. It is possible that a tube could cause this, but after trying multiple different tubes, I would start to look elsewhere.

[g1]

Does it still squeal even if the master is turned really low, so you can barely hear it?

It is not a "standing to close to the amp' feedback is it?

You can even hear the whistling with a cable attached and no guitar plugged.

Please tell us what the master volume level is set to (with gain at 6) when it starts squealing. Also, (with guitar plugged in) when it is squealing and you turn the guitar's volume off, does the squealing stop?

[jalexquijano]

Please tell us what the master volume level is set to (with gain at 6) when it starts squealing. Also, (with guitar plugged in) when it is squealing and you turn the guitar's volume off, does the squealing stop?

Okay. CHannel 2 - Gain at 7 and volume at 2 causes loud squealing. Gain at 6 and volume at 4 causes loud squealing.

Channel 1 - No squealing even with volume at 10 and treble at 10.

Should i order the 22uf 500V caps? Which source do you recommend.

I used this guys last time: Amplified Parts

Dont want to run the risk of buying faulty filter caps again.

[pdf64]

'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