Fender Twin Screeching

[yesguitarman]

Last night during band rehearsal my 2001 Fender Twin Amp starting screeching. Removing the input jack and turning the volume controls to zero did not stop it. I turned it off, waited a few seconds and turned it back on and it continued to screech loudly. I turned it back off, waited about five minutes and turned it back on, no screech. After about 20 more minutes of normal playing, the screech started again. This happened about a year ago, but didn't happen again until last night.

I'm running two pedal boards into a mixer into the input of the amp. Even though the mixer outputs are set at 0 or less than 0 dbs, is there a possibility that sound of my sound patches have too much input and are overloading the preamp? Any other thoughts, could it be a tube problem?

[MWJB]

Could just be a noisy preamp tube, disconnect all devices from the amp's input, turn it on with the volumes & tone controls fully up, gently & carefully wobble the small preamp tubes in their sockets & lightly tap them (including any hidden under metal shields), chances are you'll find the culprit...replace with same type & your preferred brand.

[Rob Mercure]

While I'm pretty sure I understood what was said, am I the only one who detected a bit of the old "Who's on First?" (Abbot and Costello) in the first post?

Rob

[yesguitarman]

Last night during band rehearsal my 2001 Fender Twin Amp starting screeching. Removing the input jack and turning the volume controls to zero did not stop it. I turned it off, waited a few seconds and turned it back on and it continued to screech loudly. I turned it back off, waited about five minutes and turned it back on, no screech. After about 20 more minutes of normal playing, the screech started again. This happened about a year ago, but didn't happen again until last night.

I'm running two pedal boards into a mixer into the input of the amp. Even though the mixer outputs are set at 0 or less than 0 dbs, is there a possibility that sound of my sound patches have too much input and are overloading the preamp? Any other thoughts, could it be a tube problem?

OK, I replaced the 12AX7s in both preamp channels (V1 and V2). When I first fired up the amp, the screech started again (nothing in the input jack). I quickly shut it down and waited a few minutes. Then I made sure the standby switch was on and turned on the power. After I turned the standby switch off, no screech and the amp played fine the rest of the day. I feel like I'm not yet out of the woods on this problem. Any more thoughts?

[52 Bill]

Any more thoughts?

My first reaction to this symptom is to look for a loose component or wire in the high voltage section of the power amp.

With all of the controls set to zero and with nothing plugged into the inputs, turn the amp on and give it a good "Enzo whack" with your fist. See if the noise will start or stop with the physical jarring of a good thump. If it does, then you will need to open up the amp to check for loose connections or parts.

[Rob Mercure]

My apologies for my last post - I must have been half asleep when I replied or some such. Anyhoo, do you have the reverb turned on? If the reverb is up this sounds like dirty connections on the reverb recovery cable. You might want to make sure that the reverb cables are plugged in with clean connections.

Rob

[52 Bill]

sounds like dirty connections on the reverb recovery cable. You might want to make sure that the reverb cables are plugged in with clean connections.

Rob

Great thought Rob, I've seen that happen a lot too.

[yesguitarman]

OK, gang, I brought the amp to The Amp Shop in West Palm Beach, Fl and they could not find anything wrong with it. The screech started again during rehearsal yesterday. I immediately unplugged the input jack and the screech continued. I turned the amp to standby (the two power LEDs were green) and back to normal, the screech returned. I turned it back to standby waited a few minutes the second time and no screech. I lowered the mixer master output to about 60% and then had no screech the rest of the day. I'm convinced the mixer was overdriving the preamp. Does anyone know the input millivolt limit for the current Fender Twin Amp? I'd like to compare it to the mixer output voltage when set at -0db.

[52 Bill]

The screech started again during rehearsal yesterday. I immediately unplugged the input jack and the screech continued. I turned the amp to standby (the two power LEDs were green) and back to normal, the screech returned. I turned it back to standby waited a few minutes the second time and no screech.

When the screech happens, does turning any of the front panel controls make a difference? Does turning down the master turn down volume or kill the screech?

If you slap the top or side of the amp with your hand or a rubber mallet can you make the screech start or stop? Turn down the reverb before you try this.

The two power LEDs on the back panel only monitor the screen grid B+ fuses. So if the amp is working they will stay green until an output tube fails.

Intermittents like these are sometimes a real pain to deal with, but if you can localize the problem based upon symptoms, you may be able to track it down.

[yesguitarman]

When the screech happens, does turning any of the front panel controls make a difference? Does turning down the master turn down volume or kill the screech?

If you slap the top or side of the amp with your hand or a rubber mallet can you make the screech start or stop? Turn down the reverb before you try this.

The two power LEDs on the back panel only monitor the screen grid B+ fuses. So if the amp is working they will stay green until an output tube fails.

Intermittents like these are sometimes a real pain to deal with, but if you can localize the problem based upon symptoms, you may be able to track it down.

Bill,

No, turning down the volumes to zero has no effect in stopping the screech. I'm convinced it's got something to do with the fact that I'm using a mixer upfront. I think I may be overloading the preamp tube making it in fact feedback. What exactly does the standby switch do? It has no effect on stopping the screech unless you let the amp sit for a few minutes. My guess is that the feedback in the preamp tube stops and then the amp play fine again. I lowered the mixer master output signal and was fine the rest of the day. Do these facts make sense in matching the symptom. I wish Fender had a direct contact to their technical team on their website but they don't.

Thanks,

Tom

[MWJB]

If turning the volumes to zero doesn't affect the screech, it can't be the signal upstream/inputs/mixer overloading preamp tubes as with the the volumes at zero you are grounding out any signal after that point (nothing, well virtually nothing, is getting through to the speaker).

The standby switch turns on the DC to the tubes, which they need to work. That DC goes to all the tubes, so it could be any one of them screeching.

Pull out ALL the 12AX7/12AT7 tubes, power up and see if the amp still screeches (if it does the problem is in the power amp/tubes), then power down and replace the preamp tubes one by one, starting with those latest in the circuit like the PI tube that feeds the power tubes, until you find the one that screeches (it could well be later in the circuit than V1/V2).

[52 Bill]

I agree, I don't think that the input mixer has anything to do with the screeching.

The volume control test is only limited by the fact that there are up to 6 tube stages in circuit post volume controls on this amp. Are you using the FX loop? Are you using the Reverb?

As MWJB suggested, try pulling the phase inverter tube, if that stops it, then you know it's pre-amp related, if it doesn't then it is power amp related.

[yesguitarman]

I agree, I don't think that the input mixer has anything to do with the screeching.

The volume control test is only limited by the fact that there are up to 6 tube stages in circuit post volume controls on this amp. Are you using the FX loop? Are you using the Reverb?

As MWJB suggested, try pulling the phase inverter tube, if that stops it, then you know it's pre-amp related, if it doesn't then it is power amp related.

Which tube (V#) is the phase inverter tube in this amp? At this point, should I just go ahead and retube the whole amp? It's about six years old.

Thanks,

Tom

[MWJB]

go to Fender.com, support, amp schematics - they'll have a schem & layout there. If yours is the pro tube twin, V4 is the PI.

Sure a full retube would be an idea, then you'd have some spare preamp tubes, but it might just be one tube that's the problem and it would make sense to work through the amp methodically to isolate the culprit.

Preamp tubes can last a lifetime (depending on their role in the amp) other times they can start making a noise after a few months - so it would be an idea to try some simple trouble shooting in case the problem crops up again in the future.

[yesguitarman]

Ok guys, I'm still not of the woods. Same symptom this past Sunday after being on for about 45 minutes. I tuned on the standy, waited a few seconds, turned standy off, screech immediately returned. Turned standby back on. Then, unfortuantely, I did two things, turned the reverb to 0 and again lowered the mixer master output, before switching the standby off. No screech the rest of the day. For giggles, I'll replace the reverb tube, which V# is it in the Fender Pro Twin?