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Thread: Fender Twin Screeching

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    Fender Twin Screeching

    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?

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    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.

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    Lifetime Member Rob Mercure's Avatar
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    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

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    Quote Originally Posted by yesguitarman View Post
    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?

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    Quote Originally Posted by yesguitarman View Post
    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.

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    Lifetime Member Rob Mercure's Avatar
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    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

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    Quote Originally Posted by Rob Mercure View Post
    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.

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    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.

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    Quote Originally Posted by yesguitarman View Post
    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.

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    Quote Originally Posted by 52 Bill View Post
    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
    Last edited by yesguitarman; 07-01-2008 at 01:23 PM. Reason: sp.

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    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).

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    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.

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    Quote Originally Posted by 52 Bill View Post
    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

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    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.

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    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?

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    Senior Member TD_Madden's Avatar
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    Probably V3 (12AT7). Does it have a tube-chart?

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    Lifetime Member Rob Mercure's Avatar
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    YGM,

    First off, as you can tell we're recommending, don't get hung up on conincidence - it can totally turn you around and in the meantime drive you crazy (ooops, did I dribble again?). And as I recommended, have you pulled the reverb connectors and cleaned them at both ends? It only takes one dirty end to make problems.

    As mentioned preamp tubes last a lifetime often so definitely hang on to your old tubes. Any recommendation to replace an entire set of tubes comes from someone who makes a profit selling you tubes - my '67 Vibrolux reverb has only had the rectifier and PI tube replaced - 31 years later, after years on stage, I'm still riding on the original output tubes (but if this amp doesn't have individual output tube bias I recommend that you get a good tech to install it).

    Have you taken a pencil and tapped each tube in the amp with the volume at a meduim level? Most often when a tube's responsible for screeching it's microphonic (has a 6SL7 once that I could actually sing into!). But to save money I'd probably only buy a 12AX7 and a 12AT7 and replace suspect tubes one by one cuz it's unlikely that you've got more than one bad tube - assuming that this is you problem.

    Rob

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    I recently fixed a Mesa Boogie that was doing a similar intermittant screeching thing, new power tubes cured it. Never known that failure mode before!
    Don't expect new production tubes to last as long as Rob's 60s vintage tubes. The power tubes in my buddy's pro tube twin amp were giving a flabby and tired after a year's hard use.
    If you have the effects loop engaged in that amp, does turning down the send or return levels have any effect on the screech? Does the tremelo modulate the screech?
    Peter

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    Quote Originally Posted by pdf64 View Post
    I recently fixed a Mesa Boogie that was doing a similar intermittant screeching thing, new power tubes cured it. Never known that failure mode before!
    Don't expect new production tubes to last as long as Rob's 60s vintage tubes. The power tubes in my buddy's pro tube twin amp were giving a flabby and tired after a year's hard use.
    If you have the effects loop engaged in that amp, does turning down the send or return levels have any effect on the screech? Does the tremelo modulate the screech?
    Peter
    I'm not using the effects loop. I haven't tried to see if the tremelo affects the screech. The screech is so loud the first reaction is to flip the standby on. Volume controls have had no effect on stopping the screech when it starts. If I flip the standby off after just a few seconds the screech is right there again, even with nothing plugged into the input jack.

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    "Volume controls have had no effect on stopping the screech when it starts. If I flip the standby off after just a few seconds the screech is right there again, even with nothing plugged into the input jack." We have known this for a few days now, what exactly have you done to isolate/identify where the screech is coming from?

    Is there any screech with all the preamp tubes removed?

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    Quote Originally Posted by MWJB View Post
    "Volume controls have had no effect on stopping the screech when it starts. If I flip the standby off after just a few seconds the screech is right there again, even with nothing plugged into the input jack." We have known this for a few days now, what exactly have you done to isolate/identify where the screech is coming from?

    Is there any screech with all the preamp tubes removed?
    Ok, I'm a true neophyte here so please be patient with me. Is it safe to pull the tubes with the amp powered? Aren't they hot, do I need gloves? Do I leave the standby switch in the normal play position when pulling the tubes? Do I start at V1 an pull the tubes in ascending numerical order?

    Thanks

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    Lifetime Member Rob Mercure's Avatar
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    Yes you can pull the tubes with the amp powered but turn down the volume controls as you pull each one cuz it will cause much pop and boom. And I'd recommend a cloth work glove.

    As to which tube to pull that probably depends on personal preference. I would first pull the 12AT7 nearest the output tubes and see what happens. This is the phase inverter (PI) and with it out the only tubes connected to the speaker (via the OT) are the output tubes - if the noise remains it is in the output circuit. If that changes nothing then I'd pull the 4th tube which is the reverb recovery and also channel mixer - 12AX7 - if the symptoms diminish I'd suspect the reverb recover section but it could be the mixer side. If this doesn't change anything then systematically start from tube 1 and remove the tubes. But, just so you can determine which actual stage is at fault be sure to restore a tube to it's original position once you've found that it doesn't make any difference. It would be easy to pull all of the tubes, have the problem fixed, and wind up with a spare set of tube with a hidden "suspect" in the spares waiting to jump out and bite you.

    As you remove and restore the tubes make sure to gently rock the tubes in their sockets a bit - this will often either make the syptom worse, pointing to a bad tube or dirty socket - or improve the syptom pointing to a dirty socket. Sockets are best cleaned with a product called DeOxit (Caig corp) followed by 99% isopropyl alcohol (to remove any of the lubricant in the DeOxit which can attract conductive dust and cause carbon tracks on the tube socket which then cause all sorts of gremlins). I think you know aleady not to operate the amplifier without a speaker to prevent damage to your output tranny and output tubes.

    Rob

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    Is this a 65 BF Twin or a pro tube twin? I seem to remember a reference to an effects loop (not a feature on a 65 Twin). If pro tube, V4 is the PI - go to Fender.com/support/amplifier schematics & print off schem & layout, you will see what number tube does what (or ask here, making sure you have a reference & can understand the answer).

    It's good practice to power down when pulling out tubes, preamp tubes don't typically getr very hot BUT the 12AT7 reverb driver will most likely be too hot to be comfortable! Pull them ALL out one by one, write on them with a sharpie where they came from.

    Replace one by one, starting with PI (V4 if pro tube, V6 if a 65), then 12AX7 reverb recovery, then 12AT7 reverb driver, then the preamp tubes one by one. Power up between each installation, volumes & tone controls set to full, and flick the end of the preamp tube - you are trying to prompt a noise. Preamp tubes V1 & V2 may give a "tink" sound when tapped/flicked...this is normal, any whistling or extended ringing sounds, screetching, frying eggs, or loud "baaaarrrrgh" sounds are not normal, replace whatever tube is dioing it.

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    OK folks, it's definitely a reverb issue. If I play the amp with a guitar plugged straight into the input jack, with no reverb, no screech. As soon as I turn the reverb control past 1, the screech returns. Which tube or tubes (V number) should I replace to address the issue. The amp is in pristine condition. There is absolutely not a spot of corrosion on the reverb spring tank RCA jacks.

    Thanks for all your help,

    Tom
    Last edited by yesguitarman; 07-28-2008 at 02:41 PM. Reason: sp.

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    If it's a pro tube twin, replace V5 12AX7 & V7 12AT7.

    If it's a 65 Twin replace V3 12AT7 & V4 12AX7.

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    Quote Originally Posted by MWJB View Post
    If it's a pro tube twin, replace V5 12AX7 & V7 12AT7.

    If it's a 65 Twin replace V3 12AT7 & V4 12AX7.
    Thanks guys, I think that did it. I replaced both V5 and V7 last night and the amp sounded great! I also replaced V1 with a new 12AX7R2 tube. The amp sound sooo much better with this tube than the standard 12AX7. Thanks for the tech at The Amp Shop in West Palm Beach for suggesting that preamp tube.

    Thanks again,

    Tom

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    Guys, this issue has returned several times. Each time I have replaced V5 and V7 and the screeching from the reverb section goes away. The amp plays fine as long as I keep the reverb at 0. What other components in the reverb section is causing these tubes to go bad. For clarity, the amp is a 2000 Fender Pro Twin (the one with a single input jack).

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    Senior Member Enzo's Avatar
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    I would ask if you removed those tubes, then tried re-installing the exact same pair of tubes?

    My first impression is feedback in either the pan or the cabling.
    Education is what you're left with after you have forgotten what you have learned.

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    Supporting Member Jazz P Bass's Avatar
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    Has anyone messed with the reverb pan?

    There is a drive end and a pickup end.

    The pickup end is marked OUTPUT.
    That end should be mounted as far as possible from the power transformer .

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