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Post subject: Urgent help - Reverb buzzing on Fender Twin
Posted: Wed Oct 24, 2012 3:30 pm
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I have a Fender Twin Amp (2002) that I am selling - delivering to a buyer tomorrow. I've just discovered, having not used it for a while, that the reverb isn't working and is buzzing.

In every other sense, the amp is working fine but when the reverb is on a buzzing sound is heard that gets louder when I turn up the reverb. No actual reverb sound is made. when I play (although the plain guitar sound remains)

I've never serviced an amp so consider me a complete beginner. I've had a look at the V7 tube (a 12AT7) which I believe is for the reverb. Nothing looks wrong with it and when turned on it glows a normal-looking orange colour.

I've also checked the connection to the spring reverb unit in the base of the amp and everything seems secure.

Is it likely to be the tube that's died or the reverb unit? What can I try (other than replacing the tube, which I will try tomorrow when I can go out and buy one)?

Basically I don't know if what I'm describing is what to expect from a tube failure or a problem with the reverb unit (or, less likely on the circuitboard).

Any help would be much appreciated asap as I've got very little time! Thanks.


Last edited by rockbadger on Thu Oct 25, 2012 2:31 am, edited 1 time in total.

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Post subject: Re: Urgent help - Rerb buzzing on Fender Twin
Posted: Wed Oct 24, 2012 3:37 pm
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Check reverb connexions ont the back of the amp and at the reverb tank.

There are RCA jack and probably need to be cleaned. Just unplug / plug wire in the jack many times at the amp and at the reverb tank . This probably will clean contact. You can use contact cleaner too.

They oxyde after many months. Sometime we must replace the jack but most of the time a good cleaning do the job.


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Post subject: Re: Urgent help - Rerb buzzing on Fender Twin
Posted: Wed Oct 24, 2012 3:57 pm
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Thanks a lot for the quick reply stratele52. I've looked at the plugs and they look completely normal. I've tried unplugging/plugging each of them a few times each but no change whatsoever. I will keep trying, since you say 'many' times, but as I say, they don't look at all wrong.

I tried this at one point with the amp on (not on standby). When I pulled the white cable (return) out at the amp-end, the amp squealed (until I put it back in, quickly). I don't know if that's to be expected or not, but I guess it might help narrow down what could be wrong?


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Post subject: Re: Urgent help - Rerb buzzing on Fender Twin
Posted: Wed Oct 24, 2012 4:16 pm
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Unplugged the send cable at the amp end and tapped - nothing.

Unplugged the return cable at the tank end and tapped - hum.

That means the tank needs fixing/replacing - is that right?

Thanks.


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Post subject: Re: Urgent help - Rerb buzzing on Fender Twin
Posted: Wed Oct 24, 2012 5:29 pm
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Could be the tank. But, tapping the cable tip that goes into the tank input won't produce any sound, that is normal. Tapping the cable tip that goes into the tank output will produce hum, and that is normal. Rough treatment of the amp can knock the springs loose, and I've even seen the wires cut from the inner part smacking the outer part and pinching the wires between. Usually, with reverb turned up, you can give the tank a good rap and hear a crashing noise if the tank and V5 are good. If you have a meter, you could check continuity on the input and output of the tank. Take the tank out to check, and see if the springs are intact.

The usual suspects are tubes though, V7 the 12AT7 for the tank input, try this one first, and don't forget V5 the 12AX7 for the tank output.

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Post subject: Re: Urgent help - Rerb buzzing on Fender Twin
Posted: Thu Oct 25, 2012 2:15 am
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Thanks for the tip.

I'm going to pick up an new 12AT7 for the V7 at lunch which I hope will fix it. I can't get my hands on a 12AX7WA so quickly though (or a reverb tank) so if it's that, it's not so great.

If it's the V5 though, wouldn't it cause problems for non-reverb sound too? I'm no expert in amp schematics but I thought that was shared with the other sound. Also, if I'm getting a consistent hum (with no unexpected noise) when touching the RCA cable, wouldn't that mean the V5 is OK?


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Post subject: Re: Urgent help - Reverb buzzing on Fender Twin
Posted: Thu Oct 25, 2012 8:37 am
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Getting the hum when unplugging and touching the cable would only indicate that the V5 is functioning, not whether it could be causing the noise that you have. A tube can function and still cause noise, or the noise can be from the tube circuit around it. Only half of the V5 tube is used for the reverb recovery, but that half could still be causing the noise when the reverb is turned up, while the other half is OK. Just guessing really, and you can't be sure without substituting another tube. Hopefully, It's only the 12AT7 in V7. If it's the tank, it isn't expensive, look up a 4AB3C1B replacement.

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Post subject: Re: Urgent help - Reverb buzzing on Fender Twin
Posted: Thu Oct 25, 2012 12:19 pm
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Got a new 12AT7 but its pins were messed up and one broke when trying to plug it in. Another's coming tomorrow.

Since there's no reverb sound coming throughat all - it's not just a buzz - unless both the v5 and v7 are dodgy (possible).

Thanks for the help.


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Post subject: Re: Urgent help - Reverb buzzing on Fender Twin
Posted: Thu Oct 25, 2012 1:34 pm
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" 12AT7 but its pins were messed up and one broke. "

Check the socket, before plugging in a new tube. You should not be bending or breaking new socket pins. May have damage to the socket. Thus, need repair or a new one.

Be sure orientation of the pins is correct with the socket. There is a gap between pins 1 & 9, that corresponds to a gap in the socket.

Good luck!

BTW: For future reference. I like the Sylvania or Philips 12ATWA (WB or WC) for the reverb driver. This is a tough military spec tube, that can handle 10mA+ RMS all day long and be quiet and stable. The triple mica ones are very good.

http://tctubes.com/Sylvania-12AT7WA-triple-mica.aspx


Image


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Post subject: Re: Urgent help - Reverb buzzing on Fender Twin
Posted: Fri Oct 26, 2012 10:27 am
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On the pins - the first replacement tube had slightly non-straight pins when I took it out of its box. I was careful with trying to gently guide it in but I could see that more than one pin was getting more out of place as I attempted this, so I had to try to straighten them with my fingers and one snapped off. The orientation was right and the old tube's pins were dead straight so I'm certain it was the tube.

Just replaced the tube successfully - but it wasn't the problem. Frustration.

Have ordered the other tube and reverb tank.


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Post subject: Re: Urgent help - Reverb buzzing on Fender Twin
Posted: Fri Oct 26, 2012 1:37 pm
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Ok, sorry about the mis-understanding. If you are going to replace 9-pin mini tubes (like roll tubes for tone), I suggest getting a tube pin straightening tool. Something like this:

http://www.ebay.com/itm/CUSTOM-BRASS-TU ... 2c6912c389

Use it before you first insert. BTW... bent pins usually = used tubes. If those were supposed to be new, I'd contact the seller.

Good luck!


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Post subject: Re: Urgent help - Reverb buzzing on Fender Twin
Posted: Tue Oct 30, 2012 3:18 pm
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Finally fixed.

In case anyone searches and wants to know, the problem was the reverb tank after all. Working nicely now.


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Post subject: Re: Urgent help - Reverb buzzing on Fender Twin
Posted: Tue Oct 30, 2012 3:39 pm
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rockbadger wrote:
Finally fixed.

In case anyone searches and wants to know, the problem was the reverb tank after all. Working nicely now.


+1


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