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Post subject: Fizz on note decay on 1965 Twin Reverb
Posted: Mon Mar 25, 2013 5:51 pm
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Hi,

My original blackface 1965 Twin Reverb has developed a sort of quiet fizz audible on the very tail end of decaying clean notes. Then it dies down. If I even touch the strings on the guitar it's enough to make it happen. It happens on both channels. It seems largely unaffected by turning volume up, although turning the Bass up makes it louder. None of the tubes are microphonic. Unplugging the reverb unit doesn't affect it.

I remember reading, and I think it was here, that someone compared it to the famous Fizz on Mustang III-V V1 amps, and said it had happened on his old Twin (?), and was a bad preamp tube. Can anyone help? Is testing the tubes on a old-fashioned tube tester sufficient?

Thanks


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Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Mon Mar 25, 2013 5:58 pm
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FernandR, welcome to Forum! How old are the main filter caps (the ones under the doghouse)? If they are less than 15-20 years old --- try swapping signal tubes (12AT7 and 12AX7) around one-by-one. And see if that changes the odd harmonics issue.

Most of the time, new lytic filter caps and a couple of fresh tubes in the gain stage(s) can get the amp sounding original, again.


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Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Mon Mar 25, 2013 5:59 pm
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If it's a pre-amp tube it can only be V6, which is shared by both channels. That may be the cause but I'd suspect a southerly-heading power tube. When's the last time you replaced them?

Arjay

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Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Mon Mar 25, 2013 8:05 pm
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Wow, thanks, guys. The amp hadn't been played in a LONG time, like maybe 20-30 years, except for a recording session or two. The tubes were being kept up the last time it was played. The power tubes glow lightly blue with B+ on. The amp sounds good except for that bloody fizz; everything works. The caps are all original. I haven't yet checked if they look bulgy, I was putting off pulling the chassis out. I guess the "doghouse" can be removed without doing that?

My next step was (is) to take the tubes down to where they have a tester. I tried a little swapping first, swapping the 12AT7s, V3 and V6 - no change.

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Last edited by FernandR on Mon Mar 25, 2013 9:54 pm, edited 2 times in total.

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Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Mon Mar 25, 2013 8:33 pm
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Super nice amp, Man!!! If the tubes were fresh 20-30 yrs ago, that shouldn't be a problem. In fact, don't disregard them!!! A thorough cleaning and a recap for the power and bias supply should do the trick. I guess all resistors should be checked. But there is probably nothing more than a good maintenance check-up wouldn't solve. Art

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Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Mon Mar 25, 2013 9:38 pm
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Be sure to tip your roadie......generously.

:lol:

Arjay

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Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Mon Mar 25, 2013 9:54 pm
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Retroverbial wrote:
Be sure to tip your roadie......generously.
:lol:
Arjay


he broken backed, gone.

The last time the amp was used, my friend the late Michael Karoli, of the German band Can fame, was doing a gig in SF ('98?), and so I had a friend of my daughter's take the Twin over to Great American Music Hall, or wherever the concert was. I forgot how heavy it was. The poor kid carried it two blocks. Michael, with all his progressive German BS, gave me this weird rant about having slaves in Amerika -- the kid was black :lol:

http://www.youtube.com/watch?v=-9nFglCyshY

BTW, How meaningful is a tube tester?


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Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Tue Mar 26, 2013 12:33 am
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Seems I do a lot of slave labor in the course of my job. Tube testers, IMO, will give a general condition reading for the tube. Not as accurate as in-amp voltage and current draw readings. IMHO, because of each amp's wiring, component values...etc, the synergy is different and a tube in one Twin may not produce the same effect in another Twin. Art

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Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Tue Mar 26, 2013 1:59 am
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I usually pull the chassis before taking the doghouse cover off. You need to access the B+ point, somewhere, and run a draining resistor line from B+ to ground (with amp off and pulled out of outlet). Need to drain any residual charge sitting in those caps.

A photo of the caps under the "house" would be nice, to help determine age.

Luckily, quality aftermarket caps are still available.

:mrgreen:


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Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Tue Mar 26, 2013 2:06 pm
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But just to inspect the caps, can the cover be removed, i.e. is it tapered metal screws into the chassis, or is it nuts & bolts, so will there be nuts floating around, with no way to put it back on without removing the whole chassis?


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Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Tue Mar 26, 2013 2:16 pm
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FernandR wrote:
But just to inspect the caps, can the cover be removed, i.e. is it tapered metal screws into the chassis, or is it nuts & bolts, so will there be nuts floating around, with no way to put it back on without removing the whole chassis?


You need to pull the chassis out:

Image
1965 Fender Twin Reverb 07 by toyranch, on Flickr

Doghouse on right:

Image
1965 Fender Twin Reverb 10 by toyranch, on Flickr

Looks like sheet metal screws:

Image
1965 Fender Twin Reverb 11 by toyranch, on Flickr

In any case, they appear to be on either side of the circuit board so should be easy to remove.

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Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Tue Mar 26, 2013 5:56 pm
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And once you pop the lid off the doughouse, if you see pimples on the side portions of the filter caps. Bulging out-of-shape bodies or fluid soaking the body of the caps --- they must be replaced. No need to even test them. :D


Image


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Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Tue Mar 26, 2013 6:52 pm
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Thanks!!!
It looks like I can do a quickie inspection with just a light and a screwdriver. But removing the chassis doesn't look as scary now, so double-thanks!

But, out of curiosity, what mechanism could cause these caps failing to cause that sort of fizz, that disappears when no signal is present?


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Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Tue Mar 26, 2013 8:29 pm
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Electrolytic caps have a solution inside the cap that acts as the lytic agent. When this solution dries out or leaks out of the cap --- you lose most of the filtering capability of the cap.

Heat, being unused for a long time, over-voltage, cap older than 15-25 years, phase of the moon ( :mrgreen: ) may lead to this problem.


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Post subject: Re: Fizz on note decay on 1965 Twin Reverb
Posted: Tue Mar 26, 2013 10:27 pm
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FernandR wrote:
But removing the chassis doesn't look as scary now, so double-thanks!


Pulling the chassis from a vintage Fender combo takes less time than typing out a step-by-step description of the procedure. If such disembowelment makes you squeamish you likely shouldn't be messing around inside an amp in the first place.

Arjay

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