Issue with '72 Twin Reverb. - diyAudio
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Old 25th September 2012, 06:39 PM   #1
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Lightbulb Issue with '72 Twin Reverb.

Hey guys, I could take this bad boy to a tech, but I know it's in my best interest to try to learn a thing or two about amp repair, seeing as how I imagine I'll be using tube amps for the rest of my life.

Here is my issue, the reverb side of my twin reverb isn't outputting any sound at all. The normal channel works just fine (and sounds great), however.

Any thoughts?

Thanks!
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Old 25th September 2012, 07:01 PM   #2
morinix is offline morinix  United States
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Start with the reverb RCA send and receive jacks on the amp. Those can go bad.
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Old 25th September 2012, 07:41 PM   #3
quayhog is offline quayhog  United States
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Reverb cables may be reversed. Does it work without footswitch? Check tubes for reveb channel. I'm assuming channel 1 works OK. This would eliminate problem in Channel 1 and PI and power tubes and power supply. SHould be an easy fix.
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Old 25th September 2012, 08:34 PM   #4
Mr. dB is offline Mr. dB  United States
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Is the entire channel out, or just the reverb effect itself?

If it's the entire channel, is there any noise or hum when you turn the volume up?

I'd start by swapping V1 with V2. See if the problem changes channels.

http://www.thevintagesound.com/ffg/twin_reverb_sf.html
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Old 25th September 2012, 09:05 PM   #5
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Old 25th September 2012, 11:46 PM   #6
Enzo is offline Enzo  United States
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I read it as the reverb channel, not the reverb itself specifically.


Look at the row of small tubes, are there any that do not have two little orange glowing spots?

I might start with an extra 12AX7, and go down the row of tubes, substituting the new tube for each one. That will reveal if you simply have a bad tube.

AFter that, we go inside, my first general check is to go down the row of preamp tubes looking at pins 1 and 6 of each for presence of high voltage. I also look at pins 3 and 8 expecting to find a non-zero voltage at each.


Those are quick simple tests that will suggest furhter action depending on what we find.

After that, there are a zillion things that could be killing sound in that channel.
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Old 2nd October 2012, 05:08 PM   #7
Struth is offline Struth  Canada
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Hi Guys

It is most likely a bad tube keeping the reverb channel from working. A tube swap as others suggest will show that the problem moves with the tube or that the problem stays with the channel.

If the problem is with the channel, check the solder connections for the tube socket related to that channel. If either pin-4 or pin-5 connection is not good, then half the tube will be without heater power and that triode won't function.

Within the channel, you can check that there is high positive voltage at each end of the 100k-1W plate resistors. The two closest the input-end of the chassis are for the normal channel, the next two are for the reverb channel in the line of components on the eyelet card. If you see the same voltage at both ends of a given resistor, then that triode is not conducting and its other wiring needs to be checked out.

Otherwise, prior to the PA is a mixing point comprised of two 220k resistors: one may be open or cracked or have a bad solder connection.

As always, the screen resistors for the output tubes should be changed to 1k-5W and the grid-stops changed to metal-film. These changes protect the output tubes and improve reliability, as our FAQ and TUT-series books detail.

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