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Post subject: 1968 Silverface Twin Reverb/Vibrato help
Posted: Fri Apr 09, 2010 9:41 am
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Hello

I just inherited a 1968 Silverface Fender Twin Reverb. I was told that the tubes have been replaced and the pots have been cleaned out. When I plug my guitar into the normal channel, the sound is great. But when I plug my guitar into the vibrato channel and I turn on the vibrato, I can hear the pulse of the amp/tubes(?). When I turn on the Reverb, there is a lot of hiss and white noise.

How would I go about fixing this issue? Is it a wiring thing or do other tubes need to be relaced? I appreciate your assistance. Thank you in advance.


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Posted: Fri Apr 09, 2010 11:12 am
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When the effects are switched off, does the vibrato channel sound okay?

If so, then the problem probably lies with the tubes that are dedicated to those effects or their associated circuitry.

You might also check the cables connecting the reverb tank to the chassis. Make certain that you have continuity and that they are correctly installed into the output and input jacks on the rear of the chassis.

Best of luck, HTH

Arjay


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Posted: Sat Apr 10, 2010 3:48 am
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Mine, a Vibrolux Reverb silverface had the same problems

For the vibrato I have to change somes resistors (I'm electronic tech) And now he is quiet.

For the reverb. I have the same problem as you have.

I have to work on it. I'll give you some news


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Posted: Sat Apr 10, 2010 7:46 am
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Your amp is due for a checkup by an amp tech.

1. Power cord - if it's a 2-prong, needs to be updated to a 3 prong and the "death cap" removed.

2. Electrolytic filter capacitors - these have dried up after 42 years and now present a shock hazard - have them replaced.

3. Try replacing your V3 and/or V4 preamp tubes with a known better tube - the one in there now may have gone microphonic. What kind of tubes are in there now?

Mike Kropotkin has a great info on preamp tubes - what to look for, what to spend the money on, and what doesn't really matter. This may be helpful:
https://www.kcanostubes.com/content/fen ... mendations


My guess is that once the amp tech has done that and ensured the amp is properly biased, you may see some other issues cropping up, like resistors, capacitors out of spec.

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Posted: Sat Apr 10, 2010 4:00 pm
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For the noise on my reverb control, I found the broblem.
I have to clean with contacts cleaner the 2 phonos plug on the reverb tank ( spring unit).

My problem was, with all amp volumes to 0, the reverb control give a hum more or less lound.
Now the amp is quiet


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Posted: Sat Apr 10, 2010 4:08 pm
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stratele52 wrote:
For the noise on my reverb control, I found the broblem.
I have to clean with contacts cleaner the 2 phonos plug on the reverb tank ( spring unit).

My problem was, with all amp volumes to 0, the reverb control give a hum more or less lound.
Now the amp is quiet


This is a common problem with all reverb-equipped amps (not only Fenders). The ground ("earth") portion of the RCA jack can develop an oxidized patina over the years, making a good contact with the plug's ground difficult. It's wise to periodically remove the cables, clean the jack with some De-Oxit or other contact cleaner, then re-install the cables and give each plug a gentle ¼ turn in either direction to ensure good continuity. The same goes for the vintage-type footswitches with RCA phono plugs.

HTH

Arjay


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Posted: Sat Apr 10, 2010 10:41 pm
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Thanks everyone, looks like I have some work to do and a few bucks to spend. I appreciate it. When I find out what the issue is I'll be sure to share it. All the best.

Jude


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Posted: Sun Apr 11, 2010 7:27 am
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Reverb problem is always there; hum when reverb control is at 4 and more.

Work in progress

Retroverbial ; you are right, some times that fix the problem.


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Posted: Sun Apr 11, 2010 3:59 pm
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Hey Stratele, if the cabling checks out, like Retro suggested, and the tubes are good, the resistors in that circuit may be suspect. Also, the coupling and cathode caps could be leaking or open. What about the lead dress and tube shields? Art

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Posted: Sun Apr 11, 2010 4:29 pm
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After studying the AB763 TR circuit for a while, I'm sorta suspecting there may be a problem at or near V4. That's the tube that sums both the normal and vibrato channels back together and acts as the intermediate gain-stage preamp before the whole show hits the phase inverter at V6.

HTH

Arjay


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