Troubleshooting a Silverface Vibrosonic Reverb

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Troubleshooting a Silverface Vibrosonic Reverb

Postby BigNath on Tue May 16, 2017 12:06 am

Hi all, first time poster here....

I have just picked up a SF Vibrosonic Reverb (SF Twin Reverb with a 15") which has some noises i'm trying to track down.

The amp has come back from a local amp guy who has previously done awesome work with my SF Bassman 10 and Hot Rod Deville that I also own.

I would describe the noise as predominantly a hum with a 'windy rustling sound' with the occasional pop/crackle. This noise is not 'overly' loud, and if playing on stage without a mic, probably wouldn't bother me much, but going through a PA i'm worried the background noise will be hear by audience...

Things that have already been done to the amp:
New Valves and BIAS done
New Capacitors/Filters (those long brown tubes inside the chassis)
Grounds checked
Pots cleaned
New 15" Fender speaker installed. (from memory it's a 'fullback' model)

Things I have done to trouble shoot so far:
Read up about valves and what they do in amps. (I'm a highly experienced musician and music teacher, and know a lot about gear, but not about the finer points of exactly how valve amps work)
I can replace valves and set the bias on my Hot Rod Deville but not on a point to point vintage amp....

My reading has lead me to pull some valves and try to isolate where the noise is coming from.
First step:
I removed valves 2,3,4 and 5 leaving just preamp stage/channel 1 and the PI (valves 1 and 6) and all power tubes. This resulted in a nice quiet amp.
I then put them all back in, and removed valves 1,3 and 5 leaving preamp stage/channel 2, reverb recovery, PI and power tubes. This brought the noise back.
I also removed the preamp valves one at a time, replacing them one at a time so that there was only ever one valve pulled at any given time, and it seems like the noise is coming from valves 3,4 and 5 (so basically the effects - Reverb/Tremelo).

I also noticed with the offending valves installed that moving the amp even gently, resulted in amplifying the 'rustling' sound. Does this shed any light on exactly where the issue might be? Just tilting the amp slightly back and forward makes this rustling sound more evident when the offending valve is installed. I can duplicate this sound by jiggling the reverb tank leads on the back of the chassis.
I'm wondering if this indicates a solder joint (ground or otherwise) is bad in the amp if it behaves like this. Or could it be something to do with the entire valve and socket assembly?
With these 'offending' valves removed, this rustling sound when moving the amp stops. (as I expected it would).

My next step is to try and replace these positions with known good valves, (keeping in mind these valves are brand new within the last couple of weeks) but is there anything else I can do to target in on the problem? For the moment i'm just gonna use the amp with them removed as I prefer the sound of my reverb from a pedal as opposed to the spring reverb tank, and I am not interested in tremolo at all.

Any help you can give would be awesome. I love the sound of this amp, and the clean headroom is amazing, I just want it to have a nice quiet noisefloor....

Kind regards,
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Re: Troubleshooting a Silverface Vibrosonic Reverb

Postby Dean Jr. on Sat May 27, 2017 5:13 pm

Pull V3...this is the reverb driver tube. IF the noise ceases, then the noise is in the reverb circuit somewhere. triode in V4 is the reverb return, so that tube could be the problem as well as could V3...or anything else in the reverb circuit. IF the noise persists with V3 out, then pull V5, which is the trem function tube. Does the noise cease?
Try this...since the NOrmal channel is quite, pull V1, which is that channels input preamp. Use this tube to sub into V2, which is the Vib ch input preamp tube. Dpes the noise cease? IF not, return V2 to its position and use the V1 tube to run V3...does the noise cease? If not, try the V1 tube in V4. Does the noise cease? And...if the noise ceased when you pulled V5, then put the V1 tube in V5. Does the noise cease?
Here is the schematic...the tubes are numbered.... IF you are going to start teching, you might as well start learning how to read a schematic. IF not, it is tech time, imho. ... _schem.pdf
Dean Jr.
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Re: Troubleshooting a Silverface Vibrosonic Reverb

Postby pdf64 on Sat May 27, 2017 11:25 pm

Do any of the controls affect the noise, eg reverb?

Are you sure that removing V5 stops the noise? It seems strange, as it's not in the signal path.
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Re: Troubleshooting a Silverface Vibrosonic Reverb

Postby Jimmy-X on Wed Jul 05, 2017 10:40 am

Is noise reduced with the foot switch unplugged (and verb still on)?
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Re: Troubleshooting a Silverface Vibrosonic Reverb

Postby stratele52 on Wed Jul 05, 2017 2:04 pm

Never see BigNath since is first post may 15 :sleepy:
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Location: Québec province, Canada

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