Vibrosonic Reverb problem

Gerard Rowan · From: Australia

Intermittent problem with 70's Fender Vibrosonic Reverb - the reverb cuts out half way through a gig. I've had this looked at by 3 different techs and they couldn't replicate the problem - the reverb worked constantly for hours on their benches. It only seems to cut out when I'm actually playing at volume. Anyone else had this annoying scenario? The amp has at some stage had a replacement reverb transformer. Every thing else - leads, reverb tank, tubes, seem fine.

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Do you have the reverb footswitch plugged in? If so, next time it cuts out, pull out the footswitch plugs. If the reverb returns, you have a short somewhere in the footswitch or it's cable. The next most common cause of cutout is a bad reverb tank. Fortunately, they're only $20 at Antique Electronics. Last on the list might be a loose reverb tube. Does wiggling the tube next to the reverb transformer make a noise?

That's about the best I can suggest.

Gerard Rowan · From: Australia

Thanks for the suggestions Donny. I might try replacing the tank. Strangely, when the reverb cuts out you can still hear a 'crash' when you shake the amp, suggesting the tank should be OK. The tubes all seem tightly seated.

Jon Light (deceased) · From: Saugerties, NY

This may seem painfully obvious but I had a problem with a Deluxe Reverb with a non-functioning reverb that also audibly crashed---seemed obvious that the reverb return was ok, so the problem had to be in the send. Simply, the fix turned out to be replacing the cords.
This doesn't explain why your works ok and then cuts out. That would make me think of something heat related.

Gerard Rowan · From: Australia

hi Jon - yes, I've tried replacing the leads to and from the tank, with no effect. Heat does seem to be a factor, which made me think the reverb transformer could be at fault?

Ken Fox · From: Nashville GA USA

The reverb works like this:

-A sample of the signal is gathered from tube 2 (V2)(12AX7)
-It is sent to V3 (12AT7, both halves) to be amplified and sent to the reverb transformer.
-The reverb transformer provides proper impedance match from the tubes parallel plates to the 8 ohm input of the tank.

The above is the reverb "drive circuit"

-The signal is delayed through the tank and goes to the 1/2 of V4 for recovery and amplification
-It then goes to the Reverb Control pot and then onto the other 1/2 of V4 (12AX7), where it is mixed with the dry signal from V2.

If you can hear spring noise, your reverb recovery circuit, V4 and the reverb pot are OK, the problem is more like in the drive circuit (including the input transducer in the tank). Problems here could be a bad tube (V3), dirty tube contacts, cold solder joints on the tube or drive circuit.

If you hear no spring noise, have good tank and good wires, then suspect the reverb "recovery" circuit. Problems here could be a bad tube (V4), dirty tube socket, dirty or bad reverb control pot, cold solder joints in the reverb circuit. One thing to replace is the 25uf/25WVDC cap on the cathode of the reverb recovery tube. These can go bad and cause a tremendous howling sound. Just plain good to replace an old one and prevent future problems. Also make sure the RCA plugs (female on the tank and amp) and male (cable ends) are clean. These tend to oxidize very badly.

You will get no reverb if the tank is plugged in backwards, but will get spring noise. For a Fender amp, the point marked input on the tank goes to input on the amp and the point marked output on the tank goes to output on the amp.

[This message was edited by Ken Fox on 26 February 2004 at 07:15 AM.]

Michael Brebes · From: Northridge CA

Since you can still hear the springs when you move your amp after the reverb dies, it either has to be in the drive circuit or the input side of the spring assembly. The most likely candidates on the drive circuit side would be the tube, the transformer, and the actual connection to the RCA connector that goes down to the spring pan. Since you replaced the cable, we know that is not the problem. You can visually inspect the pan. Remove it from the bottom of the amp and examine the internal wires, leading from the RCA connector marked IN to the input transformer, for any breaks.

John Floyd · From: R.I.P.

Used to be you couldn't hardly damage the transducers on a reverb pan, but these days if the leads get accidentally swapped connecting the receiving transducer to the send side of the loop can open the coil on that transducer up be very careful how you connect the pan up, could save you some bucks

Tim Rowley · From: Pinconning, MI, USA

These are a great little amp and I've had a bit of experience with them. Generally the problem you described is due to one of two things. If it was me, the first thing I would do is disconnect the foot switch and see if that fixes the problem. If so, I would then repair the bad connection or broken cable on the footswitch. If that's not the trouble, the very next thing I would do is pull the reverb tank out of the amp and check the fine lead wires from the RCA connectors to the tiny transformer on the pan spring assembly. These are very prone to vibration fatigue when the amplifier is being transported. As Michael Brebes has correctly stated, since you can hear the springs crash through the amp, the broken connection will likely be found on the input side of of the spring assembly and may not appear broken until you touch it with a toothpick or similar object. Carefully repair the connection with a 25-watt iron and a minimal amount of fine solder and you should be back in business. Good luck.

Tim Rowley

Gerard Rowan · From: Australia

Thanks for all the advice, guys. I have taken the reverb pan out and couldn't see any broken connections, and I don't use a footswitch. The search goes on...

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