Silverface Twin Reverb: Reverb pot adding distortion

[jvm]

I have a Silverface Twin Reverb with a reverb circuit issue. When increased, the reverb pot adds distortion but has little or no effect on the reverb level itself. The reverb sounds like it is full on except when the pot is turned completely off. I have subbed tubes, reverb cables and reverb tank with no change in the problem. Reverb pot tests ok at 100K and varies with no glitches. I have found no leaky caps in the circuit. Anyone seen this before?

[Mick Bailey]

First check the DC voltages on the recovery tube against the schematic.

Kill the reverb recovery signal with the foot switch. Does increasing the reverb level now add distortion? If so I would still suspect DC leakage from V4b's coupling cap which would skew the bias on V4a.

[pdf64]

Even then, it may be that the board is 'leaky', rather than the coupling cap.

[Mick Bailey]

Good point. I've had a few amps where a leaky board has made me think the amp is haunted and defying logic. DC leaking onto the grids can give lots of problems. Sometimes a hot air gun or hairdryer to heat the board so the wax glosses over can effect temporary fix.

[jvm]

Voltages on V4 are both cathodes at 2.3VDC and both plates at 259VDC. I killed recovery side with footswitch. Reverb pot still increases distortion. I can find no leakage at any of the caps. I can measure up to 1.3VDC on the eyelet board itself in various places, the highest voltage nearest the input end of the board.

[Jazz P Bass]

I would heat that eyelet board up.

[g1]

Which version of silverface is this? Any pull-boost knobs or master volume? Can you post the exact schematic?
The reason I'm asking is that some used a reverb driven boost circuit. So maybe something related to that could be defective, or maybe someone could have modified it to have such a boost.

[jvm]

Here is the schematic. It is a master volume non boost model. twin_reverb_aa270.pdf

[jvm]

I heated the board with a hair dryer and it took care of the distortion I had at high reverb level settings. However, the reverb still seems to be very strong at 2 on the pot setting. It does increase as the knob is turned up but at 2 is very wet. Any way to tame this? I subbed a 12AU7 for the driver tube. It seemed to help slightly.

[52 Bill]

Any way to tame this? I subbed a 12AU7 for the driver tube. It seemed to help slightly.

If it's really that bad, you can remove or reduce the value of the cathode bypass cap of either the driver tube V3 or the recovery tube V4B.

[Mick Bailey]

Now you've heated the board, just check for DC on the preamp tube grids to see if there's still a problem affecting the reverb operation. If the circuit is stock, the reverb should be fairly progressive.

[jvm]

I removed the V3 cathode bypass cap and left the 12AU7 in place. The owner of the amp is happy with his reverb now. For what it's worth, where I was measuring 1.3vdc on the eyelet board before heating it, I now have not quite .2 vdc. Heating the board eliminated the distortion being added by increasing the reverb level. Thanks for all the help everyone!

[52 Bill]

I removed the V3 cathode bypass cap and left the 12AU7 in place.

Good to hear that owner is happy.

In thinking about the circuit, because the two cathodes are connected to one resistor/cap, removing the bypass cap on V4 will also change the gain of the second channel. The choice of removing the cap from the drive tube will only change the reverb signal.

[pdf64]

When the V4 sections are in cascade and share cathode bias, it has to be adequately bypassed, otherwise a positive feedback loop is created and oscillation ensues as the reverb control is turned up.
A failing bypass cap here may have it howling.

In order to get good control resolution for low reverb levels, I like to use an audio taper pot for the reverb control.