18Watt.com

So I have a Silverface Twin that I got as a basket case. Got everything up and running except the Vibrato channel is weak.. that is until I turn the reverb up. The reverb causes massive volume and if the reverb is engaged then it is like the reverb effect is dialed to 10. I have looked at every connection possible with varying schematics and everything looks good. Ohmed out the 3.3M reverb mix resistor and it looks good. All wiring looks good. All grounds are going to ground. Swapped preamp tubes around and no difference. Lifted the board to make sure there were no shorts and didnt see any. This is making me pull my hair out. Any ideas? Does not have a push pull Master on it anymore. Was doing the same thing before that was removed too.

Ok so just got it figured out. So the resistor that comes directly off of the reverb pot center lug , the attenuation resistor, was supposed to be 470k. Pulled the resistor out of circuit and it was a 470ohm resistor!! Replaced it with the 470k and my reverb stopped controlling the volume of the channel. Also, I noticed that my vibrato would not work unless the reverb was turned on. That little resistor made all the difference. Now I am going to analyze and see how it was doing that. If anyone has any idea how it was causing the symptoms it was experiencing then please put in your two cents. I would love to hear anyone's thoughts. Thanks.

I'm not super familiar with the circuit, but at quick glance, here's my guess. It looks as though the tremolo and reverb are both connected to each other at the grid of V4A.

Now consider that essentially those are two resistors in parallel to ground. 470K + 220K in parallel is about 150K. 470R + 220K in parallel is about 470R. That's assuming that the reverb pot is turned down all the way.

If you turn the reverb pot all the way up, you have 100.47K with the pot and 470R resistor in series. In parallel with the 220K that's about 69K. Not nearly what it's supposed to be, but significantly higher than 470R.

Thanks,
Josh