I have this silverface Bandmaster Reverb that I've been working on restoring for the last couple of months. Things have gone really well up until this point. I've put in new filter caps, esentially re-done the entire pre a little at a time, got the trem sounding right... now I've got this 60 cycle hum in just the reverb that has crept up out of nowhere. It goes away when the reverb control is turned off or if I disconnect the reverb return (not if I only disconnect the send). The reverb works otherwise, meaning that the effect is still there. Does anyone have any tips or tricks that I might be able to try.
I am brand new to this forum. Thanks for taking a look at this.
Clay
Hum in my 1971 Bandmaster Reverb.
Moderators: pompeiisneaks, Colossal
Re: Hum in my 1971 Bandmaster Reverb.
The reverb return goes right to the grid of the reverb recovery amp and can hum badly if the return cable is not grounded properly. Make sure you have the right tank: it should be 4AB3C1B. This has an inulated input and a grounded output. If you aren't making a good ground at both ends of the cable, the cable itself or the tank housing will not be shielded. Check for continuity from the amp chassis to the reverb tank chassis. If you are getting resistance, clean and retighten the reverb return jack and make sure that the ground connection on the "output" jack is good. You have to get inside the tank top do this. It's a crappy, contact ground to begin with, but you can usually get it to work. Also make sure you don't have a bad cable; the shield sometimes gets frayed and breaks.
Re: Hum in my 1971 Bandmaster Reverb.
Turns out that everything checked out fine. Flipping the tank in the bag did work though. Maybe some change that I made kept that hum from being canceled out?
I did run into another interesting question. While digging back into the silverface schematics, I noticed that the placement of the .002 cap to ground jumps from one side to the other of the reverb tank, if it is there at all (depending on reverb amp model). The 220k resistor seems to stay on the output side if it is there. Any known reason for this?
I did run into another interesting question. While digging back into the silverface schematics, I noticed that the placement of the .002 cap to ground jumps from one side to the other of the reverb tank, if it is there at all (depending on reverb amp model). The 220k resistor seems to stay on the output side if it is there. Any known reason for this?
Re: Hum in my 1971 Bandmaster Reverb.
Ah, the old "flipping the amp in the bag" trick. In reverb heads, where the transformers are too close to the reverb tank, it's crucial to keep the output side of the tank as far from the tranformer fields as possible. And you may have restablished a ground just by moving things around -- the RCA plugs Fender used tend to get a bit corroded over time. I usually roll a tube of emery cloth inside them to polish up the ground connector.
The .002 bleeds off some unnatural sounding high-end and probably make more sense on the output side of the tank, though, as you noticed, it's sometimes on the input (and sometimes missing altogether). The 220K, though, is the grid resistor for the reverb recovery tube and needs to be where it is. The standalone Fender reverb unit didn't have this and the recovery tube will go into runaway if the reverb tank is unplugged.
The .002 bleeds off some unnatural sounding high-end and probably make more sense on the output side of the tank, though, as you noticed, it's sometimes on the input (and sometimes missing altogether). The 220K, though, is the grid resistor for the reverb recovery tube and needs to be where it is. The standalone Fender reverb unit didn't have this and the recovery tube will go into runaway if the reverb tank is unplugged.