nasty hum in deluxe reverb

Brian Davis · Post by **Brian Davis** » 16 Mar 2008 2:23 pm

hello,
my deluxe reverb has developed a nasty hum which seems to be directly related to the reverb effect. the hum seems to vary in intensity depending on the location of the amp, but when i switch the reverb off the hum always goes away. is this an indication that the tube for the reverb effect is becoming microphonic? i live pretty close to a large radio antenna as well as some power lines for the public bus system. any ideas? Thx.

BD

Ken Fox · Post by **Ken Fox** » 16 Mar 2008 5:14 pm

I had a Twin Reverb with an issue like that in the ship. Like to drove me nuts!

First thing to check is the 12AX7 at V4. Thats the reverb recovery tube.

Heck, even unplug the tank and check it. If no hum it is like a tank, cable or V3 issue.

Mine ended up being a well overheated filter choke coil that had seeped nasties inside the cover. That was allowing it to couple like a capacitor to ground and causing the power supply ripple to the reverb circuit.

Brian Davis · Post by **Brian Davis** » 17 Mar 2008 10:06 am

I tried unplugging the tank and removing the tubes, the hum is still there if the reverb control is on and it gets worse if you turn the reverb up.

Ken Fox · Post by **Ken Fox** » 17 Mar 2008 3:06 pm

Sounds like time for some bench analysis and work.

Glad to do it if you want to send the chassis down to Georgia.

Let me know!

Clyde Mattocks · Post by **Clyde Mattocks** » 17 Mar 2008 3:44 pm

Here is a hum situation I ran into acouple of weeks ago on a Princeton Reverb. I noticed if I touched the transformer, the hum disappeared , so my first thought was loose laminations. Upon pulling the chassis, I discovered a ground strap fastened to one one of the transformer mounting screws. I took the screw out, sanded the spot on the chassis and tightened it back down. Hum gone. This may or may not be applicable to your problem, but lots of times,
it is these type of non textbook scenarios that can drive you crazy.

Brian Davis · Post by **Brian Davis** » 19 Mar 2008 8:41 am

hey thanks guys. I need to turn this around in kind of a hurry, so mailing out the chassis is not really an option this time. I dropped it off to a shop yesterday and while testing the reverb it was acting a bit differently than at my apartment. It seemed to only cause trouble when the tank was plugged in, and the nasty hum I heard changed into more of a pure oscillation and only when the reverb was cranked. Perhaps the proximity to those transit power lines at my apartment was having an exacerbating effect on the cause(s) of the oscillation.

Eric Jaeger · Post by **Eric Jaeger** » 19 Mar 2008 8:45 am

I've used The Amp Lab in Redwood City (around the corner from Gelb Music).

-eric

John Billings · Post by **John Billings** » 21 Mar 2008 11:23 am

When my tech hot-rodded my Princeton Reverb, he did the same thing he does to every PR that comes into his shop. He re-routed a wire. Might have been one that went to the pot. Can't really remember. gets rid of the hum. All I recall is that, with the chassis opened, the pots at the far side away from you, it's a wire at the far right end of the amp. I've seen him do it on other Fenders too.