5E3 first stage hum

[MikeT]

This 5E3 that I'm building from parts, not a kit, has crazy hum in the preamp. It only happens with the volume turned up and goes away with the volume down. The amp does function but the hum makes it un-useable. I pulled the first preamp tube and the hum went away so I changed the tube but the hum was still there. Then I double checked all the wiring, filamenmts, b+, signal, and cathode grounds and they were all good. The lead to the grids is shielded and grounded only at the jack end, not the tube end. Pins 2 and 7 are strapped and isolated with resisters so both volume controls work on a single input jack. I did separate them to check for no hum but that was not it as it still had the crazy loud hum.. Any suggestions would be great. Thanks.....

Mike

[echuta13]

Maybe a ground loop?

[dkevin]

First stage hum

I agree with the last post. I just finished a Gibson GA-17RVT Scout rebuild and I had a hum that increased with the amp volume too. I tried splicing in electrolytics and cathode bypass caps and just about every other possible source of the hum. Then I started looking closer at the amp grounds and discovered I had created a ground loop by adding a 3 wire power cord and grounding it at a power transformer lug. The rest of the amp used an unused pin on the rectifier tube to ground the power transformer. The only place where the circuit was actually grounded on the chassis was at the input jacks. When I lifted the ground at the PT and joined it to the other grounds at the rectifier pin, the amp settled down and behaved itself.
In my experience, it's usually something silly like this that is the cause of hum problems. I should have recognized that when I began to question the health of the brand new electrolytic capacitors I had just installed.

[passfan]

This 5E3 that I'm building from parts, not a kit, has crazy hum in the preamp. It only happens with the volume turned up and goes away with the volume down. The amp does function but the hum makes it un-useable. I pulled the first preamp tube and the hum went away so I changed the tube but the hum was still there. Then I double checked all the wiring, filamenmts, b+, signal, and cathode grounds and they were all good. The lead to the grids is shielded and grounded only at the jack end, not the tube end. Pins 2 and 7 are strapped and isolated with resisters so both volume controls work on a single input jack. I did separate them to check for no hum but that was not it as it still had the crazy loud hum.. Any suggestions would be great. Thanks.....

Mike

Check that you have it grounded just like the Fender deluxe schematic. I had the same problem and you can find a thread on it here: 5E3 Proluxe problems - The Fender Forum

Good luck

[loudthud]

The cathode resistor(s) for the first preamp tube need to be connected to ground at the input jacks. Also, the B+ filter cap for the preamp needs to be grounded there.

[MikeT]

I found a real dumb thing that took care of the real nasty hum although what remains when the volume is turned up is still unnacceptable. The solder joint on the cap to ground on the tone pot was broken...sorry guys, but I think anyone who has been doing this for awhile can relate to most problems being the absolutely dumbest thing.
I have everything except the ground leg of the AC supply grounded to a lug on the PT on the back side. The ground leg of the AC is grounded to the front nut of the OT. Could be a ground loop as per echuta13. Lots of good info on your thread, passfan. And loudthud's idea sounds worth a try. Maybe I'm trying too hard and should just put it back to original 5E3 specs....All you guys are real helpful... I'll keep you posted...thanks again...

Mike

[echuta13]

I feel that it's part of the learning process to some degree. I think you need to figure that you'll make mistakes, but as long as you learn from them (and don't kill yourself in the process...), you're moving forward! Congrats Mike.

[MikeT]

The cathode resistor(s) for the first preamp tube need to be connected to ground at the input jacks. Also, the B+ filter cap for the preamp needs to be grounded there.

I grounded the first stage cathode to the input jack and now the amp is virtually silent!!! Thanks loudthud......!