Deluxe 5E3 HF oscillations

[CB Amp]

Hello,

I've got a problem on a 5E3 (Ceriatone kit) : when I plug a guitar in the Normal input, I can't turn the Bright volume more than 3 quarters (with the Normal volume and tone from 3 to 10).
The amp begins to make a very high sound with the Bright volume at 8/9. Not really a feedback, but someting like oscillations, as if there was too much gain.

When I plug in the Bright channel, I can turn all the pots full clockwise, nothing wrong happens.
All the tubes, circuitry and components are ok.
Have you got an idea how to fix it ?...

Thank you.

[Darkbluemurder]

Sounds indeed like an oscillation. Check the layout, especially whether a grid wire is too close to a plate wire which is in phase with the grid wire.

In his third book Gerald Weber describes oscillation problems with the 5E3. I do not have experience with that circuit but it is interesting reading.

[rfgordon]

One of the problems with a lot of amps, particularly Fenders, is the long grid wires. Basically, once the signal that comes off the plate passes thru the coupling cap, it's in a grid wire. Grid wires should be as short as possible. Here are two approaches you can take with this amp (or any other).

1. Replace the plain wires that connect the coupling caps to the tone/vol pots with shielded wire, tying all the shields together at the pot end and grounding them in one spot on the bus. Ground only one end of the wires! To test to see if this would work, carefully wrap the wires, one at a time, if you like, with aluminum foil, then ground it with a clip lead from the foil to ground. BTW, this is a neat trick to chase down hum anyway.

2. Take the coupling caps off the board and solder them to the tabs on their respective caps. That way, the wires that run from the plates to them are plate wires, and the HT on them rejects signal and prevents parasitic oscillations.

Good luck.

[Darkbluemurder]

Take the coupling caps off the board and solder them to the tabs on their respective caps. That way, the wires that run from the plates to them are plate wires, and the HT on them rejects signal and prevents parasitic oscillations.

Good luck.

Good point. I guess you meant to say "... tabs on their respective pots."
This is also addressed in the Gerald Weber book. I did this to both my modded Bassman and Bandmaster. The Bandmaster got less hum that way. The only slight concern I have with this is whether such arrangement would hold up in an amp that is on the road and gets pushed around a lot. But then again vibration and rattles would probably kill tubes first.

Also important to combat oscillations are the grid wires to the power tubes. They should be as short as possible and as far away as possible from the negative feedback wire and the OT primary wires.

[CB Amp]

Thanks for your help !
You are right : the grid wires to the 6V6 were faulty.
I will try the other improvements you propose to reduce hum.
Thanks again

[rfgordon]

Oops, yeah, "their respective tabs" is what I meant to say.

As for doing the mod in an amp that gets knocked around, That can be a concern, depending on the physical layout. I've done Gerald's mod on Blackface amps, and used Orange Drops, silicon gluing them upside down to the floor of the chassis next to the pots. That way they ain't moving.

[delux1959]

I have had lots of luck with my Hoffman layout 5e3. Take a lookhttp://www.el34world.com/