5E3 low feedback/oscillation, stumped???

[Twangman]

I've got a 5E3 that goes into a loud low feedback/oscillation (no chord plugged in) when you tap on the chassis.

I have:

(1) Replaced both preamp tubes with good ones (12AY7 and 12Ax7)
(2) Replaced the 1 Meg resistors on the input jacks
(3) Replaced the 68K dropping/input resistors and wired them directly to the 12AY7 via shielded cable

STILL GOES INTO OSCILLATION!!!!!!!!!!!!!!??????????

I HAVE NOTICED THAT WHEN I HOLD ONTO AND PUSH/PUT PRESSURE ON EITHER PREAMP TUBE, THE OSCILLATION STOPS. I know the tubes are good, so I'm really stumped. I'm not a full-time tech, but have tinkered with amps for about 10 years. I figured that there's probably some folks out there who can lead me in the right direction.

Any help would be GREATLY appreciated.

[Enzo]

Do any of the controls have ANY effect on this noise?

Put your chassis on the bench and connect a speaker to it. Fire it up. Use something insulated, like a chopstick, and gently probe each part and wire on it. When you tap the chassis, you send a mechanical shock through the entire system. Try to move things without doing that. Looking for maybe a wire with crummy solder, or a loose tube socket pin.

[Twangman]

I did the chopstick test, but didn't check for cold solder joints. When I push on ANYTHING, it goes into oscillation, so it's hard to isolate. I'll check each volume control independently. When I turn down the volume, it goes away, but I didn't do them indepently.

I try that and get back.

[marcfrom]

low frequencies oscillation problem in a 5E3

I have a low frequencies oscillation problem in a 5E3, even with unplugged inputs.

When i turn the position of the amp or when i move the clothes speaker cable it disappear.

Someone have an idea ?

Marc

[MWJB]

Any flourescent lights nearby?

Check integrity of all ground connections at input/output jacks, pots & circuit board connections to ground.

Is this a 5E3 that you have bought, or built? If the latter, please post pictures.

[marcfrom]

It's a 5E3 i built for a friend,

I'm sure about grounding because i use to build 5E3 and i ground it always on this same schematic to avoid any hum.
There is absolutely no Hum or something like that, but something like a static oscillation in very low frequencies, and it stop depending how i oriented the amp.

My friend take the amp yesterday because he have some gigs for this week and want to use it.

The speaker is wirered with 18AWG clothe wire.

So i thinked it's maybe caused by one of the tubes, or a AC wire pass over another wire, but i can't check it until he bring back the amp.

I would to know if somebody already has this problem, and give me an idea what i have to check.

Thanks guys

Marc