Weird feedback---Crate

[Valvehead]

Im working on a Crate Blue Voodoo tube head. The amp has a crazy and loud high freq. oscillation / feedback. It does this even with nothing in the input.
Ive swapped out ALL tubes...I was assuming it was a microphonic tube.

The more gain and volume i add , the louder it gets. Tapping on the chassis can set it off.

The jack and everything is pcb mounted, so theres no shielded wiring that could be bad.... so any ideas what would cause this ?? Ill be pulling the pcb out to inspect.

Bad ground ?

also--if i pull V1 out----no feedback...so its in the first 2 stages, and does it on BOTH channels.

i will try to attach a video.

Crate Video by solrizzo | Photobucket

[Valvehead]

computer probs...trying to upload video

no luck...anyway---the oscillation is about at 6.3k to 8k...sounds like a microphonic pre tube.

[Enzo]

Well, it couldn;t hurt to tell us what model amp it is.

If pulling the first stage tube stops it, scope the B+ node for that stage and see if the oscillation signal is present there.

[Valvehead]

No scope here...

its a blue voodoo , 120 watt , model BV120

[The Dude]

Does the amp still pass signal, or do you get only the oscillation? I'd start by measuring DC voltages on V1. It's quick, easy, and often telling.

[Valvehead]

yes--amp works fine--sounds good--just has the squeal.

V1 measurements :

PIn 1. 155v
2. 0.5mv
3. 2.3v
4.heater ok
5.heater ok
6. 168v
7. 3mv
8. 2.3v
9.heater ok

V2 looks good also

[Valvehead]

i have the pcb pulled out--everything looks great....

any suggestions ??

[g1]

Schematic here: http://music-electronics-forum.com/a...e_bv120_sm.pdf
V1A is before the volume control. When on clean channel does turning down the volume make the feedback stop?

[Valvehead]

yes

[g1]

Do the tone controls (clean ch. again) have any effect on the pitch or volume?
If it does, try connecting a jumper from the R1/R2 junction to ground.

[Valvehead]

ill have to put it back together to try that. Might do that tonight

[Valvehead]

ok....before i put it back together, i re-soldered 2 of the main ground points and ran a couple wires from the gain pot and cl vol pot to the main ground. The amp is working fine now---no oscillation !

Of course its still partially torn apart...so i will have to see if its ok when together.

So.. maybe that ground was dodgy ???

[Valvehead]

And sure enough....... I have a new noise. random loud static/light popping sound . This goes away if i pull V1. I tried new tubes.

ill try to attach a video of it.


Noise Video by solrizzo | Photobucket

[g1]

Have you resoldered the socket for V1?

[Valvehead]

if i didnt, i inspected it.... but, i can pull the pcb again and re flow