Express Whistling

[populartsl]

I'm getting a high pitched whistling sound almost unbearable. Even when i have no guitar lead plugged in. It gets even worse when i adjust the presence knob.

Any ideas of the problem chaps

Ben

[stoo]

Did this just start on it's own or is this a new build?
stew

[UR12]

Ben

You might want to try a different tube in the preamp section. Sometimes a bad tube can cause this. It sounds like you are experiencing an oscillation and if it turns out not to be a preamp tube then you need to look at your wiring to make sure something isn't running to close to something else. Use a wooden stick (chopstick or skewer) to move the wires around to see if the oscillation changes.

[loverocker]

It gets even worse when i adjust the presence knob.

Any ideas of the problem chaps

Ben

Reversed plate leads from the OT to the valve sockets? (you'd have positive feedback instead of negative feedback) Just temporarily disconnect the NFB lead to check.

[populartsl]

Ok i'll try all these things out and get back to ya

Thanks Guys

[populartsl]

Stoo its a new build

Ben

[stoo]

What loverocker says

[keithrick]

I would tend to think that it is wiring related. I had this problem as well albeit only when cranked.

As a suggestion try moving around the plate and grid wires on V1 (pins 1 and 2). My amp responded well by adjusting this area. If you still get occilations try using shielded cable from V1 grid to the Volume pot.

If the OT taps were reversed it would howl so loud that you would crap your pants.

[populartsl]

If i touch the chassis it stops. Do you think because its a high gain amp it could be sheilding problem.

[keithrick]

Could be caused by the speaker cab. Try moving the amp off the cab. Might solve your problem. I believe the "big Man" himself had said to do this or to use a foam pad to deaden vibrations.

Hope it works!

[populartsl]

Thanks for your suggestions i'll give a go and hopefully get it sorted

Thanks Ben

[CathodeBiased]

While grid/plate coupling is a large source of oscillation problems, one also needs to make sure that one does forget to look for plate/ground coupling as well.

[populartsl]

Tried a piece of aluminium plate to sheild components seems to have stopped the problem. Just building my cab now so fingers crossed it should be ok

[Zippy]

If i touch the chassis it stops. Do you think because its a high gain amp it could be sheilding problem.

Depends on whether you are damping vibration, changing capacitive coupling, or just providing a better ground.

[lastwinj]

check your grounds.

jeremy