6505+ Fault Microphonic squealing / whistling, but stops when a pedal is plugged into front end!

[GrantsV]

Hi,
I have a 6505+ amp and the Crunch Channel only whistles and howls like an old kettle. The pitch of the whistle changes as the gain knob is altered across its range.

I have swapped out all 6 preamp tubes.
Master volume does not affect the problem.
Does it with all guitars.
Problem only occurs on the Crunch channel, Lead works fine.
Is louder with Bright Switch engaged.
Only occurs when guitar is plugged directly into the input of the amp

This is the weird part, when I put a tubescreamer between the amp and guitar, the whistle is gone. Regardless of whether the tubescreamer is engaged or not!
Its almost like the pedal being put before the input acts as a passive buffer, preventing the howl.

Any advice would be much appreciated. Thanks

[Helmholtz]

This is the weird part, when I put a tubescreamer between the amp and guitar, the whistle is gone. Regardless of whether the tubescreamer is engaged or not!
Its almost like the pedal being put before the input acts as a passive buffer, preventing the howl.

A tubescreamer has an active bypass and always acts as an active buffer (emitter follower output) regardless if engaged or not.

Seems the whistle depends on amp input termination impedance. Does the amp work as it should with the TS?
Does the whistle also stop with guitar volume at zero (without TS)?

Please post schematic.

[GrantsV]

Hi, the high pitched whistle quietens when I drop the guitars volume, until at about guitar vol 6 it stops altogether.
The amp works absolutely fine with the TS in the chain, whether the TS is active or not!???

As soon as a guitar is plugged directly into the front input, and guitars volume is above 7, it whistles. Doesnt matter what the master vol is set to (preamp fault only).
The other channel (LEAD) works perfectly.

All new preamp tubes, any guitar I try.

Thanks for asking.

[Mick Bailey]

Peavey refers to the channels as rhythm and lead. Crunch is an additional gain boost function of the rhythm channel, not a channel in its own right. Does the fault occur on the rhythm channel without the crunch switch engaged?

[GrantsV]

Hi, yes it only occurs when the Crunch button is engaged. Its get even worst with the Crunch and Bright both engaged.
When I turn the gain knob on the preamp the whistle pitch changes like a synths analog filter. But soon as I insert a pedal before the input, engaged or not, the amp is completely normal!

The Cleaner sound on the Rhythm, and also the Lead channel are absolutely fine.

[Helmholtz]

Master volume does not affect the problem.

But changes volume?

any guitar I try

I guess it also whistles when you insert just a plug or guitar cable without a guitar?


Need schematic to see how the crunch switch is wired.

[Mick Bailey]

This should match. Also, do any of the lead channel controls have any influence on the problem? They should be inactive of course when the rhythm channel is selected.

Peavey-6505PLUS-112-Schematic.pdf

[GrantsV]

Hi, thanks for your replies.
The master vol works fine, its purely the preamp section on the channel.
It doesn't whistle unless a guitar (any) is plugged in, and also the guitars volume must be turned up past half way (e.g. high frequencies present at input)

I think it might be input impedance / input resistance problem on the channel.
Impedance fault is the only thing I can think of that explains why its ok once a pedal is put between the guitar and amp. I guess its an RMA.

Thanks again everyone.

[Helmholtz]

I think it might be input impedance / input resistance problem on the channel.
Impedance fault is the only thing I can think of that explains why its ok once a pedal is put between the guitar and amp.

Yes that's why I thought that an open input would also cause the whistling.

Might be a bad relay contact. Please verify that S6 pin 5 is grounded in crunch mode.