Squeeling PV Century

[drewl]

Any tips for taming a PV Century B series preamp that is squeeling with all the controls maxed?
I know, I know, turn it down some but these kids crank all the controls.

Turn down the gain, saturation or highs a bit and it's fine.
It's especially bad when in the cabinet closer to the power amp.

Are these supposed to have grip washers on the pots and jacks for better chassis grounding?

This one doesn't, thinking that may help as other wise it plays and sounds great.

[Enzo]

First, does it do this with NOTHING plugged into the input jacks? If it only does it with a guitar, turn the guitar volume control to zero, does it still do it?

So are both 15v rails even? Scope the rails when it does this, is there oscillation on the rails? Does it ONLY do this if the speaker is nearby? Bang on the chassis, is it at all microphonic? Run a signal into the power amp in, does bypassing the preamp still do it? Run a signal into the input and feed the preamp out to some other amp. Disconnect our speaker. Now does the preamp still do this coming out the other amp? You have three op amps, I bet they are in sockets. I know you have a spare one somewhere, sub it in each spot to see if one is wacky.

[drewl]

Well if it was something easy I wouldn't be asking, but that doesn't mean it wasn't something stupid

Originally it would oscillate with nothing plugged in.
Oscillation on the rails, fine when plugged into the power amp in jack.

My test speaker is across the room about ten feet away.
No noise when banging on amp.

It seems the power amp had some bad PC board connections and a flaky connector to the preamp.

Everything aimed with no input - fine now
Everything aimed, guitar plugged in with volume turned down- fine now


Everything dimed, guitar cranked, some squeel before normal feedback- normal enough.
Only does this on high gain input.

[Jazz P Bass]

Are we down to 'single coil pickup' feedback?

[drewl]

No, humbuckers.

This and a PV MKIV were stumping me this week, but they're both sorted out now.

The MKIV was losing signal at the power amp quad op amp

Power supply problem.
Rule #1: Always check power supply first!
I say this only to drum it into my head again!

[Jazz P Bass]

"Power supply problem.
Rule #1: Always check power supply first!
I say this only to drum it into my head again! "

I am saying it again to drum it into my head.

[TimmyP1955]

Are the supply lines to the ICs bypassed near the ICs? If not, you might add some. Also check that the caps in the feedback loops are still good.

[drewl]

The 15 volt supplies were good, but the 2k resistor for the feed was open, so opamp supplies looked fine so I thought the others were too.
Rookie mistake.


Rule #1: Always check power supplies first.