another PT issue

[gary sanders]

I have an old PT out of a Carvin x amp acting crazy.My bias current goes up and down when you turn the presence pot.Triple checked and all wiring is good/All my secondaries are showing continuity so that means its cooked,right?Heater,to B+tap,to center tap no matter what you check,its short.Any idea what could have caused this?

[FYL]

If the amp works somewhat the PT isn't toast. If bias wobbles when you play with the presence pot this could very well be a stability problem.

[gary sanders]

Thats what stumped me,it works and sounds fine til you mess with the knob and then stuff gets hot.Im not an expert especially on transformer science but I thought it was odd that any wire on the secondary side had continuity.

[sluckey]

but I thought it was odd that any wire on the secondary side had continuity.

That's very normal for a PT with a center tapped HT winding and a center tapped filament winding. With both center taps connected to ground, you will have continuity between ALL leads of those two secondary windings. You should not have zero ohms between the HT winding and the filament winding.

[gary sanders]

Well thats good news!I was afraid it was dead

[FYL]

Well thats good news!I was afraid it was dead

The PT isn't dead but the amp is sick.

[phsyconoodler]

Bias supply issues.Diodes,capacitors.If I were you I'd replace all those components.The reason I say ALL is because you likely have to remove the circuit board to do anything on the bias supply and missing something is a pain later.

[kriswel]

If it changes when moving the presence knob you might have a parasitic problem that is not audible. I have had my fair share of parasitic issues and have spent days on end trying to fix them, it's so frustrating. In fact you could probably search the forum and find 4 or 5 threads that I started trying to deal with the problems. Is this a stock amp? any mods, got pictures?

Gain mods can be bad for this because even if it doesn't cause the problem it might just be amplifying a problem that already existed.

Show us what you got. Pictures, layout, schematics...

[gary sanders]

Sure dont,after i got totally fed up of trying to find the problem it got gutted and i started the amp all over.It started when I re-arranged my filter caps for more chassis space and ended up redoing the phase inverter too, so that may be where it could have been but i looked a thousand times and couldnt find a thing wrong.I was worried about the PT because mainly because I was worried my bias tap was bad and checked the secondaries and got continuity but since thats ok,i will use it.In the process of replacing all the filter caps and power supply components.

Parasitic?!?!?! What exactly does that mean and what causes it?Its not like a gremlin is it?

[FYL]

Parasitic?!?!?! What exactly does that mean and what causes it?Its not like a gremlin is it?

The amp is unstable, it can oscillate at high frequencies depending on the presence settings. Oscillations are ultrasonic, you don't hear them except if you're a dog or a bat but can see them on a scope.

 

[kriswel]

Google "parasitic oscillation" but if you are starting again from scratch first look up "good tube amp lead dress" lots of info out there but it doesn't make it any less frustrating when it happens to you. It's more fun to smash your head with a brick. I now know for the most part how to avoid these problems and or can find the reason pretty quick if it does happen but I have spent hours and days learning the hard way.