Carvin PI problem

[drewl]

I have a Carvin Legacy 2x12, it seems that only one half of the phase inverter is conducting.

the top half of the PI is the "B" section of the tube, pin 6 has the full voltage on it as seen on the other side of it's plate resistor.
The odd thing is pins 7 & 8 have their normal voltage, the same as pins 2 & 3.

All the resistors in the PI circuit test good, any ideas of what to look for.
The socket is making good contact with the tube pins, everything else is normal and functioning properly.
I checked the output coupling cap on pin 6, I'm wondering if the reverb circuit is screwing it up somehow.
Thanks.


http://www.carvinmuseum.com/pdf/amps...gacy%20Amp.pdf

[Chuck H]

I don't see how the reverb could be screwing up the PI looking at the schem. Have you tried a different tube in the PI socket? It's clear that the input triode isn't drawing current. If it's not the circuit then it must be the tube.

P.S. What is the measured value of C69? I'm on another thread with a guy who used this schem as a plan for a rebuild and the value is simply indicated as "10" on the schem. If it were a polar cap I would expect a polarity indication. So I don't think it's 10uf. If it were 10pf the NFB loop wouldn't do much. So I'm wondering what the actual value is.

[drewl]

Of course I tried a couple different tubes.
C69 is a 10uf, it says NP (non-polarized).

While reading through some other posts I started jumpering across the little 1/4W resistors in the PI circuit, the gain goes up and the output signals are better balanced, but it's still not conducting properly.

The reason I was wondering about the reverb circuit is I wasn't sure if the schematic matches this amp exactly, and jumpering across the tank connections (tank is out of circuit, mounted in the cab) was making the PI conduct a little more, but that doesn't appear to be the problem.

I'll swap out the rest of the 1/4w resistors for half watt tomorrow and see if that helps, as well as replace the caps in the circuit.
There's only so much stuff there!

[Chuck H]

+1
on the flip, if it's not a tube problem, it's a circuit problem.

I've seen 1/2 watt resistors fail as the bias resistor in LTP PI's. I can't imagine a 1/4watt in this circuit.

Just to be clear, I interpreted that your reading the full supplied B+ on the input triode plate with no drop across the load resistor?

[drewl]

[QUOTE=Chuck H;214962
Just to be clear, I interpreted that your reading the full supplied B+ on the input triode plate with no drop across the load resistor?[/QUOTE]

Correct.
But usually the other pin voltages will be way off when the tube is not conducting, that's what's odd.

[drewl]

Bad 10uf.

Looks like the 10uf non-polarized feedback cap is bad.
Went home for lunch, pulled one end of the cap and the plate voltage dropped to what it should be, the other side went up to it's labelled 300v and the output is a nice clean sinewave.

I'm still going to replace the 1/8W resistors in the PI circuit with 1/4w ones to prevent further failures.

[Gtr_tech]

I'd go half watt....maybe even 1w on the PI plates....

[drewl]

DOH!
yes, it has 1/4 watters in there stock, gonna bump em up to 1/2w.