Princeton Reverb kit Screen Grid Problem

[rdavy]

Hello all,

I just built a Mojotone PR kit for a friend with small mods (no tremolo, added mid control and pre-phase inverter master volume) and everything tested fine until I put the chassis into the cabinet for a final run-through. I turned the amp on and it started to smoke. I opened it up and the first dropping resistor (1K, 5watt) in the B+ was burnt.
This led me to think something was running the B+ to ground. Turns out, it's one of the 6V6 power tubes screen grid pin 4.
I pulled the tube out and I no longer had continuity to ground. I put the tube back in, still no continuity to ground. As a side note, I put 500 ohm, 5 watt screen grid resistors on the power tubes.
I replaced the burnt 1K resistor and tested all B+ points for continuity to ground after and got no continuity. I also tested the power tube pins out of the amp for any continuity and I got nothing with the exception of the filament pins 2 and 7. Still, the new 1K resistor started to smoke. They supplied me with "Mojotone Electronics" branded power tubes, which I'd never seen before. Is this just a case of a bad 6V6?
Any help is greatly appreciated, thanks!

[xtian]

I pulled the tube out and I no longer had continuity to ground.

Confusing. Continuity between WHAT and ground?

At this point, pull out power tubes and see how amp is running at idle. Measure B+ and bias (pin5) of both power sockets.

[rdavy]

Continuity between pin 4 and ground was what I was referring to.
With the power tubes out, my B+ off the rectifier tube is 415vdc.

V6 - 6V6
pin 3 413vdc
pin 4 408vdc
pin 5 -24.5vdc

V5 - 6V6 (the tube in question)
pin 3 413vdc
pin 4 408vdc
pin 5 -28.9vdc

[xtian]

I would expect the same bias voltage on both pins 5. That's a little weird.

Is your cabinet shielded? I wonder if you shorted something when you put your chassis in the cabinet.

[rdavy]

I would expect the same bias voltage on both pins 5. That's a little weird.

Is your cabinet shielded? I wonder if you shorted something when you put your chassis in the cabinet.

I thought so too, one of the 1K5 grid resistors I put in must not be in spec from the other (will investigate). The cabinet is not sheilded and it burned a new 1K dropping resistor when I tested it out of the cabinet.

[xtian]

After you get the bias voltages figured, assuming voltages are stable with NO power tubes, I'd try a different set of power tubes.

[rdavy]

Will do and will update with results. Solved the bias issue. A 1.2meg resistor got sorted into my 1k5 bin and I missed the color code when installing it on the tube socket. Replaced it with the correct resistor, dumb mistake on my part.

[rdavy]

Well, new JJs made all the difference. Amp works as it should. It's frustrating because the tubes they supplied the kit with were "NEW" tubes. Is there a chance that when the tube heated up, the screen grid connected to the suppressor grid and that's what grounded it? Either way, thank you xtian for all your help, I really appreciate it!

[xtian]

I have seen a hand full of failures where DC arcs from plate or screen grid to heaters, which are close to ground, so an easy target for internal tube arcing. And this only happens when tubes warm up and start conducting.