PI is suddenly red plating - HELP!!

[greg]

Hello

I have a little 15 watt amp using a Matchless DC30 preamp which has been working well for a while - until last night. I decided to put a 12AT7 in the PI, (which was untested) to see if I could clean it up a bit.

And then, I powered it up and forgot to hook up my speaker for about 5 minutes. Oops...

Once I plugged the speaker in and started playing, I noticed A) a large decrease in headroom, and B) the sound was sort of fading in and out a bit - not on and off, but definite volume fluctuations.

I then put the 12ax7 back in the PI (noticing that the 12AT7 was VERY hot), and this seems to have resolved the volume fluctuation thing, but it still flat out does not sound right. Distorting more than it did previously - in a bad way - is the primary symptom.

I also then noticed that the 12ax7 in the PI was red plating.

I don't see anything in the way of visual inspection. Nothing smells or looks burned.

Can anyone help me with where to start my troubleshooting without doing more damage??

Thanks in advance.

Greg

[Alex/Tubewonder]

What PI configuration do you have in the power amp section? Long tail, split load, or what? A schematic will make it much easier, without it there's no sense in divining - can you post it?

[greg]

It is a standard long tailed-pair. Sorry, but I don't know how to post a schematic (nor can I find a full schematic), but here is a DC-30 Power amp:

http://www.freeinfosociety.com/elect...chlessdc30.pdf

Mine is just missing two power tubes and likely has some different coupling cap values, as it is a 15 watter.

Here is a layout of the full DC30.

http://www.ceriatone.com/images/layo...0Ceriatone.jpg

Thanks!
Greg

[Alex/Tubewonder]

Hm, red plating 12AX7. Both plates or just one? There's too much plate current flowing, check that 1k2 resistor in the PI to begin with. If it's shorted it would cause excessive plate current.

[greg]

Well, the plate resistors on the PI are fine. I put in some old crappy tubes and powered it up - no more red plating. I then reinserted the original set of tubes, still looked fine, and played with it cranked for about 30 minutes with no problems.

So this is all good, but I still don't know what caused the problem originally. Is it feasible that when I had it unplugged from the speaker load for 5 minutes or so, that it overheated the OT, stressed the power section and PI and simply needed to cool off??

Thanks,
Greg

[Steve Conner]

It sounds like one of the plate resistors in your PI was shorting out intermittently. The 12AT7 draws enough extra current that it could have overheated the plate resistors and made one of them unsolder itself and move to short out on something it shouldn't. A shorted plate resistor would make one plate go red and also cut off drive to one of your power tubes, which would explain the bad tone.

[greg]

That makes sense to me. As always, I learn the most when something breaks - with the help of this excellent website and the folks who are always willing to lend their expertise.

It is appreciated.

Greg