Bad Preamp Tube?

[splawnboy]

I was playing my Competition in 2nd gear with the OD switch on, and suddenly experienced a major loss of gain and volume. Both power tubes and the 3 preamp tubes are still glowing, but my high-gain sound is lost. I noticed that the OD switch no longer seems to have any effect.

(1) Does this sound like a preamp tube going bad?
(2) What type of preamp tubes are recommended?
(3) Does anyone know which of the 3 tubes corresponds to which gear and OD (when viewed from the back).

Thanks again.

[splatter]

sounds like a power tube not a preamp tube . Just because the tube still glows doesn't mean its good .

[splawnboy]

Thanks for the quick response. I just did the pencil test by gently tapping the power tubes and one is microphonic. I'm taking it to an amp repair place and see what they find. Thanks again.

[splawnboy]

The roar of my competition is back! I took my amp to King Amplification in San Jose, California for a checkup. Although it was Saturday, Val King was working and examined my Splawn on the spot.

He cleaned the tube contacts, check each tube on his tester, replaced my marginal pre-amp tubes with factory JJ's, and re-biased my power tubes a little hotter. I was glad to see an experienced amplifier repair shop in the San Francisco Bay area. Val King is an experienced professional and quite courteous. He definitely knows his business and has a great setup with dummy speaker loads, signal generators, and oscilloscopes to diagnose issues. He even sells his own line of handmade style amplifiers with a Fender Blackface vibe.

I brought my Competition back home an hour later and it rocks once again.