SF Vibrolux tube question

[steadygarcia]

Finally got around to replacing the speakers in my SF Vibrolux (replaced the pair of 16 ohm Celestion G1050's with pair of 8-ohm Jensen P10R re-issues). Turned it on and the whole amp had gone what I believe is called microphonic. I would describe it as a ringing-feedback-ish sound. Any metal I touched on the amp would "ring". Flipping the standby switch back and forth was accompanied with a loud-ish "ping". There was a low feedback sound, that continued even after I turned the volume on my guitar all the way down. I tapped the tubes and yes, they would ring, especially the first two pre-amp tubes, which are listed on the tube diagram as 7025's. I switched off the amp and took them out to find that one was a 12AX7 (which I know is interchangeable with a 7025), but the other was a 12AT7, which I don't believe is the same as a 7025, correct?

Anyway -- I had some work to do on my Strat to clean some dirty crackling pots and PU switch, which I did. Then I turned the amp back on to see if the pots and switch on the Strat were quiet (they were). Lo and behold, now the amp seemed fine. The microphonic sounds were gone.

But I'm still guessing I may have bad tubes. My question is, do I need to replace the 12AX7 and 12AT7 that are currently my first and second pre-amp tubes (the diagram lists these as 7025's) with a matched pair of 12AX7's? Or for that matter, a matched pair of 7025's? What damage, if any could having a 12AX7 and 12AT7 in those slots cause?

FYI - the tube diagram for a SF Vibrolux is as follows:

7025 -- 7025 -- 12AT7 -- 7025 -- 12AX7A -- 12AT7 -- 6L6 -- 6L6 -- 5UA

Thanks!

[mbratch]

You don't need to match pairs of 12AX7's or 7025's.

As far as what to replace, it's very hard to tell without diagnosing the amp further. You should always have at least one spare 12AX7 or 7025 around (they're the same tube really), and a spare matched pair of 6L6's. Also a spare 12AT7 doesn't hurt. Basically something of each.

The rectifier is a 5U4, not a 5UA ;)

If you have a spare 12AX7 and 12AT7, you can systematically change one at a time to see if one of the tubes is the problem. If the problem only occurs on one of the channels, then it's V1 or V2. If the problem only occurs when reverb is on, then it might be V3 or V4. If you have tremolo (vibrato) issues, it's V5.

[Blue Strat]

See if either volume control affects the microphonic behavior. Also the reverb control.

This will help narrow down the problem a bit.

[steadygarcia]

Thanks, It's good to know I don't need a matching pair. I'm thinking of picking up Tung Sol RI 12AX7's from KCA. But I believe I have spares kicking around that I can experiment with to focus in on the culprit.