will
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Scott,
I don't know the ZP3. But my Torii IV has OA3s as a regulator/filters for the power tubes, and OC2s for the input tubes. After a few years of a lot of tube rolling, on one channel the OC2 would flash on and off, mainly at startup and shut down if I recall correctly. But this was not normal for the Torii.
Just after turning off the amp power, if I pulled the input tube on that side, the OC2 would stop flashing. Steve suggested it might be a loose input tube/socket connection. I don't think it was, as I carefully tightened the socket pin grabbers to no benefit. And later, it was solved after I did some amp modifications...I can't remember exactly what, but probably something in the power supply.
But this makes me wonder about yours being a loose tube connection, though not necessarily the OA3.
The ZP3 writeup says "OA3 tube regulation for entire circuit." Speculation, not being able to look inside, but I am guessing, this is referring to the entire signal circuit, and not the rectifier.
Since your pre was acting normally at first, the OA3 strobing only on startup and shut dow, and sounds good...I wonder if you had cleaned up the 12AX7 and 12AU7 tube pins too, and checked for firm socket connections?
I suppose it may be that you are getting oscillation from a slightly loose/weak connection in the circuit the OA3 is feeding. Other than your rectifier, your tubes probably run pretty cool. You may be able diagnose a poor socket connection by very gently grabbing your small signal tubes with a cotton glove or rag, and while holding it vertically, very gently put tension on the connections by turning each tube just a little in its socket? If the flickering stops with one or another tube tighter, that might show that a tube socket adjustment would help.
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