What do defective phono tubes sound like?

Why not replace the old arm and see what happens. If the problem (popping from the speakers?) remains, then the new arm is most likely not the problem.
But, if your back to vinyl heaven, then perhaps it's time to deal with the new arm.
Hang in there. Keep us posted.

Dear Sirseedy, very strange you post this problem now, because I have the SAME problem occuring recently, and we have quite similar systems ! I am also on the diagnosis pathway, and I never thought of a tube problem . I swapped
recently the 12AX7A NOS Telefunken for military grade Mullards , and wonder if they are the culprits.It is like a big electrical discharge occuring every 10 minutes, but only with my analog system . I borrowed a PS Audio Premier Power plant and proceeding by elimination, apparently it is the phonostage headamp who is picking up AC noise when my REFRIGERATOR switches on. When I unplug the frig this not occuring anymore. Hope this helps.

Another possible source is the unshieled proximal part of the tonearm cable, Purist Venustas in my case, hooked to Graham Phantom, being too close to phonostage and picking ground loops and discharging loud pops. When I separate the tonearmcable from phonopreamp the noise and pops disapear

SirSpeedy,

Any chance it's mistracking? (I know you'd have recognized that, but thought I'd cover all the bases.)

Diagnostic question: do these pops happen in both channels or just one?

I'd put a small wager on a loose connection at the cartridge pin(s). Check those carefully for tightness and clean the pins and clips. For cleaning gold, see the last paragraph.

We had a similar LP-only problem last winter, in the R channel only, which made diagnosis easier. Turned out to be slightly spread connectors in one pin holder of one tube socket in the phono stage. I tightened the two clips carefully with a fine tweezers and the problem was resolved. So when you pull your tubes check the connectors in each hole before reinstalling, and clean all the pins and pin holders of course.

For cleaning gold connectors Paul recommends (and we use) a very gentle rubbing with a paper towel. Nothing stronger or more expensive is needed, nor does it require any chemicals. Gold oxidizes in a layer that's basically one molecule thick and it only takes a gentle buffing to remove. Do NOT use a Magic Eraser! ;-)

Good luck,
Doug

What is suggestesd to use to clean tubes and sockets?