Preamp 12ax7 failure caused by a problem in circuit?

[six el six]

I had a Cameron 4 X el34 in the other day that the client said he had replaced the 2nd position 12ax7 twice in. Both tubes went bad (apparently) before he brought it to me. I checked voltages on all the tubes and even the bias in the power section, replaced the tube in the second position and everything was fine.

My question is what could possibly cause this other than bad tubes?

Is it possible he had just put in two tubes that couldn't handle the plate voltage?

He did have fancy labeled "boutique" tubes that I see a lot of high gain aficionados using.

It worked fine when I replaced the tube and sounded great for a fire breather amp so I left it alone.

Any thoughts would be greatly appreciated.

[12AX7]

One possibility is if there is a cathode follower at V2, there are at least 2 or 3 tubes on the market that can't handle the cathode voltage in that circuit. One is the new production tung sol. I can't remember the others, but one of those sovteks, i think the "LPS" and one other i can't recall. But they will burn right out sometimes in seconds. I lost at least one that way.

[terminalgs]

One possibility is if there is a cathode follower at V2, there are at least 2 or 3 tubes on the market that can't handle the cathode voltage in that circuit. One is the new production tung sol. I can't remember the others, but one of those sovteks, i think the "LPS" and one other i can't recall. But they will burn right out sometimes in seconds. I lost at least one that way.

+1 on that.   new Mullard 12ax7s  supposedly can't cut it in a CF.,,  like the second half of a top boost.

[FYL]

All Sovtek-sourced 12AX7s - Sovtek, Tung-Solski, Mullardski, etc. - suffer from the same problem: they'll fail if cathode to heater voltage is more than 100V or so. CFs, cathodynes and other stages with high cathode to heater voltages kill them.