NOS Blackburn Amperex/Mullard FX2 anomilies.

[Thermionic/EL-34]

I scored 2 matched pairs of Mullard Blackburn works Amperex labeled FX2's from a private seller. He had had them many years but never used them. Bought them from a tube vendor tested. They biased fine & test good, just comfirmed testing them today again but i have a intermittent strange noise that happens every once in a while the kind you {don't} want to hear in a tube amp.

So i pulled them & re-installed a matched quad of Tungsol EL-34B's that sound great.

My question is how can a tube test good but still be microphonic ?

Its NOT the amp, ive never had an issue with the amp.

I don't want to get stuck at a gig with bad tubes but i wish i could put them back in my JMP.

[xtian]

No tube tester I know tests for microphonics...

[Thermionic/EL-34]

I put the XF2 quad back in and biased the amp. I will play the amp tomorrow and see how it goes.

Not ready to give up yet.

[Thermionic/EL-34]

So amp sounded great this morning, can't really crank it but it really sounds good, no issues so far.

One of the Amperex EL-34's runs a little hotter than the other 3. I knew this from before so i biased the hotter one @ 37-38ma @ 450v {70% dissipation}, that puts the other 3 a little lower biased about 60% dissipation. Got the cold clipper thing going but the amp has much more clarity a tad colder.

IDK if the anomaly i heard before was dirty power or what or if the bias drifted as it was a little hotter before.
I do use a Furman Power Factor Pro and it works great.

I will find out at rehearsal later this week how it performs.

[Thermionic/EL-34]

After 2 weeks the amp is still doing fine. The issue i had was likey RFI coming thru the wireless.

Was not the Amperex/Mullard XF'2.