Why is V1 slighlty Microphonic?

[Jerry2013]

Was playing my Marshall JMP 100 watt head early this morning ( night owl- about to go to bed), and no matter what tube I put in V1 ( farthest to left?) when I tap it with a pencil, its slightly microphonic , a lot moreso than the other pre-amp tubes. All of the power tubes are okay. IS there something wrong with the amp? It will squeal at real high volumes ( and I do like to crank it at times), or if I get to close.
BTW, this Marshall absolutely smokes ( maybe I shouldn't say that on this forum lol). I settled on a 7025 Phillips I had- really breathes life into the beast.

[eddie25]

It's probably just because that tube gets the most amplified from the amp since it goes through every other stage. The other tubes sound less microphonic because they only go through the stages further down the line. Hence why people always look for low microphonics on V1 because it will get amplified the most. Hope that makes sense.

[Structo]

In my experience, I think most preamp tubes will react to tapping with a pencil.

The true test is how it performs when the amp is turned up.
If you hear ringing or other noise when you play, then yes the tube isn't the best for that location.

And since a 12ax7 has a amplification factor of 100, everything gets boosted.

[Bob S]

If you think V1 is touchy in a Marshall 4 holer...
Get you an Express.
I still twitch when I think about V1 in a an Express.
Roll tubes & try at high volume.
One will work, even on 11.
Be Lucky,
Bob
Oh & don't tap it with a pencil.

[cbass]

V1 in the express makes a great tube tester

Don't tap your tubes with a pencil

[Reeltarded]

Yeah. I use tiny tube hammers I got from a hollow elf in a cookie tree.