Pre amp tube issues

[Twitch]

Ok, so Im reasonably sure the following is a pre amp issue but want a second oppinion:
My X100B has a random crackling static noise thats only apparent when cranked at loud volume on the lead channel. If my memory serves me correctly this would be due to a failing 12ax7. Ive tested for microphonics another tell tale sign of failing preamp tubes and found the V1 position to be far more microphonic than V2(all tubes are microphonic if you tap them hard enough), however, could this V1 microphonics issue be due to it being the primary gainstages for the amp i.e. since V1 is being run hotter than V2 could this also explain the accentuated microphinics in V1 rather than a failing tube?

I know I could just troubleshoot this with swapping known good for suspected bad, but I want a 2nd opinion before I waste an hour trouble shooting.

[dannyr]

I agree it sounds like a tube issue. Try this...Turn the gain all the way up on your lead channel, then slowly turn your master volume down. If the noise persists after the master is down, it's an output tube problem. If noise goes away after turning master down, it's a bad preamp tube. Hope that helps!

[DonP]

Swapping tubes is an easy method, and probably should be your first.

My gut tells me it's shot noise from carbon-comp resistors however. Preamp tubes are under a small load and don't go out very often.

[Twitch]

Swapping tubes is an easy method, and probably should be your first.

My gut tells me it's shot noise from carbon-comp resistors however. Preamp tubes are under a small load and don't go out very often.

Interesting theory, never heard of this, could you elaborate?

[DonP]

Interesting theory, never heard of this, could you elaborate?

If your amp is using carbon comp plate resistors in the preamp, these cause shot noise.

I'm not sure what more I need to elaborate on?

[Twitch]

That worked(the elaboration). I didnt know what resistors you were refering to. Well if it is plate resistors on my 12ax7s then that wont be a tough fix, hell for s&g I may just up the values of them like I did on my 900s.

[Cygnus X1]

What year is your X100B?
And it takes an hour to swap preamp tubes on it?

[Twitch]

What year is your X100B?
And it takes an hour to swap preamp tubes on it?

Its one of the reissues, and not to swap them, but to dig out the old tube tester and set it up then test all the 12ax7s then decide which ones I want to put in which position. Im too thorough when it comes to pre amp tubes.

[Cygnus X1]

Its one of the reissues, and not to swap them, but to dig out the old tube tester and set it up then test all the 12ax7s then decide which ones I want to put in which position. Im too thorough when it comes to pre amp tubes.

OK.
I test mine, label them, and stick them in a drawer.



(Hickok 6000 tester).
I'm thinking that while DonP is correct in what he wrote,is it very likey the RI would have carbon comps in it?
More likely metal film.
I'd be interested in a gut shot if you do open 'er up.
Thanks.

[Twitch]

OK.
I test mine, label them, and stick them in a drawer.



(Hickok 6000 tester).
I'm thinking that while DonP is correct in what he wrote,is it very likey the RI would have carbon comps in it?
More likely metal film.
I'd be interested in a gut shot if you do open 'er up.
Thanks.

Im not sure what my tester is. Its ancient, like 60s 70s era, wooden case, two scrolls on the bottom with test voltages, amp and other test specs. If I have to tear it open Ill try to remember to take pics.

[DonP]

OK.
I test mine, label them, and stick them in a drawer.



(Hickok 6000 tester).
I'm thinking that while DonP is correct in what he wrote,is it very likey the RI would have carbon comps in it?
More likely metal film.
I'd be interested in a gut shot if you do open 'er up.
Thanks.

No, it's very unlikely to have carbon comps. The OP didn't mention it was a newer one in the original post.

[Cygnus X1]

No, it's very unlikely to have carbon comps. The OP didn't mention it was a newer one in the original post.

Or just to confuse matters even more...carbon film.
Which could cause just what you described.
Not as likely as composition but possible.

[Twitch]

Well I finally got around to fiddling around with this issue and it turns out that all 3 of the 12ax7s in this amp had that static anomaly, so out of 7 tubes I had in the house, only 3 were good. Which 3 were they you ask?? 3 of the 6 stock chinese 12ax7s that came out of the B52 AT100 I used to own. Anyway shes silent as a mouse now when the guitar volume knob is rolled down.

[Cygnus X1]

Chinese 12AX7's are underrated.
I shouldn't say anything.

I find plenty test fine, sound great in high gain applications,
and last forever.
But don't tell anyone.