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[Ayles]

Just need to verify my thinking. I have an amp with 4x 12au7, 4x kt88, pre has 6x 12ax7. Have some noise that doesnt change with the volume. I found that two of the 12au7s in the amp are very microphonic.

I assume thats the first set replace?

[908/930]

How old is the amp? could also be a cap getting old, possibly in the power supply side.
Is the noise in both channels?

[Ayles]

Yeah, its more of a white noise. But there is one funky tube in each channel as it sits now.

Amp is probably no more than 10 yrs old.

[Gogar]

Swap positions until you have 2 "funky tubes" in s single channel and determine if the noise moved to "only" that channel.

In addition just "exercise" the sockets a little bit, could just be a little dirty.

[908/930]

Did the noise just start all of a sudden, any other changes in equipment?
Are those tubes original or have they been rolled? VAC amplifier?

[Eric Coffey]

Originally Posted by Gogar

Swap positions until you have 2 "funky tubes" in s single channel and determine if the noise moved to "only" that channel.

In addition just "exercise" the sockets a little bit, could just be a little dirty.

This.

You can also isolate the worst offenders by lightly tapping the tubes with a soft object while the amp is on, but no music playing. Start with the volume really low.

[Ayles]

No changes to the system. I have swapped a set of tubes into the pre and the noise persisted. I just dont have any spare 12au7 or kt88 laying around. The 12au7s are definitely microfonic, just did the test above with a pencil. A couple were more so than the others. I switched from triode to pentode and it intensified as well. The kt88s were not microfonic.

With music playing its not terrible and pretty happy with how it sounds. But its apparent during quiter passages and seems like the noise has intensified over time.

[Gogar]

Maybe it's time for some new au7s and ax7s. Not expensive.

[Steve Carlton]

(lighting cigarette) Good tubes. Certainly not great tubes.

[Ayles]

Thanks for the input

I'll probably start with a new set of 12au7, the manual for my preamp states that two of the six need to be low noise. So I'll have to do some digging as it seems off the shelf tubes won't do.

Reading up on this stuff can send you into a rabbit hole of seemingly endless variables.

[Gogar]

It's mostly hot air. Buy some regular priced au7s and put them in there and see if that's your issue.

iMO

[beepbeep]

All tube amps have inherent thermal noise. It is perfectly normal...signal/noise ratio is seldom better than 80dB, depending on design.

Troubleshoot this way:
Disconnect everything but speakers from the amp, short the input connectors. Noise still there? If it is white noise (and not 60Hz hum), and roughly same level on both channels then I would say there is nothing wrong except that tubes are maybe getting old. Record the noise and upload it here otherwise.

First stage tubes are always microphonic...it is the nature, they are amplifying very weak signal, thus very sensitive for vibration.

[Ayles]

Hi there, thanks for that test idea.

Can you go into a little more detail on shorting the input connectors? I am not familiar with how that would be done.