Can a faulty amp make a 6L6 become microphonic?

[markusw]

15 years ago I've built a Dumble style 50W head
https://ampgarage.com/forum/viewtopic.php?f=4&t=7506
In June 2024 I replaced the SED Winged C 6L6 I had in the amp with a pair of "Old Style 6L6WGC-STR BLACKPLATE small bottle TAD PREMIUM Matched" RT811-OS
https://www.tubeampdoctor.com/old-style ... um-matched since one of the two SED 6L6 became microphonic and up and then did some crackling noise when warming up.
I barely played the amp since June 2024 (maybe 48h in total) after I put in the TAD 6L6. It also was never moved from my living room.
Now I noticed that one of the two new 6L6 is slightly microphonic again. I hear a slight noise when I tap the tube with my finger no matter in which socket the tube is, much less then with the old Winged C though.
As far as I recall the tube was not microphonic when I put in into the amp in June 2024.
The other 6L6 is fine.
Might there be an issue with the tube socket or the components soldered to it that makes the tube become microphonic?
Both tubes run at 38 mA at ~450V, btw.

Best,
Markus

[Stevem]

Is this tube socket the closest to a / the speaker?

90% of the time vibrations and expansion and contraction rates makes them go microphonic.

To extend output tube life the amp should be warmed up in standby for 3 minutes ( especially if it was in cold car or room in winter) then the amp can be put into play mode.

When you go to turn the amp off and especially if you have been playing it cranked you should let the amp sit for two minutes even before you put it in standby.

Once in standby let it sit for 3 more minutes until you power it off completely.

Also placing a small stand alone fan on the rear of the amp will help greatly with tube life.

[markusw]

Thanks for you reply, Steve!
The tubes are in a head with tubes up and the rear cover is not even on the amp head for years.
Therefore, I think the heat should be able to dissipate from the amp head rather easily.
The amp head sits on another amp head both are on top of the cab.
I will keep it in mind to keep the amp in standby for 3 min before I put it in play mode.
I didn't do this in past. Usually, I waited just for maybe 30 seconds but the amp was always in a warm room.
I swapped positions of the two 6L6 now to check if the other one becomes microphonic too.

[B Ingram]

Might there be an issue with the tube socket or the components soldered to it that makes the tube become microphonic?

A tube is "microphonic" when when its internal elements are able to move in a way that changes its electrical output, generally as a result of external mechanical movement (like "sound waves shaking the tube" or a tap on the glass). That happens as a result of how the tube is constructed, and the approach the manufacturer took to produce a rigid structure that avoids vibrating at problematic frequencies. See the writeup by Tomer and Mullard
from the 1960s.

I don't see any feasible way the electronic factors of a circuit make a tube's construction less-good over time. But I bet you can imagine how something that is "not tied down well" could become "less securely tied down" over time.

[xtian]

When you go to turn the amp off and especially if you have been playing it cranked you should let the amp sit for two minutes even before you put it in standby.

Once in standby let it sit for 3 more minutes until you power it off completely.

I don't have time for that! Don't use stand by switches either.

[markusw]

A tube is "microphonic" when when its internal elements are able to move in a way that changes its electrical output, generally as a result of external mechanical movement (like "sound waves shaking the tube" or a tap on the glass). That happens as a result of how the tube is constructed, and the approach the manufacturer took to produce a rigid structure that avoids vibrating at problematic frequencies. See the writeup by Tomer and Mullard
from the 1960s.

I don't see any feasible way the electronic factors of a circuit make a tube's construction less-good over time. But I bet you can imagine how something that is "not tied down well" could become "less securely tied down" over time.

Thanks for your explanations and the links! I guess then it's just bad luck

[B Ingram]

Sadly, a lot of modern tubes are microphonic. JJ output tubes seem especially prone to it, so much that it's amusing and sad that their distributor sells a lot of dampener rings to try to help mitigate the rattles.

A lot of old production tubes were microphonic, too, but specific tube types were developed to attack the root-causes as much as feasible.

For example, tubes used in early digital computers were either in a high-current ("On") state or a low-current ("Off") state. There wasn't in-between as in audio, and also no speaker, so controlling microphonics was a non-issue. I outsmarted myself one time buying 5 dual-triodes to "get good American 12AY7s cheap" by picking a tube that seemed to have similar characteristics. They were like $2 each, and I didn't understand the data sheet note about their application in computers. Once in my amp, simply turning the amp's Volume to 1/4 of the way up (with no guitar plugged in) was enough to make the amp emit a full-power howl! I learned then what extraordinarily microphonic tubes sound like.

[markusw]

Sadly, a lot of modern tubes are microphonic. JJ output tubes seem especially prone to it, so much that it's amusing and sad that their distributor sells a lot of dampener rings to try to help mitigate the rattles.

A lot of old production tubes were microphonic, too, but specific tube types were developed to attack the root-causes as much as feasible.

For example, tubes used in early digital computers were either in a high-current ("On") state or a low-current ("Off") state. There wasn't in-between as in audio, and also no speaker, so controlling microphonics was a non-issue. I outsmarted myself one time buying 5 dual-triodes to "get good American 12AY7s cheap" by picking a tube that seemed to have similar characteristics. They were like $2 each, and I didn't understand the data sheet note about their application in computers. Once in my amp, simply turning the amp's Volume to 1/4 of the way up (with no guitar plugged in) was enough to make the amp emit a full-power howl! I learned then what extraordinarily microphonic tubes sound like.

Seems I'm having luck then.
I have 6 of the "Old Style 6L6WGC-STR BLACKPLATE small bottle TAD" running in my two amps and only 1 is slightly microphonic.
Don't know what experience others have made with this tube.

[Helmholtz]

I have 6 of the "Old Style 6L6WGC-STR BLACKPLATE small bottle TAD" running in my two amps and only 1 is slightly microphonic.
Don't know what experience others have made with this tube.

Be aware that a 6L6WGB cannot replace a 6L6GC as it's screen voltage is limited to 300V (absolute max. rating) vs. 450C (design max.) with a 6L6GC.
In case of doubt, ask your dealer for a binding datasheet.

Edit: Sorry, I misread WGC for WGB, because there was no original 6L6WGC.

[B Ingram]

I have 6 of the "Old Style 6L6WGC-STR BLACKPLATE small bottle TAD"

Edit: Sorry, I misread WGC for WGB, because there was no original 6L6WGC.

I had the same problem a couple years ago when TAD decided to give their tube a type-designation that never existed before.