NOS tubes and microphonics

[Lambertini]

Hello!

I just bought 3 NOS tubes for an AB763: RCA 5751 for V2, GE JAN 12AX7WA for V4 and Mullard CV4024 for V6.
The Mullard behaves correctly, no problem.
The RCA tube is very sensitive to microphonics: when slightly tapped, you can hear a loud "poc" in the speakers, much louder than any new tubes I A/B tested.
The GE JAN is so microphonics that it enters in oscillation when tapped, producing a sort of larsen.

I already experienced 2 NOS tubes, RCA and GE, and both were so microphonics that I sent them back to the seller.

Does all NOS tubes are concerned by this issue or am I just very unlucky?
Do you already experienced perfectly quiet NOS tubes?

Thank you!

[tubeswell]

If you find a NOS tube from the 50’s or 60’s these days, you’ll be scraping the bottom of the barrel. They’ve been plundered for 60 or 70 years.

[Lambertini]

The RCA might be from 60, the GE is from 75.

[Dave]

Any can be microphonic. New or old. Not sure about the science. I wanted to address the one that oscillates though.


One time I had a tube that was behaving the way you describe and after a long period of trouble shooting, I just happened to reach over and turn my soldering iron off and it stopped oscillating. Just saying.


Dave

[Lambertini]

Any can be microphonic. New or old. Not sure about the science.

Based on my meaningless but experienced statistics, 75% microphonics on one side, and 10% on the other. And I'm not even sure how old is the Mullard, and how microphonics sensitive is the V6.

One time I had a tube that was behaving the way you describe and after a long period of trouble shooting, I just happened to reach over and turn my soldering iron off and it stopped oscillating. Just saying.

As soon as I switch the amp off, the oscillation is gone, just saying :)

[Esquirefreak]

I have a decent stash of NOS tubes and I rarely think they are better than current production, in terms of microphonics and noise floor. My favorite current production tube is the Sovtek 12AX7 LPS.

/Msx

[Lambertini]

On one hand, I feel like these tubes sounds warmer. On the other, I can't touch the amp anymore without hearing some crystal glasses clinking. Even touching the input jack generates noise...
I'll do further testing tomorrow with fresh ears.

[passaloutre]

I couldn’t imagine paying NOS prices. I’m happy with JJ tubes. I’ve ended up with a lot of vintage tubes from buying vintage stuff and I’ve had a lot more GEs, RCAs, Mullards, and Amperex fail than JJs.

[MrPink]

Tubes might act microphonic in one position and fine to use in another as well

Any can be microphonic. New or old. Not sure about the science.

[Esquirefreak]

Whenever I have a strong noval with microphonics, it goes in the "PI & tremolo tube box" 

/Max

[HotBluePlates]

Does all NOS tubes are concerned by this issue or am I just very unlucky?
Do you already experienced perfectly quiet NOS tubes?

I have roughly 800 old-production American & European tubes.  While they are not uniformly-free of microphonics, none are bad enough that they cannot be used.

I have previously encountered truly, horribly microphonic tubes:  no "tap" was needed, they simply caused the amp to go into a full-power howl when the amp's Volume control was turned above 1/4.

I just bought 3 NOS tubes ... The RCA tube is very sensitive to microphonics: when slightly tapped, you can hear a loud "poc" in the speakers ...
The GE JAN is so microphonics that it enters in oscillation when tapped ...

I already experienced 2 NOS tubes, RCA and GE, and both were so microphonics that I sent them back to the seller. ...

The most important factor when buying tubes is that you buy from a vendor with a strong Return Policy, such that you can be refunded if the tube is defective.  You will encounter noise, hum, microphonics, and short-circuits if you sample enough tubes whether they were made 70 years ago or yesterday.

[dude]

I’ve had good luck with used old tubes made in USA, Germany, other countries. Most were good, lots of used 6v6’s off ebay were decent after matching by current draw in a fixed bias amp. Mostly better than new Russian 6v6’s.
Better buys ten years ago on ebay, now most of the tubes are a crap shoot.
But all in all, l’ll take a good used American tube over current production any day, worth the search especially at flea markets. Good buys are hard to find today but you can get lucky. My 2 cents, FWIW.
Also, l find a lot of used old American used tubes make noise tapping on them but sound fantastic.

[MrPink]

I have found that all tubes in my ab763 V2 turned microphonic before changing the plate load resistors. i did swap some around for sound comparison and they all were non/not badly microphonic in the beginning. some hours of playing later, some turning off and on the amp or days to pass all my old tubes (only about 10 or so) had turned microphonic in V2. made me mad. i changed the plate load resistors to counter hiss as learned here https://el34world.com/charts/fenderservice1.htm
and after that none of them was giving me any noise any more. don't know if this is any scientific but it's exactly what happened, making me really happy i didnt throw them

btw telefunken win beeing in a different league (subjective as can be)