Microphonic power tubes?

[jdurango]

I just purchased these vintage GE 6L6GC tubes from eBay and had them installed by my tech with 40+ years of experience working strictly with tube amps. They seem to be microphonic, at least 3 of the 4, one is probably an acceptable level, but what do you think?



The seller is claiming this is a normal amount of microphonism for a 60's Fender tube amp (mine is a '66 Twin), which may be true? But he's also saying the tubes could've been damaged in shipping, or by my tech, so he seems(?) to also be acknowledging there's an issue here. I've certainly heard tubes that are more microphonic, but the vast majority of tubes I've heard are far LESS microphonic. The last time I had tubes replaced on this amp was about 10 years ago, and they were brand new tubes so I didn't even think to check. I could buy all new tubes to compare, but that'd cost $150 for the tubes, plus $40 to my tech for the install, that's a lot of time and hassle just to find out if I should be returning these GE tubes, especially since I'm already out $260 for the tubes + $40 for the install/biasing. Thanks in advance for the help!

 

[metalockpick]

1) power tubes shouldn't be microphonic. Sounds like something is loose inside the tube (that's bad) If the preamp tubes are microphonic, you can scrape the pins until they're shiny and wrap a few turns of electrical tape around the glass to get them to behave..

2) The cost to properly store tubes in a cool, dark area for 40/50/60 years is more than the profit you'd make selling the tubes...

3) If they're not stored properly, the performance and reliability will be compromised...

4) ...assuming that it's not a used, worn out, repackaged tube. Does it have visible signs of wear, such as browning from overheating, or the edge of this chrome "getter" turning white?
There are companies and people that sell quality vintage tubes. They ain't eBay.

You could keep them so that when you sell the amplifier, you have "period" tubes to go with it.
Other than that, I can't think of a good use for those tubes.
I run selected current production tubes in vintage stuff. Certain brands and models of tubes will be better then others in particular applications. Finding the best tube for the application is the trick.

 

[jdurango]

1) power tubes shouldn't be microphonic. Sounds like something is loose inside the tube (that's bad) If the preamp tubes are microphonic, you can scrape the pins until they're shiny and wrap a few turns of electrical tape around the glass to get them to behave..

2) The cost to properly store tubes in a cool, dark area for 40/50/60 years is more than the profit you'd make selling the tubes...

3) If they're not stored properly, the performance and reliability will be compromised...

4) ...assuming that it's not a used, worn out, repackaged tube. Does it have visible signs of wear, such as browning from overheating, or the edge of this chrome "getter" turning white?
There are companies and people that sell quality vintage tubes. They ain't eBay.

You could keep them so that when you sell the amplifier, you have "period" tubes to go with it.
Other than that, I can't think of a good use for those tubes.
I run selected current production tubes in vintage stuff. Certain brands and models of tubes will be better then others in particular applications. Finding the best tube for the application is the trick.

Thank you very much for the help! I'd love to hear more opinions. It looks like the seller is being combative about this. If I'm wrong and these tubes are perfectly acceptable, I'll hold onto them, but I don't think I'm wrong. Thanks!

 

[Brian Roth]

@jdurango​

Your video link says "private" here. Would love to see/hear it.

Bri

 

[drtechno]

Thank you very much for the help! I'd love to hear more opinions. It looks like the seller is being combative about this. If I'm wrong and these tubes are perfectly acceptable, I'll hold onto them, but I don't think I'm wrong. Thanks!

It could be from oxidized cathodes from it just sitting in storage. Many Nos suppliers cookout that before selling them. So what you do, Is run the heaters of the tube (AC voltage) without the plate voltage for a week or until the static resistance from plate to cathode with heaters are the same or near the same as a new tube. I do this on a test chassis I built for the different tube base configurations I use. so I would recommend you build a burn in jig so you can revive NOS tubes you buy from someone.

But Mishandling a tube while in storage can cause a tube to be monophonic as well if its structure support is compromised by mechanical shock. Keep this in mind if burning off oxidization doesn't help.

 

[jdurango]

Bah, sorry guys. Youtube has been default setting all my videos to private. Should be visible now. Thanks again everyone!

 

[emrr]

Those seem totally normal to me. No ongoing ringing oscillation tones, etc. Unless they're making extra sound when you're playing, I'm not concerned.

 

[jdurango]

Those seem totally normal to me. No ongoing ringing oscillation tones, etc. Unless they're making extra sound when you're playing, I'm not concerned.

Hmmmmm, well one person saying it's abnormal, one saying it isn't.....can anyone else chime in? I just don't wanna force through a return on this poor guy if there really is nothing wrong with these tubes....but at the same time, $300 is a lot for bad tubes. Thanks again!

 

[CJ]

Nprmal

 

[Tubetec]

Ok presumably you have bought a set of used tubes ,
what do you expect?
some microphonics in powertubes is to be expected ,
connect an instrument , advance the gain , does it interact in a bad way with the speaker/cab on any particular note ?

 

[Tubetec]

Abolutely yes ,
one of the quartet of used tubed you bought is more prone to microphonys ,

 

[Tubetec]

Might be why you ended up getting caught paying over the odds .

 

[Tubetec]

On measurement a microphonically defective tube could show as well minus the influence of external vibrations , tube testers dont test for that , the ears are the final arbiter-on test under actual usage conditions .

 

[Tubetec]

Take pot luck offEbay of a better 'single used'specimen' maybe?
make a good quartet.
a single used tube going for considerably less than a proportedly matched pair on the instruments .

 

[Cqwet Dbdfte]

"Matched" sets of tubes is suspect,
matched for what? Bias voltage, leakage, gas, gain, or transconductance? Tube testers mainly check transconductance, which is a "quality" factor, affected by wear out.
As Tubetec said, microphonics is not tested. Tubes can also be noisy or have self-oscillation needing an o'scope to measure, not included in typical measurements.
If a bay seller cannot provide actual test data numbers expect not to be surprised.
Good manufacturing practices of tubes included rejecting bad tubes, crushing bad ones, I suspect this did not happen sometimes in the eastern bloc where production quotas had to be met.
Tube selection is getting harder as factories close.

 

[Wavelength]

In most cases with used tubes I bet they are not matched anymore. I have all the old peerless transformer data and was looking over some of their 20-20+ and they spec very low current difference in matching like less than 5ma.
In my experience (since 1981) a couple of reasons for that to happen. #1 is always grid current. If the tube is drawing grid current even a little it will cause microphonics. You can measure the grid resistor for DCV to see if that happens.
It can also be as above how are the matched or really almost anything, high voltage (Fender usually runs higher than spec), low voltage.
I don't sell tubes even to customers, because really my cost of NOS is usually higher than the customer. I usually have to buy (and or return) more tubes because most of these guys are just selling crap.

 

[rmburrow]

EL34's notorious for being microphonic..

 

[Tubetec]

The back of a guitar amp isnt an easy life for tubes due to vibration from sound but also due to transportation .
Marshall use a vibration damping system on the heads , that helps a lot as tubes age , combos tend to be worse than seperate head/speaker cabinet set ups ,
Often in a studio setup isolating the head from the cabinet by placing it on a carpeted floor helps reduce microphonics .

On a few sessions I had to pull the amp section out of combos and set them on the ground nearby because there was some gremlin,buzz or rattle that manifested itself on a particular note.
A JC120 can sound like a bucket of if its cranked and thats transistorised

 

[jdurango]

Thanks for the info, but I'm just trying to figure out if these specific tubes are within acceptable range so I can either return them or not. I'm not aware of any standardized test for microphonism, but this seems like a concerning amount....like it'd definitely pick up the vibrations into the cabinet, then back into the tubes, creating a little feedback loop. The tubes aren't feeding back or anything, at least not currently, but I have a feeling it's affecting tone by adding a little microphonic feedback circuit. Is this normal? Thanks!

 

[metalockpick]

Nprmal

On finding quality US made 6L6GC tubes being microphonic in a 66 twin as "Normal" ???

Strongly disagree.

I've got old 6L6G, GC, real tung sol... None of them do that in guitar amps.

EL84 tubes being microphonic? Well...yeah.