Micro phonic EL34's...common?

[fredcapo]

I repaired a JTM60 that we acquired on a trade-in. There were a myriad of problems with mostvserious being loss of bias voltage due to open pcb land. This fried the OT. The repaired amp works well with new OT andh a known good pair of EL34's. However being the reckless type, I installed an untested used pair of EL'si which are micro phonic and actually create a sustained oscillation when tapped. Are micro phonic output tubes common? Is this trying to tell me I created a phase reversal with the OT installation? Thanks.

[Leo_Gnardo]

Yes I've heard EL34 (and other power tubes) go microphonic with a Big Ben bonnngggg. Sometimes it sustains and sometimes damps after a short while but not what you want in a guitar amp. Can those microphonic ones - they're good for target practice. fred you reckless so & so... good work on getting that amp up & running. Marshall 600's not commonly seen, an idea that came & went, fast. They can sound pretty darn good.

Since the amp works well with a known good pair of output tubes you needn't worry about an inadvertent phase reversal.

[Enzo]

And it kinda doesn;t matter how common it might be, it happened either way. It might be one in a million or it might be one in three, but nonetheless it happened to you here.

[fredcapo]

And it kinda doesn;t matter how common it might be, it happened either way. It might be one in a million or it might be one in three, but nonetheless it happened to you here.

Well it would be nice to know that I am not the only one it happened to...if I was the only one I might be emotionally crushed...or maybe think I have a special "musical" set of EL34's

[Leo_Gnardo]

Well it would be nice to know that I am not the only one it happened to...if I was the only one I might be emotionally crushed...or maybe think I have a special "musical" set of EL34's

Microphonic tubes are sorta like bells, typically the ringing frequency corresponds with the size of the tube - but not always. As you continue to repair amps & gather singing tubes you can get enough to play scales on...

Just for amusement I've kept some of the 12AX7's that have made the most outrageous noises - gas music from Jupiter.

Don't be crushed, far from it, set up to make some avant garde music from accidental sources. Found sounds. Aleatoric and all that. Meanwhile, yes you're definitely not the only one it happens to. Had to replace a pair of real honkers last week. Back to work!

[fredcapo]

I don't fully understand how the gain of the stage relates to the level of microphonic, or if one half of the PP stage is singing how it would sound...but yes as I work on more amps these anecdotal occurrences will be understood and expected.

So far my favorite defective tube is a sparking Chinese 6V6. I plan to install it in an amp every July 4th.

[Leo_Gnardo]

I don't fully understand how the gain of the stage relates to the level of microphonic, or if one half of the PP stage is singing how it would sound...but yes as I work on more amps these anecdotal occurrences will be understood and expected.

If it's the output tubes, you could remove the drive tube and still hear them moan, groan, klonk & bonk. And of course, try just one at a time in the amp, to separate the sheep from the goats. Naaaayyyyy ---- baaaahhhh.

So far my favorite defective tube is a sparking Chinese 6V6. I plan to install it in an amp every July 4th.

YEAH That's the spirit!

[Mick Bailey]

Very common nowadays -especially JJs and the last TAD-EL34B-STR I bought.

JJs are so bad right now they should come with a guarantee - if they don't go microphonic within 90 days you get an exchange or your money back.

How long has mankind been making Tubes? and Zippers? and other stuff that used to work reliably and predictably but doesn't any longer?

[fredcapo]

BTW the microphonic pair of EL34 in my OP were used tubes made in Japan. Don't recall or know name or date. I was not aware of tubes manufactured in Japan.

I think it's worth the effort to locate and test/screen used vintage tubes considering the problems with the new ones...

[Leo_Gnardo]

BTW the microphonic pair of EL34 in my OP were used tubes made in Japan. Don't recall or know name or date. I was not aware of tubes manufactured in Japan.

I think it's worth the effort to locate and test/screen used vintage tubes considering the problems with the new ones...

Some very good tubes were made in Japan. Matsushita bought the GZ34 tooling from Mullard & Matsu's are the poor man's Mullet or Bungle Boy, available at a fraction. Not bad at all.

As Mick mentioned there's a rash of bonky JJ's but they're not the only culprits. I've seen rattly, honking output tubes from practically every brand, whether new or used. Now that CE won't guarantee against microphonics, the game is on. They KNOW something's wrong and don't want to get stuck with bad product. I understand that but not happy about it, because they have to make a profit. OTOH a company that moves that many tubes ought to be boppin' their suppliers over the head until they supply tubes suitable for audio use. The case has been made on MEF threads recently that Eurotubes delivers JJ tubes that are nearly problem free, so you could try ordering from them. AFAIK Euro doesn't have a wholesale pricing for us repair shops. Could rationalize the extra cost as 'peace of mind.'

In my workshop I have an old Heathkit W-4 mono amp I use for "real world" testing of output tubes & rectifiers. Unlike most tube testers that only put @ 175V on plates, the Heath is about 450V. Also has jacks near the output tubes for bias current measurement. I leave these permanently plugged into a pair of 100 mA meters. The Heath is self-biased and one of these days I'm going to add a fixed bias option with several choices of bias voltage. You could make a similar tester out of a scrap amp.