tap testing tubes.

[joesatch]

for preamp tubes specially V1 how accurate is tap testing them for microphonics? I see many folks doing this but i think in some amps even good tubes will emanate a sound upon tapping on them

[Ronquest]

I'd say it's accurate enough to let you know it makes noise when you hit it.   
Some tubes make lots of noise when you tap them and some barely.  A few of my favorite tubes (12ax7 and 12at7) are microphonic when you tap them or the chassis, but they don't feedback or sound glassy through the amp when playing.  My amps are  mostly heads, so I can get away with more than a combo.

[WimWalther]

for preamp tubes specially V1 how accurate is tap testing them for microphonics? I see many folks doing this but i think in some amps even good tubes will emanate a sound upon tapping on them

Most tubes are microphonic to some degree, this is just the nature of the tech. The internal electrodes act as the plates of a capacitor. Impinging noise causes the plates to move, producing a signal in exactly the same way a capacitor microphone does.

As far as tap testing, this is exactly how the pro tube graders that I know test them. Just don't use your fingers, use a plastic pen like a Bic or a Sharpie to do the tapping.

[PRR]

> don't use your fingers, use a plastic pen

There IS a tool for that.

https://el34world.com/Forum/index.php?topic=22130.msg313288#msg313288
keep reading

[glass54]

Improvise (steal an appropriate device from the kitchen) 

[joesatch]

i ask this cause i put in a brand new JJ in v1 on a build and tapping it definitely gave a loud clank sound. other tubes in v1 of this amp too clanked. the only one that didn't was a ruby 7025 i guess cause they are lower gain. i think tapping preamp tubes is hogwash.

[sluckey]

The 7025 has the same gain as a 12AX7. But it is like a high quality 12AX7 and has low noise/hum characteristics. There was a time when Fender used 7025s in the signal path of their amps.

http://www.tubebooks.org/tubedata/HB-3/Receiving_Tubes_Part_2/7025.PDF

[WimWalther]

> don't use your fingers, use a plastic pen

There IS a tool for that.

https://el34world.com/Forum/index.php?topic=22130.msg313288#msg313288
keep reading

I have to admit, this one is new to me. Amazing. And silly.

But the CBS/Hytron pin straighteners are great, fwiw.

[scstill]

is it true that short plate 12ax7 are less microphonic than long plates?
have also heard that long plate 12ax7 "sing" better that short (assuming sing might mean more sustain?)

[acheld]

is it true that short plate 12ax7 are less microphonic than long plates?
have also heard that long plate 12ax7 "sing" better that short (assuming sing might mean more sustain?)

I dunno.   It makes "sense" that the long plates might be more microphonic though after thinking about this some years ago I don't think it really does.

I bought a fair number of Sovtek 12AX7LPS tubes awhile back by mistake -- did not register the "LPS" somehow.  When I got them I was concerned that the long plates would be susceptible to feedback, so went about testing them at various positions in my builds.   Never found any problems . . . and those tubes have stood up very nicely over time.

[Ronquest]

i ask this cause i put in a brand new JJ in v1 on a build and tapping it definitely gave a loud clank sound. other tubes in v1 of this amp too clanked. the only one that didn't was a ruby 7025 i guess cause they are lower gain. i think tapping preamp tubes is hogwash.

Do those same tubes "clank" in your other amps?  The amount of "clank" is circuit dependent.

[kagliostro]

https://www.audiotubes.com/tubetap.htm



Franco

[mresistor]

Quit messing around and get a real tool for the job!

[WimWalther]

Quit messing around and get a real tool for the job!

"If I can't fix it, I'll fix it so it can't be fixed."

[Lectroid]

My motto:

"If it jams, force it.  If it breaks, it needed replacing anyway."

[AlNewman]

Some are certainly more microphonic than others.  You did your due diligence in trying other tubes with the same result.
I think a fingernail works for detecting Jimi type feedback on the 1st stage.

[mresistor]

https://www.ebay.com/itm/305254162437

[kagliostro]

Since the first day this thread started I remembered that I had some documentation in my archive but I only managed to find it now


Franco

[kagliostro]

Other

Franco

[kagliostro]

Other

Franco