A question about microphonic tubes

bongomania · #1 07-31-2011, 01:14 PM

Can they act as speakers? That is, if you run a strong enough signal through them, can you hear the signal physically outside the tubes, coming from the tubes?

**B-string** · #2 07-31-2011, 01:35 PM

Don't know about consumer use, but used to build/test a 1000 watt AM transmitter. Had 4 big (beautiful glass) tubes. Made a sweet tweeter (not very loud though). Should have checked if it was the modulator or RF tubes, but never did.
That is not "microphonic" though.

bongomania · #3 07-31-2011, 02:58 PM

The connection I made there is that a diaphragm and a passive transducer work in both directions. Most headphones can be used as dynamic microphones, and you can force audio out the "in end" of a dynamic mic as well.

If vibrations picked up by the glass could be transmitted into the audio path, then it seemed plausible to me that the reverse could occur.

johnk_10 · #4 07-31-2011, 03:00 PM

i've never really heard preamp tubes emit sound (although it's possible), but i've definitely heard output tubes put out sound like a super tiny speaker.

**B-string** · #5 07-31-2011, 03:11 PM

The outside vibrations are transmitted through the glass, to the spacers (mica) to the grids. In those transmitting tubes the path was from the grids to the plates and through the connection to the plate cap on the glass? Just thinking out loud.......

JimmyM · #6 07-31-2011, 11:34 PM

What a bizarre question, but one I'm interested in nonetheless. I think microphonic tube speaker technology could very well save us from the predatory pricing of Chinese neodymium.

T-Bird · #7 08-01-2011, 05:22 AM

Hi.

Yes, they eventually do, but that is usually discovered on bench with a dummy load. Not while playing.

The pre-amp tubes are relatively silent, but power tubes can be heard on certain frequencies.

In a case of a tube acting as a loudpeaker, the retube has been long, long overdue though.

Regards
Sam

staindbass · #8 08-01-2011, 05:53 AM

they make a high ring, like a feedback through the speakers. they also do it when you tap the tubes with the end of a pencil, so you can find out which one it is. having the head on the speaker vibrates the head, setting the tubes off. i think something in the tubes vibrates, and then that sound gets amplified.

Groover · #9 08-01-2011, 07:16 AM

Sounds like you may need to get a dog involved in this one...

Off the top of my head, I would think whatever sound the tubes emit would follow the waveform of what ultimately comes out of the speakers, but as far as the actual sound/tone itself goes, I would think it sounds more like a current buzz if anything. Just thinking out loud here, no studies to back-up any of what I said..

Maybe try a stethoscope?

Bassamatic · #10 08-01-2011, 08:18 AM

I would say NO.

The reason a tube gets microphonic is because some outside vibration causes the plates to move inside. As the distance between the plates changes, the gain of the tube changes by a small amount, and you hear it. There is nothing in the tube that is magnetic, that can be moved by the signal and generate sound.

Your question is a lot like - can you put voltage into a condenser mic and hear something? - NO.

**IvanMike** · #11 08-01-2011, 08:44 AM

Quote:

Originally Posted by Bassamatic

I would say NO.

The reason a tube gets microphonic is because some outside vibration causes the plates to move inside. As the distance between the plates changes, the gain of the tube changes by a small amount, and you hear it. There is nothing in the tube that is magnetic, that can be moved by the signal and generate sound.

Your question is a lot like - can you put voltage into a condenser mic and hear something? - NO.

I'm going with this - different events are happening in tubes and transducers.

Surely some level of sound can get generated by a power tube (as empirical data show), but it's not going to be of transducer quality, just whatever resonant frequency the tube has depending upon the physical properties of the materials involved and the disturbance.

Great question though, the more we know about how our stuff works, the better.

bongomania · #12 08-01-2011, 02:06 PM

Quote:

Originally Posted by T-Bird

Yes, they eventually do, but that is usually discovered on bench with a dummy load. Not while playing.

The pre-amp tubes are relatively silent, but power tubes can be heard on certain frequencies.

In a case of a tube acting as a loudpeaker, the retube has been long, long overdue though.

It was in fact during a bench test, with a dummy load, that I heard it. The reason I asked "is it possible" was to help rule out insanity, indigestion, or other reasons I might hear something so unusual.

For the record, it was not a hum, a pinging, a ringing, a buzz, or anything like that. It was a quiet, but entirely intelligible, amplification of the music I was running into the amp under test.

I have some new tubes on order.

johnk_10 · #13 08-01-2011, 03:00 PM

that is completely normal, and the tubes don't need to be microphonic to do it.

**B-string** · #14 08-01-2011, 08:07 PM

I keep searching the brain cells but I think the ones I'm looking for died?

I think I remember it was a radio station engineer that heard transmitter tubes producing audio. He fathered the short lived electro-static speakers. They sounded (from reviews) pretty good, BUT the large size and very high voltage supply was their undoing?

will33 · #15 08-01-2011, 10:10 PM

That's it, I'm not figuring out speakers anymore, just micing my output tubes

. Got that more gear than I can carry thing figured out.

Really though, like to hear more about this from those of you who really do know about/work with this stuff.

**B-string** · #16 08-01-2011, 10:26 PM

Here is site on electrostatics. Frequently Asked Question: Electrostatic Speakers | MartinLogan

Bassamatic · #17 08-01-2011, 10:55 PM

I don't know if there can be enough voltage in a tube and enough modulation of that voltage to behave in the same way as an electrostatic speaker. In a tube there is a section that acts as a "gate" to the electron flow, so the high voltage remains constant. An interesting concept though.

MAMMOTHvolume · #18 08-02-2011, 04:50 AM

Quote:

Originally Posted by bongomania

Can they act as speakers? That is, if you run a strong enough signal through them, can you hear the signal physically outside the tubes, coming from the tubes?

potentially, but before that happens they tend to just go into oscillation

**IvanMike** · #19 08-02-2011, 08:10 AM

Quote:

Originally Posted by bongomania

It was in fact during a bench test, with a dummy load, that I heard it. The reason I asked "is it possible" was to help rule out insanity, indigestion, or other reasons I might hear something so unusual.

For the record, it was not a hum, a pinging, a ringing, a buzz, or anything like that. It was a quiet, but entirely intelligible, amplification of the music I was running into the amp under test.

I have some new tubes on order.

that's cool - the big question is - what was the frequency range?

Was it like that sound putting your ear to the plastic cover over a record player where you could actually hear the vibration of the needle on the vinyl but it was odd sounding due to the frequency response curve on records? That's how I'm imagining it.

bongomania · #20 08-02-2011, 08:48 AM

It was very much like--have you ever pressed 'play' on your stereo and wondered why the music was so quiet, only to realize there were headphones plugged in, and you were hearing them on your desk or floor? Sounded exactly like that, such that I went around and made sure all headphones and speakers in the room were unplugged.