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Identifiying a Microphonic Tube & Moding a Tube Radio

post #1 of 9
11/25/09 at 10:25pm
Thread Starter 
I have a vintage tube radio that has a microphonic tube that I can't figure out which one it is. Does anyone have any experience with one of these tubes and could tell me which one it may be:

12SA7
12SQ7
6BJ6
35Z7 GT
50L6 GT

If no one knows, could some one walk me through figuring it out which one it may be.


Has anyone modded a tube radio to add a headphone jack option? How would one do so? Dedicated audio transformer for headphones at X impedance. I'd love to hear other options or better methods.


P.S. I posted a similar article on another Head-Fi forum but I feel the tube microphonic question, which is a shared question between threads, might be better answered by the DIY builders.
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post #2 of 9
11/25/09 at 10:50pm
Quote:
Originally Posted by TheKisho View Post
I have a vintage tube radio that has a microphonic tube that I can't figure out which one it is. Does anyone have any experience with one of these tubes and could tell me which one it may be:

12SA7
12SQ7
6BJ6
35Z7 GT
50L6 GT

If no one knows, could some one walk me through figuring it out which one it may be.


Has anyone modded a tube radio to add a headphone jack option? How would one do so? Dedicated audio transformer for headphones at X impedance. I'd love to hear other options or better methods.


P.S. I posted a similar article on another Head-Fi forum but I feel the tube microphonic question, which is a shared question between threads, might be better answered by the DIY builders.
There's no tube that has a penchant for microphonics over another, AFAIK. As best I can tell, something happened when they were originally assembled at the factory. The plates or the getter - or some other part of the metal interior besides the pins - was assembled so that it was touching the glass. The result is that any vibration that the glass can pick up gets transmitted into the signal.

Which one is it? Simple. While you're listening to the radio, give a flick with your fingernail to each tube in turn. The microphonic one should ring like a bell.
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post #3 of 9
11/25/09 at 11:10pm
BMF and a few others have more knowledge with old radios. Identifying a microphonic tube is tricky but most microphonic tubes will "ping" if you tap them - while you have the amp running you could lightly tap the glass envelope of each tube to see which one elicits the greatest noise at the output. Be careful not to touch anything that may have high voltage in it..dB


edit - this is what happens when you open a window to rpond and then get distracted , Tomb slips in and gets to the point
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post #4 of 9
11/26/09 at 5:48am
Rather than poke about with your finger, use a pencil...
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post #5 of 9
11/26/09 at 11:23am
Well, if we're getting picky about it - the pencil tip could break, fall in and cause a short somewhere. You guys make a good point, though - better not use a finger unless the tubes are well exposed and there's no chance of touching anything else. I sometimes forget because all of our tube amps always have the tubes exposed outside the case.

Find something non-metallic/non-conductive and try rapping each tube with that.
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post #6 of 9
11/26/09 at 3:50pm
Thread Starter 
Thanks for all the suggestions. Figuring out which one may be, might be a pain in the ass due any slight vibration or loud sounds usually sets it off. Sometimes vibrations will make it ring and some times it won't; it's just strange. The keyboard on this laptop will almost always set it off if I'm on a roll [continues typing]. I'm in no danger of high voltage, same as the millet amps all the tubes are mounted on the top part of the chassis (even if it wasn't I'm more then careful enough and have some experience with high voltage live circuits to know what to do and not do). Could it not be a sign of an aging tube or tube that is in need of replacement?
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post #7 of 9
11/26/09 at 5:46pm
Thread Starter 
It's the 12SQ7, It's the only that make any noise. Um it's not a glass envelope tube [metal]. I'll probably end up replacing it. It may be on the way out. I don't have a tube checker to check any of the tubes (if they are in spec still). Does anyone know if I get all of these tubes made by either Tung-Sol or Sylvania? Or does anyone have a recommendation on good quality tube brands that isn't RCA or GE.
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post #8 of 9
11/26/09 at 7:37pm
Several, including Tung-Sol, on Ebay.
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post #9 of 9
11/26/09 at 10:37pm
Thread Starter 
Oh wow that's a nice price, thanks! I got to see what brands and price The Tube Store has for these tubes [local (Southern Ontario)]. Raytheon tubes any good [just thought of another brand]? I seen someone replace the 12SQ7 with a 12AV6. It apparently is less microphonic then the typical 12SQ7 but needs shielding (shielded socket). It requires an adapter due to the fact that it's miniature tube but I checked the datasheets for both of these tubes and match up all but these two specs:

12SQ7:
Plate Resistance 110,000 ohms
Transconductance 900 microohms

12AV6:
Plate Resistance 80,000 ohms
Transconductance 1,250 microohms


This shouldn't make a big difference should it? People use the 12SQ7 GT in place of the 12SQ7.

12SQ7 GT:
Plate Resistance 91,000 ohms
Transconductance 1,100 microohms

And these aren't the only differences between the 12SQ7 GT and 12SQ7. More so the the 12AV6.

Should I try the 12AV6 or is any thing I can do to reduce microphonics of a metal case tube? I assume the tricks one uses to reduce microphonics of a glass envelope tubes will not work on metal tubes.


P.S. For now I'm just going to borrow my friends 12SQ7 as he hasn't fixed his humming radio [humming (bad filter capacitor)].


12SQ7 for 12AV6 Swap Example Link
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