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Old 17th January 2005, 09:03 AM   #1
s2kov is online now s2kov  Canada
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Default Microphonic

What cause of this? Is it the tube or the circuit?
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Old 17th January 2005, 09:10 AM   #2
arnoldc is offline arnoldc  Philippines
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the construction of the tube and the external factors...

i used to have an Anthem Pre 1L that uses 6DJ8/6922. i bought Amperex Gold pins at a price only to find it to be microphonic. i can run my fingernails in the pre's front panel and able to hear a very small jet plane taking off :lol:

i can also walk on my wooden floor and hear a "dinosaur thud" from the speakers at every step :lol:

i then replaced them with Sovtek 6922 and no microphonics whatsoever
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Old 17th January 2005, 08:08 PM   #3
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well, as far as i recall microphonics are often caused by tubes or/and capacitors..
-for example, i had two capacitors of the same brand but out of different batches, one was like a microphone and the other where totally without any tendency to microphonics.
so.. knock lightly with the plastic shaft of a screwdriver and listen..
-but watch out for hazardeous voltages!!!
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Old 18th January 2005, 01:38 AM   #4
s2kov is online now s2kov  Canada
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arnold/Mr. Triatic,

thanks for all your inputs.
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Old 19th January 2005, 05:12 AM   #5
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Default I was going to post something similiar,...

12B4 is notoriously known for its microphonics, otherwise it would be a lovely tube for preamp. Anyone has experience on taming those valves? How would damping the chasis help, and the methods to do it.
Thanks
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Old 19th January 2005, 05:50 AM   #6
arnoldc is offline arnoldc  Philippines
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i am planning to use that in my office amp
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Old 19th January 2005, 06:48 AM   #7
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Microphonics in a valve is caused by external vibrations modulating the distance between the grid and cathode, changing the amplification factor (and hence causing the familiar sound).

The amplification factor changes with the cathode-grid distance because of the way a valve functions: electrons are 'boiled' off the cathode and are accelerated toward the anode by an electric field. The grid is inserted somewhere in between, with a negative voltage applied wrt the cathode, which repels the electrons, controlling the flow. The further away the grid is, the faster the electrons are travelling (the acceleration between the anode and cathode over a distance by the electric field), and hence the less electrons it can stop, hence a lower mu. On the other hand, when the grid is close to the cathode, the electron velocities will be relatively small, and the grid will have a larger effect on the electron flow; hence a higher mu.

Hope this explains things.

Chris.
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Old 19th January 2005, 07:16 AM   #8
rdf is offline rdf  Canada
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Factor local sources too. No point sweating airbourne micophonics just to bolt a vibrator (read: PS transformer) on a thin chassis right next to the tubes.
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Old 20th January 2005, 01:38 AM   #9
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Quote:
Hope this explains things.
yes,

So its an internal defect which cant really be defeated? Are the 12b4 microphonic in all manufacturers?

Quote:
Factor local sources too. No point sweating airbourne micophonics just to bolt a vibrator (read: PS transformer) on a thin chassis right next to the tubes.
Transformer in separate chasis?
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Old 20th January 2005, 02:40 AM   #10
rdf is offline rdf  Canada
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Your call. I don't have enough experience to say it will sound better, but I do know in taking my first run at an amp that, putting an ear to the prototype chassis, the vibrations of the PS trannie were clearly audible and would swamp by orders of magnitude anything airborne short of sitting the amp in front of a K-horn. I tried to ameliorate the effect by bolting the tsfmr to a block of brass and rubber isolating the entire assembly from the main chassis with rubber hole grommets and rubber bumper standoffs. It was successful in that PS tsfmr vibrations are now just at the treshhold of audibility with an ear to the new chassis. If you're worried about mechanical vibrations reaching the tube it worth considering.
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