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 🙂
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 🙂
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!!!
-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!!!
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
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
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.
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.
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.
yes,
So its an internal defect which cant really be defeated? Are the 12b4 microphonic in all manufacturers?
Transformer in separate chasis?
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.
For my preamp project I was planning to suspend the complete tube-part with springs. A separate power supply was another idea, but that will take too much space.
real said:
Yep, but also due to variations in cathode to anode distance as well. This changes the acceleration for a given anode voltage and hence the valve characteristics like m. Granted, the anode is generally much more rigid than the fine wires of the control grid.
High gm (transconductance) valves are especially vulnerable, since the control grid must be wound very close to the cathode where the electrons have a lower velocity, so the variations of control grid voltage have a larger effect on anode current. The result is a small variation of grid to cathode distance being very significant since the initial distance itself is very small. For instance the control grid to cathode distance of the WE416C is only 20mm (gm = 65mA/V).
skyraider said:
You can do things to reduce its effect, like making a subchassis to isolate the vibrations (think trampoline). Also, I have not used the 12B4 myself - it's on a far too long to-do list, but I have read that one of either GE or RCA is microphonic while the other is not. The problem is I can't remember which one 🙁 A little googling might yield the answer though.
Microphonics can vary between manufacturers because the internal structure of the valve may vary somewhat (i.e. some may be more rigid than others). The valves with the less rigid internal structures tend to be microphonic.
Microphonic caused by loose lug!
I am currently working with an Eico HF-14 and I had severe microphonics. At first I suspect one of the tubes. Then as I narrowed it down it was the speaker/OPT ground lug nut that was loose and causing it! I tightened it and silence!!
so I guess its not always tubes....
I am currently working with an Eico HF-14 and I had severe microphonics. At first I suspect one of the tubes. Then as I narrowed it down it was the speaker/OPT ground lug nut that was loose and causing it! I tightened it and silence!!
so I guess its not always tubes....
It used to be common practice to suspend the low-level stages in high quality tube gear, but you'll need to watch the spring resonances.
Sorry, made a mistake:
GE 12b4a IS microphonic. I got rid of all the hums on my preamp 2 days ago, and turned the volume up max... can hear all wind noise.. tapping on the chasis can be heard clearly on the speakers
GE 12b4a IS microphonic. I got rid of all the hums on my preamp 2 days ago, and turned the volume up max... can hear all wind noise.. tapping on the chasis can be heard clearly on the speakers
x27 type triodes (327, 227, 127, some 27) are very microfonic tubes too.
Some ECC88/Ex88CC are microfonic too. But in most of the cases the problems could be solved with careful tube positioning and gridstop resistors.
Some ECC88/Ex88CC are microfonic too. But in most of the cases the problems could be solved with careful tube positioning and gridstop resistors.
skyraider said:
I put some on an amp of mine and they actually seem to help! I just used high-temp silicone automotive o-rings instead of "real" 😉 tube dampers. They are somewhat stretchy and a deep red color.
I wouldn't say that there was a huge difference in the regular sound but there is less microphonic sound if the amp gets bumped slightly while operating. (I've since moved the amp to a better location where bumping doesn't happen but I left the o-rings on. They don't hurt.
"I put some on an amp of mine and they actually seem to help! I just used high-temp silicone automotive o-rings instead of "real" tube dampers. They are somewhat stretchy and a deep red color.
I wouldn't say that there was a huge difference in the regular sound but there is less microphonic sound if the amp gets bumped slightly while operating. (I've since moved the amp to a better location where bumping doesn't happen but I left the o-rings on. They don't hurt."
Hi, you mean the o-ring is placed in between the tube and the socket? That dampens the tube-socket contact right?
But basically the vibration from the chasis to the socket is not dampened right? And the vibration can reach the tubes through the pins i guess..
I wouldn't say that there was a huge difference in the regular sound but there is less microphonic sound if the amp gets bumped slightly while operating. (I've since moved the amp to a better location where bumping doesn't happen but I left the o-rings on. They don't hurt."
Hi, you mean the o-ring is placed in between the tube and the socket? That dampens the tube-socket contact right?
But basically the vibration from the chasis to the socket is not dampened right? And the vibration can reach the tubes through the pins i guess..
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