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Old 9th October 2008, 12:01 AM   #1
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Default microphonic sub-miniature tubes?

I'm trying to use a number of sub-miniature tubes for a mobile amp project, however all of them are extremely microphonic. I found this strange since some of them were used in mobile radios and such... Any ideas about making them more stable?

Here are a few models I tried, some of them are more noisy than others:
547DX, 6088, 5672, CK512
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Old 9th October 2008, 01:11 AM   #2
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Just a couple of thoughts from a rank amateur.

Tiny little elements are probably not as stiff as larger ones possibly making them more prone to vibration.

If these tubes were primarily RF tubes used in tuned amplifiers then being microphonic in the audio range is probably immaterial to their intended application.
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Old 9th October 2008, 01:13 AM   #3
wa2ise is offline wa2ise  United States
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I've noticed the same thing with filament (directly heated) sub-mini tubes, that they are microphonic. I don't have a solution for making them less microphonic either. I suspect that it's the filament itself bouncing around as compared to the grid structure inside.
Even the old 7 pin battery radio tubes tended to be microphonic, like the 1R5, 1U4 and so on. I sometimes could get such a radio howl with accoustic feedback from the speaker to one or more microphonic tubes...

Indirectly heated tubes are much less microphonic, but they are power hogs for portable battery work as compared to filament tubes, which I'm sure is a major reason why you wanted to use the filament tubes.

Mounting the pre-amp or line amp stage circuits on a separate chassis or circuit board that has shock mounts (like springy rubber grommets) might help some. Placing transformers or the batteries and other heavy parts on that board would add inertial damping to it as well.
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Old 9th October 2008, 03:02 AM   #4
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I cured nuvistors gluing still nuts around by epoxy.
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Old 9th October 2008, 12:51 PM   #5
mwiebe is offline mwiebe  United States
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Try running your filaments undervoltage a bit. That usually works for me.
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Old 9th October 2008, 03:25 PM   #6
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wa2ise - what you are saying is correct, i could use indirectly heated tubes, but that would make the power consumption too high to stay mobile (with reasonable amount of batteries). i thought about using soft silicon rings around the screws that hold the PCB, maybe that will give the tubes some insulation and I might stick some "heavier" components and maybe something additional on the PCB.

mwiebe - i'll try that!

Wavebourn - i might try that as the "additional" thingy on the tube PCB. from my short experience with nuvistores they are not as microphonic as some of the sub-miniature tubes but i can understand how using nuvistores in a non-stationary format can be a nightmare!
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Old 9th October 2008, 04:46 PM   #7
kevinkr is offline kevinkr  United States
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Yeah, I've found the 6088 an 5672 to be quite microphonic, the key seems to be to clamp them firmly to a pcb and to suspend the pcb on rubber bands, bushings or similar in order to decouple them from the rest of the chassis. Make sure all wires leaving the board are quite flexible.

Reducing small dht filament voltage slightly often also results in slight improvements in linearity, just make sure you are not running them too near to maximum rated plate current in order to avoid damage to the filament. (Depleted space charge around the filament can result in ion bombardment damaging the filament.)

Steve Bench writes about this and many other issues pertaining to submini battery triodes on his site here: http://members.aol.com/sbench101/ Note that sadly this site is gone at the end of this month, something going on at AOL.. Have contacted Steve Bench about this..
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Old 9th October 2008, 08:07 PM   #8
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I tested many (many) when I designed a portable tube hybrid headphone amp:

(http://www.ttvj.com/index.php?main_p...oducts_id=1062)

Due to battery life constraints, I was restricted to low-power filamentary tubes. Some of them are so microphonic that they can actually self-sustain (enter an electromechanical oscillation) if you thump them with your finger.

In general, the longer the tube is (tip to seal where the wires come out), the worse it is. So look for very short tubes. Also it seemed like the lower filament voltage tubes (0.625V) were a little better than higher filament voltage tubes.

Beyond that, I didn't find any magic answer. I never found a good way to damp them mechanically (at least in a small portable enclosure).

The best I found was the 534AX. Sorry, but I think I bought up the entire world supply of them . You might find a few here or there...

If I remember right, the 6419 was OK as well. 512AX and 6088 are pretty bad... the 6418 is very bad.

Interestingly most of these tubes datasheets do not list an abs max plate current limit. I suspect that the filament is operating at such a low temperature (they do not glow) that it is impossible to damage it with excessive current - it can only emit so much.

Pete
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Old 9th October 2008, 08:30 PM   #9
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Quote:
Originally posted by space-cake

Wavebourn - i might try that as the "additional" thingy on the tube PCB. from my short experience with nuvistores they are not as microphonic as some of the sub-miniature tubes but i can understand how using nuvistores in a non-stationary format can be a nightmare!
I did not try sub miniature glass tubes, but nuvistors were Ok both in condenser microphones and guitar amps, though always wanted a nut that made their bodies heavier and stiffer.
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Old 9th October 2008, 10:23 PM   #10
Sheldon is offline Sheldon  United States
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Originally posted by kevinkr
http://members.aol.com/sbench101/ Note that sadly this site is gone at the end of this month
Nooooo!!

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