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Old 4th March 2004, 04:43 PM   #1
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Default neon bulbs as grid voltage clamps

I've been using them in my dc coupled OTLs since I lost some input and driver tubes due to grid to cathode overvoltage breakdown during warmup and they work great as sort of a 'gas' transzorb without the crap solid material dielectric issues when connected either between the grid and cathode, grid to clamp reference voltage or cathode to clamp reference voltage. Just wondering why I've never seen mention of their use in the literature. They seem to work fine at clamping up to a few milliamps from, say the long tailed cathode resistor in a differential input stage until the tube warms up.
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Old 4th March 2004, 05:06 PM   #2
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This is possible, yes.

Don't forget that neons add a lot of noise when working.

What I do see however, are diodes to perform the same task.
See for example the TubeCad magazines at www.tubecad.com

Dick.
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Old 4th March 2004, 05:44 PM   #3
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Other than the highish turn-on voltage, neons are better to use than diodes- lower capacitance, no nonlinear capacitance. Since in normal operation, they're not on, the noise is a nonissue.

The first time I saw neons used for this application was in the Marantz 9, IIRC.
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Old 4th March 2004, 07:09 PM   #4
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Quote:
Don't forget that neons add a lot of noise when working.
Hi,

As SY pointed out, the neons will only be 'working' when the circuit they're protecting isn't (eg, warming up). But, the neon bulbs could probably be used as a low performance, low current alternative to a voltage regulator tube (I haven't tried this), in which case the noise would become an issue, as with the dedicated voltage reference tubes. What I've done for OA2s, etc. is to series them with maybe a few hundred ohm to 1K resistor then put a film cap across the combo which seems to pretty much remove audible noise without hurting stability of the regulator tube. Then to really improve input rejection and to more than compensate for any regulation stability lost with the series resistor, I've (effectively, not necessarily by a direct connection to the grid) summed a small amount of the input rail to the grid of the first tube in a two tube regulator, which, when trimmed out gives me 20 db or more improvement upon the input rejection capability basic regulator circuit I started out with, including compensating for the delta voltage across the resistor in series with the regulator tube. By doing this with my phono/line level preamp regulated supply, I was able to get a comparable audible improvement in ac supply immunity to running the entire preamp off of a dedicated harmonic neutralized ferroresonant transformer line regulator.
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Old 4th March 2004, 08:42 PM   #5
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I agree, they are superiour than diodes.
Even saw them as coupling devices in a tube opamp, as they have a pretty constant voltage drop ( was around 70V if I recall right ).

The noise is indeed a nonissue in this application, as I mentioned "when working".

Apparantly, as Thoriated explaned quite well, the noise can be filtered out effectively.

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Old 4th March 2004, 11:07 PM   #6
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Hi,

Quote:
But, the neon bulbs could probably be used as a low performance, low current alternative to a voltage regulator tube (I haven't tried this),
Sure, you can do that. There used to be an enteire series of neons that were used in tube gear as voltage refernces.

The most common being the Ne2. You saw these in older US radios and some military stuff.

If you're lucky, Antique Electronic Supply in Tempe AZ may still carry some of those.

I also noticed that John Broskie of Tubecad often puts a protective diofe (SS) between grid and diode.

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Old 5th March 2004, 01:20 AM   #7
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Just made me think, it might be cool to mount them in the front panel as "tube death" indicators.
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