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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 17th Apr 2019, 3:28 pm   #1
trickie_dickie's Avatar
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Default Voltage Regulator Valves eg OA2

Are these valves noisy? I was thinking of using them in an amp in series with the screens on the output pentodes. Also interstage coupling instead of capacitors. Any thoughts?
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Old 17th Apr 2019, 3:44 pm   #2
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Default Re: Voltage Regulator Valves eg OA2

Yes. I'm afraid they are somewhat noisy. In some cases the 'special quality' versions have their noise properties specified (not necessarily reduced very much, just specified). Depending on the details you're interested in they can be quite a bit less noisy than Zener diodes of the same voltage rating. But the advantage of Zeners is that the noise can be filtered out using a large parallel capacitor. The discharge regulators, by contrast, have a negative resistance region in their characteristic and putting more than a few tens of nanofarads (perhaps a hundred nanofarads, if you're brave) directly across them can cause them to burst into catastrophic oscillation. I've never seen it but I'm told it's pretty exciting to watch, from a safe distance of course.


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Old 17th Apr 2019, 4:48 pm   #3
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Default Re: Voltage Regulator Valves eg OA2

Yes, they are noisy - indeed in some equipment discharge-tubes have been used as a source of wideband noise, either to drive 'jammer' transmitters or to feed random-number generators. (In later years photomultiplier tubes were found to be ideal for this application)

As mentioned, paralleling them with too much 'C' can produce a nice relaxation-oscillator. I use a series-connected pair of such valves to provide the regulated screen-supply for a parallel-pair of 6146 in a linear-amp, and there's only 5nF paralleling them. So far no oscillations!
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Old 17th Apr 2019, 5:32 pm   #4
Craig Sawyers
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Default Re: Voltage Regulator Valves eg OA2

Ernie 1 used cold cathode valves (like an OA2) as the random noise source. The particular unit used was a QS92/10, which is a 92V regulator valve.

Ernie 1 schematics and description of the design are here

So yes - regulator valves are noisy!
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Old 17th Apr 2019, 6:39 pm   #5
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Default Re: Voltage Regulator Valves eg OA2

I have used them on 807s in series to drop voltage due to the limited voltage rating of the screen, I used 85A2 IIRC.
couldn't actually hear any significant noise in the speakers.
If you drive the amp well into clipping though it could cause some problems.
ie when the anode voltage drops below reg voltage.

For measuring conducted emissions in the 9kHz-30MHz range we actually use a pair of neons across the line as a noise source.
at 150kHz the emission peak value is pretty much on the conducted emission limit~66dbuV however these are on AC50Hz therefore theres a lot of switching noise when they strike. never tried one on DC, must have a go now its been brought up.

Last edited by Megatron; 17th Apr 2019 at 6:47 pm.
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Old 18th Apr 2019, 12:20 pm   #6
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Default Re: Voltage Regulator Valves eg OA2

Using them as a dropper means they act as a voltage deregulator.

Interstage coupling might be interesting. Apart from the noise and current flow (typically 5mA minimum) they are sluggish to respond to voltage changes, which is why you can get away with a low value parallel cap but not a bigger cap. You might get a built-in tone control.
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Old 19th Apr 2019, 11:43 am   #7
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Default Re: Voltage Regulator Valves eg OA2

You can make an oscillator using a neon lamp and a resistor/capacitor combination. The oscillation is due to the striking voltage being higher than the normal discharge maintenance voltage. So you need to be careful if you are adding parallel capacitors to a normal dropper resistor combination in case you unwittingly create an oscillator.
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