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Simplest possible tube regulator -- will it work?
Simplest possible tube regulator -- will it work?
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Old 9th August 2016, 03:31 PM   #1
rongon is offline rongon  United States
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Simplest possible tube regulator -- will it work?
Default Simplest possible tube regulator -- will it work?

I was searching for uses for LM317 in a high voltage regulator (for a tube preamp or headphone amp) and came across something interesting in John Broskie's blog where he introduced his PS-1 regulated power supply.

PS-1 Solid-State Regulator Kit

This is what caught my attention:

Click the image to open in full size.

I understand that a MOSFET pass element will give far superior performance, but what if I want to use something super simple and durable to get about 300VDC at 40mA?

There are some common power pentodes that when triode-strapped will work with 90V to 100V plate-cathode, with grid-cathode voltage of 5V to 10V. That would provide enough voltage between the IN and OUT pins of the LM317 (or TL783). Likely tube choices would be EL86, 6P3S (the cheaper version), or 6AV5GT. 6AS7 or 6080 would also work, of course. (6V6 or EL84 won't give enough Vg-k for the LM317 to work.)

- Will the LM317 survive this? The pass tube should warm up slowly, which should keep too high a voltage from appearing between the IN and OUT pins of the LM317 -- right? Or am I wrong about that?

- Would a tube rectified B+ solve the problem? It would mean another voltage drop. It would be nice to be able to get a regulated +300V from a raw B+ of +450V.

I suspect that if this type of regulator was any good it would have been used by now. What am I missing?

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Old 9th August 2016, 04:02 PM   #2
rayma is offline rayma  United States
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Originally Posted by rongon View Post
I suspect that if this type of regulator was any good it would have been used by now.
That's a conceptual circuit, a practical one would have additional diodes, capacitors, resistors, etc.
TI has a HV regulator app note. Audio Research and others have used a tube series regulator,
based on tubes like the 6550 and 6080.

Last edited by rayma; 9th August 2016 at 04:12 PM.
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Old 9th August 2016, 05:08 PM   #3
rongon is offline rongon  United States
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Simplest possible tube regulator -- will it work?
OK, that is true that it's a conceptual circuit.

A tube series regulator would need a tube error amplifier, which means another cathode to heater voltage limitation that can't be violated. That could mean two separate heater supplies would be needed, one for the pass tube and the other for the error amplifier tube.

How about if the suggested protection diodes, etc. were in place? Something like the attached...
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Old 9th August 2016, 05:32 PM   #4
rayma is offline rayma  United States
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Originally Posted by rongon View Post
How about if the suggested protection diodes, etc. were in place? Something like the attached...
That's more like it, but these kinds of circuits take some development before they will work reliably.
Usually there's a compensation cap across R1.

Last edited by rayma; 9th August 2016 at 05:34 PM.
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Old 9th August 2016, 05:50 PM   #5
rongon is offline rongon  United States
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Simplest possible tube regulator -- will it work?
Quote:
Originally Posted by rayma View Post
Usually there's a compensation cap across R1.
Looking at the various Maida-style regulators I've seen, there's often a diode across R1, but not a cap. Here's a link to the original Maida application note:
http://www.ti.com/lit/an/snoa648/snoa648.pdf

Here's a more fleshed out conceptual schematic (attached).

Here's a schematic of a 2000VDC regulator using a pass tube and an LM317. It's attributed to Jim Williams, who I think was the founder of Linear Systems.

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I'm not sure if that's a working circuit or not. If it's capable of delivering 2000VDC, I'm sure it could be scaled down to provide 350VDC.

So, does that mean this is a viable idea for a simple circuit? What I find attractive about it is that it gives me a use for a couple of 6AV5GT tubes I have on hand that I won't have a use for otherwise.

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Old 9th August 2016, 05:56 PM   #6
rayma is offline rayma  United States
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Originally Posted by rongon View Post
Here's a schematic of a 2000VDC regulator using a pass tube and an LM317.
It's attributed to Jim Williams, who I think was the founder of Linear Systems.
If Jim Williams designed that, I'd definitely go with it, carefully adapted.
Remember the load must drain a certain minimum current for the Zener.
You may need an extra load resistor to establish that.

Last edited by rayma; 9th August 2016 at 05:59 PM.
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Old 9th August 2016, 08:59 PM   #7
rongon is offline rongon  United States
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Simplest possible tube regulator -- will it work?
Question for the moderators... Would this topic be a better fit for the Tubes/Valves forum? Or does it belong here in Power Supplies?

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Old 12th August 2018, 01:21 AM   #8
denny9167 is offline denny9167  United States
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Here’s a regulator I built using a 6L6GC tube works marvelously. One of the best power supplies I’ve ever built.
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Old 12th August 2018, 01:39 AM   #9
rayma is offline rayma  United States
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Originally Posted by denny9167 View Post
Here’s a regulator I built using a 6L6GC tube works marvelously. One of the best
power supplies I’ve ever built.
Looks good, do you use it for a preamp, or for an output tube screen regulator?
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Old 12th August 2018, 01:43 AM   #10
denny9167 is offline denny9167  United States
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Quote:
Originally Posted by rayma View Post
Looks good, do you use it for a preamp, or for an output tube screen regulator?
Iím using it in a line stage. I was really surprised how quiet
It is!!
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