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Old 8th February 2013, 07:06 PM   #11
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Thanks Rod,
Not looking for a design, I was just trying to learn what I could from your perspective. I'm inching towards a shunt reg that will regulate something just under 500VDC and while I have a couple of regultaor tubes that can stand the voltage my inclination is toward a simple transistor circuit if I can arrive at one that results in the sound I'm after. Just fishing for educational thoughts.
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Old 12th February 2013, 04:33 PM   #12
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Quote:
Originally Posted by Rod Coleman View Post
Hi Erik,

Yes. differential bipolar transistor error amplifiers are the way to go.

If the grid current of the pentode is say <10uA, you can run the differential pair at 500uA. This in turn will keep the base current low. The tail can remain a resistor if the -100V is regulated - it will be constant current anyway, and CMR is not a real problem.

As for transistors, please try the excellent NXP PBHV series of 500V parts in SOT23 and 1W SOT223:

PBHV8540Z :: NXP Semiconductors

A "lead" cap can be used from B+ to FB, but it may be necessary to compensate the diff pair with a cap, also.
Hi Rod,

thanks for your advice. I am 'in the field' now (living in Mozambique) and unable to build anything, so for the moment lots of reading, theory, and some exercises in how to tackle a Shunt regulator. But I am quite sure these circuit will be put together when I have access to my stuff again
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Old 12th February 2013, 04:44 PM   #13
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Chokes do matter for tubes. Morgan Jones Valve perfect book for the matter.
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Old 12th February 2013, 05:22 PM   #14
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Quote:
Originally Posted by suntechnik View Post
Chokes do matter for tubes. Morgan Jones Valve perfect book for the matter.
Hi Suntechnik

Are you trying to say that I should use chokes instead of (shunt) regulators?
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Old 9th March 2013, 02:23 PM   #15
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In a bright moment I remembered that one can also drive a pentode (tetrode?) using G2 with G1 and cathode at ground potential. G2 will need a 'power driver' (mosfet), but this will probably allow one to build the shunt element without the need for a negative rail.
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Old 7th November 2015, 09:46 PM   #16
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Reviving this old thread with some new info.

I still got this idea of building this shunt regulator, so I made some experiments today.

To start I traced some G2 driven curves of the GU50 with the utracer, and found the following set of curves, where G1 is at fixed -9V and G2 varies, very interesting for this application.

CCs would be set at 150mA max, and I think that 40mA through the GU50 during operation would be a good value, so I draw both points, to find that G2 will vary between 100 and 190V. So I made the regulator circuit, based on the LND150 depletion fets, and used 50V as reference voltage.

I can say it works, but haven't done extensive tests, yet. With G2 at 100V, which will be the case during operation, G2 pulls about 1mA of current, so the Mosfet feeding the G2 current doesn't need heatsinking. I also want to put a gridleak resistor in series with the 9V battery and a cap from B+ to grid, so the GU50 does some "Ripplerejection" work as well.

decision to change drive to G2 is mostly because it makes it possible to have everything powered from a single supply (with the exception of the -9V at G1, but well, a battery is a simple solution, otherwise three 3V zeners in serie with the cathode, and G1 to gnd).
Attached Images
File Type: png GU50 G2 mode, -9V at gate.png (78.5 KB, 205 views)
File Type: jpg regulador shunt.jpg (50.9 KB, 206 views)
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Old 8th November 2015, 01:16 PM   #17
Ketje is offline Ketje  Belgium
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Another way to do things ?
And perhaps your roots are there
Mona
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Old 9th November 2015, 05:32 AM   #18
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Hi Ketje,
I am Dutch indeed An uncle did some research into our family name, and found both "de beste" (dutch for "the best") and "de beest" (dutch for "the beast). And then he got scared, and stopped his search

Thanks for the schematic! I have looked into something similar, well, actually I made a lot of sketches, but decided for the above circuit because it does really put all the heat on the GU50, and the SS stays cool. During operation the FET feeding G2 passes about 1.5mA at 200 to 300, so between 300mW and 450mW, which is comfortable for a TO220 (maybe I ll put a small heatsink on it).

Best, Erik
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Old 10th November 2015, 01:05 PM   #19
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I expect if you run the numbers, you'll find that the overall system efficiency is worse than if you use a stable (or even regulated) amplifier in the first place.

That is to say: class A is inefficient to begin with, but as its bias current varies little (and could be servoed to keep stable, or even regulate the supply if desired, with a little effect on audio compression/expansion), it will do the same thing as you propose, without wasting an extra tube (and regulation overhead, and heater, and bias, and all that).

Even back in the toob days, everyone understood that pass regulation is just so much superior to shunt regulation, when any more than a few watts are at stake (i.e., whatever a 0A2 / 0D3 can handle). The overall efficiency still won't be much better, of course.

Tim
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