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HV Regulation using variable series LV supply?
HV Regulation using variable series LV supply?
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Old 23rd May 2012, 12:21 PM   #1
Cathode Follower is offline Cathode Follower
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Join Date: May 2012
Default HV Regulation using variable series LV supply?

Hi there,

I'm currently tinkering with a valve amp that runs 4xEL34 at 800v plate/400v screen for a 200W output (theoretically). However regulation on the supply is poor and actual voltages under full draw are more like 730v plate, 360v screen, producing only 155W output (yes I know, still not bad for a quad of EL34's but I want MORE!)

I figured I needed more supply volts to get to 200W, so I used a smaller 47V transformer I had lying around, wiring the secondary in series with the lower of the two 400V secondaries on the main PT. (It's a standard stacked bridge arrangement, used on such amps a the carlsbro 200TC - Carlsbro 200 T.C. Amp Schematic).

This gives me a nice supply at full draw, and I can get 200 watts now no problems, but the voltages at idle are much too high - roughly 870v plate and 470 screen - for the valves.

Rather than regulate the supply as a whole, messing about with high voltage regulation etc, would it be simpler and easier to seperately rectify the smaller transfomer, and use it as a variable 'auxillary' supply to help retain the 800v/400v when required?

I can't find any information on this type of regulation. Anyone have any ideas/suggestions/info etc?

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Old 23rd May 2012, 03:25 PM   #2
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Join Date: Dec 2011
Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina
Which is the power capability of such transformer? If the xformer has small power cacity of that the demands of the load, voltage sags an will overheat.
Osvaldo F. Zappacosta. Electronic Engineer UTN FRA from 2001.
Argentine Ham Radio LW1DSE since 1987.
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Old 24th May 2012, 04:09 AM   #3
takao21106 is offline takao21106
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Join Date: May 2012
Eventually a Thyristor pre-regulator could be used at the secondary side of the 42V transformer, instead of a rectifier.

That in turn give tension to a capacitor, use a feedback loop from the 800V, 2 MOhms voltage divider.

Or alternatively, control this Thyristor with a small microcontroller, 50Hz is not difficult.

If you want any of this stuff inside a tube amplifier, anyway.
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Old 24th May 2012, 07:22 AM   #4
RJM1 is offline RJM1  United States
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Join Date: Jan 2010
Location: Titusville, Fl.
Something like this?
1N5388 = 200V 5W zener, as drawn dissipating 359mw.
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Last edited by RJM1; 24th May 2012 at 07:27 AM.
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