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Old 23rd May 2012, 12:21 PM   #1
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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?

Cheers!
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Old 23rd May 2012, 03:25 PM   #2
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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.
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Old 24th May 2012, 04:09 AM   #3
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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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Something like this?
1N5388 = 200V 5W zener, as drawn dissipating 359mw.
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File Type: jpg 800Vreg.JPG (77.6 KB, 46 views)

Last edited by RJM1; 24th May 2012 at 07:27 AM.
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