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Cathode Follower 23rd May 2012 12:21 PM

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?


Osvaldo de Banfield 23rd May 2012 03:25 PM

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.

takao21106 24th May 2012 04:09 AM

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.

RJM1 24th May 2012 07:22 AM

1 Attachment(s)
Something like this?
1N5388 = 200V 5W zener, as drawn dissipating 359mw.

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