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Old 16th October 2013, 01:21 PM   #1
kimbal is offline kimbal  Australia
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Default Building A Really BIG Variable Tube Power Supply.

Hi All;

I recently sourced on eBay a good condition mains transformer capable of the following: >

Input 0-240 Volts Ac @ 50 cycles.
Output 0-50-700-800-900 volts Ac at 3.5 amps continuous.
Size is aprox - 12 inches x 6 inches by 9 inches - using 2 large C-cores

Likewise I also got a large choke of around 0.25 henry at 2 amps, uses 2 large C-cores and measures about 8 x 7 x 8 inches.
Pictures are attached.

The transformer weighs about 35 Kg and the choke about 10 kg.

My intention is to build a massive bench power supply for testing transmitter tubes operating in audio mode.

A pair of 833a triodes in push-pull would require some 2,500-3,000 volts and at least 1 amp plate current. Similar figures would be found for other large power tubes.

From my calculations - the power transformer is around 3.2 KVA and able to deliver 2,520 volts Dc at aprox 2 amps continuous from a standard voltage doubler rectifier. Greater voltages ( with less current ) can be obtained with another stage of doubling and with a bridge rectifier a healthy 1 kv Dc at close to 3 amps can be supplied. Because of this capability I may adapt various rectifier combinations via some suitable switching to achieve these other outputs.

I have some NOS 3B28 Gas Rectifier Tubes, which 4 tubes in parallel ( 2 paralleled in one stage and 2 in another ) will deliver the required 2,500 volts at 2 amps when wired as a half wave voltage doubler, ( with peak load currents of 5 amps at a reverse voltage of 10KV. )

[ Because of the sensitivity and difficulty working with semiconductors at these voltages - personal experience talking here - I would prefer to not use semis and stick just with vacuum tubes. The reliability would be far greater under extreme operating conditions with using tubes. Size and weight is not a concern. ]

I also have a huge 15 amp / 240 volt variac which will control the mains input to the transformer - effectively making it a variable DC supply.

Adding the 0.25 H / 2 Amp choke and a suitable capacitor network, will clean up the DC as smoothed supply.

BUT;

I would though prefer to have a more conventional vacuum tube regulator tacked onto the output of the filter choke and use that as the variable Dc output rather than waste a good 15 amp variac on such a job.

Does anyone have any proven tube regulator circuits which will vary the output from say 500-2,500 volts at 2 amps Dc ?

As for the series pass tube, it too, has to handle the full 2 amps. If smaller tubes are used, ( maybe as a series of stacked regulators ) many will be required.

In reality - the whole unit will have to be rack mounted on castor wheels, as it will be way too big for a normal bench supply.

I have the the volt meters ( 3kv Motorola panel meter ) and current meters.
I have the rectifiers and some of the smaller parts.

I'm open to design suggestions.
Attached Images
File Type: jpg 900v @ 3.5A Power Transformer.JPG (344.9 KB, 123 views)
File Type: jpg Large Transformer - Voltages.JPG (168.2 KB, 120 views)
File Type: jpg Larger Power Transformer.JPG (417.3 KB, 119 views)
File Type: jpg Large Choke.JPG (317.3 KB, 118 views)
File Type: jpg Choke 1.JPG (226.4 KB, 115 views)
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Old 16th October 2013, 02:06 PM   #2
SY is offline SY  United States
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The 2004 Cotes de Ventoux was a better balanced wine than the 2005. Just sayin'.
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Old 26th October 2013, 10:05 AM   #3
glennb is offline glennb  Australia
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Quote:
I also have a huge 15 amp / 240 volt variac which will control the mains input to the transformer - effectively making it a variable DC supply.

Adding the 0.25 H / 2 Amp choke and a suitable capacitor network, will clean up the DC as smoothed supply.

BUT;

I would though prefer to have a more conventional vacuum tube regulator tacked onto the output of the filter choke and use that as the variable Dc output rather than waste a good 15 amp variac on such a job.

Does anyone have any proven tube regulator circuits which will vary the output from say 500-2,500 volts at 2 amps Dc ?
Hi Kimbal S,
For variable DC output the variac will be much much simpler that the fairly extreme effort required to implement a series pass tube regulator. The regulator tube will need to be bigger than the tubes you are going to test!
Cheers, Glenn B.
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Old 26th October 2013, 10:12 AM   #4
glennb is offline glennb  Australia
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I have a pair of 5771 if you are interested. I don't think I will ever use them.
5771, Tube 5771; Röhre 5771 ID32477, Transmitter Triode, liq

Glenn.
Attached Images
File Type: jpg 5771-463.jpg (13.3 KB, 17 views)
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