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Old 22nd October 2010, 09:46 PM   #1
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Angry tube rectified psu

Hi,
My I put 740vct on anode ez81.I need only 50mA.I found max. is 2x350VAC,but for 150mA.Isnt it?!
Many thanks.
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Old 23rd October 2010, 12:30 AM   #2
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The voltage rating is independent of the current rating. So you can't put too much on the plate just because you aren't drawing maximum current.
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Old 23rd October 2010, 01:25 AM   #3
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Misterno,

You may get away with this, particularly if the 1st filter cap. is small. However, why take a chance on destroying a useful EZ81/6CA4?

Set up what I call a "cockeyed bridge". Wire 2X series connected pairs of UF4007s. Snub each pair with a HIGH WVDC 10 nF. capacitor. Connect the cathode ends of the SS assemblies to the ends of the power trafo. Ground the anode ends of the SS assemblies. Connect both EZ81 plates to the CT of the power trafo. Connect the PSU filter to the EZ81 cathode. In the configuration described, the "sand" diodes take all the reverse voltage stress. The vacuum rectifier keeps any SS switching noise out of the B+ rail and provides a "soft" start.
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Old 23rd October 2010, 01:58 PM   #4
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Many thanks,
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Old 23rd October 2010, 02:20 PM   #5
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HI Eli,
Could You be so kind to drew it,for those people like me,English is not mother lenguage and we beter understand shematic.
Thank You.
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Old 23rd October 2010, 03:18 PM   #6
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come to think of it, Eli's suggestion does a very nice job of keeping SS noise out,
if the anodes of the tube (and CT of transformer) are "rested" below the rectified voltage, by a resistive divider or even just a resistor from the anodes of the tube to ground, that way, in order to rise the CT and anodes, the solid state diodes have to start conduct first (while the tube is still "open"), that way, fargoing isolating the SS noise indeed.

it may also work without resistor, but then stray capacitances may let the tube slightly conduct when the SS diodes switch, and hence pass a little switching noise.

/me inspired ;p
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Old 23rd October 2010, 06:07 PM   #7
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Crude schematic attached.

On their own, snubbed UF4007s are acceptably quiet. What little noise remains, the vacuum rectifier blocks.

A small added benefit of the 10 nF. snubbers is slight suppression of ringing in the power trafo rectifier winding.
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File Type: jpg Cockeyed_Bridge.jpg (221.2 KB, 98 views)
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