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Vintage threads from the first ten years

Re: Tube Power Supplies (Design)

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5/19/1999 8:26 AM
Santiago	Re: Tube Power Supplies (Design) I think that 12V 500mA would do because if you run the filaments at 12V each one would need 150mA not 300mA, with two tubes it makes a total of 300ma, with 200mA for the rest which I think is more than enough.

5/19/1999 10:17 AM

Michael Burgundy

Guess we all have the datasheets lying around; 6.3V/0.3A and 12.6V/0.15A for a 12Ax7. Two of these would rate at 0.3A at 12V. Actually, I heard some VTwins have a seperate 6.3V secondary, but what the heck.
Here´s the catch (aside from always, always building in safetymeasures, currentwise);
Ever notice how lightbulbs always blow when you switch them on? This is mainly because the cold resistance of the filament is much, much lower than the hot R, so it will draw a LOT of current for an instant when it´s switched on. Same goes for tubes. You´ll want ample margin, for safety. I like big margins, but 400mA would be enough safety (I´d still like 600).
B+ was around 200V, right? That´ll probably draw something like 1mA per triode, so 4 or 5 mA in total. Wow. Adds up to an additional, say, 120mA at 12V. add a generous 100mA, why not, for all the solid state gear, and this adds up to 400+120+100=620mA.
Okay, okay, 500mA *might* work fine, and it will probably run hot, and...
FWIW, as far as I know, MESA rate (generously)
600mA, 200mA, 100mA respectively (this is hearsay!!)

5/19/1999 10:35 AM
Santiago	That´s right Mike, and in this case we are not saving much money in the difference from a 6VA transformer to a 12VA transformer, so the 12V 1A is a BETTER option.

5/18/1999 11:32 AM
C. Hütter	Hi! I guess the best way to get high voltage is to use two small 12V transformers back to back as done in the Real McTube article: http://members.xoom.com/aronnelson/mctube/ CU Carsten

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