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Old 23rd October 2010, 10:53 PM   #1
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Default 300b filament

could rectify the filaments 300b with the voltage of 5 volts ac transformer, how I can do? Consumes more amps the transformer,
made with the 6SN7 and warmed over, someone can help me, thanks
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Old 25th October 2010, 09:27 PM   #2
tomchr is offline tomchr  Canada
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I'm not sure what you're trying to do here. Are you trying to build a DC supply to run the filament of a 300B? If that's the case, then I suggest looking at an LM317 voltage regulator IC. Or you can follow what I did in this thread.

You will need a rectified DC voltage of at least 7.5 V at the input of the LM317 (or LMZ12002). Use a 9 VAC transformer that can supply the needed current (1.5 A per 300B). You'll need a diode bridge that can handle the current with plenty of margin - KBPC2504 would be practically indestructible in your application. Then add a 10000 uF cap on the output of the diode bridge. The cap should be able to handle at least 25 V.

The LM317 will dissipate up to 10 W so it will need a fairly big heat sink.

For the 6SN7 filaments, you can go with AC -- or if you insist on using DC, you can do the same as above. However, as the 6SN7 needs 6.3 V, you'll need a transformer that can supply 12 VAC. The feedback resistors on the LM317 will be different than those used for 5 V. There's an equation in the data sheet for calculating those resistors.

Does this help?

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Old 25th October 2010, 11:00 PM   #3
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The 10000 uF cap should be of really good quality because of the continuous current.

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Old 26th October 2010, 01:52 AM   #4
tomchr is offline tomchr  Canada
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Yeah, the cap should be able to handle the ripple current. Low ESR is a plus. I'd also put a polypropylene cap, 100 nF ~ 1 uF in parallel with the big one for better performance at RF. The LM317 (and LMZ12002) requires an input cap for stability anyway.

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