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previous: Cliff Anyone know how to convert an A.C. ... -- 7/24/2000 1:35 AM View Thread

Re: filaments on D.C.

7/24/2000 3:19 AM
Re: filaments on D.C.
Well, you have to rectify the AC voltage to DC.  
You will need a bridge rectifier, and a rather large capacitor, say 10,000uF/16V.  
You will get 1.414 * the AC voltage you put into your bridge. which will be probably 8.9V if you use a 6.3V tap. This needs to be dropped back down. There are a couple of ways to do this. You can use a very small value of resistor, and fiddle with it until the voltage and the current balance out to what you need. -- Say a 12AX7 takes 300mA at 6.3V, so use V=IR and figure out what size resistor you would need. If you have other tubes, you will draw more current in parallel, so account for that.  
You can also use Diodes. Si will drop about 0.6V depending on how much forward current you put through it. As long as you don't exceed the forward current rating of the diode, you can string them up until the voltage is within 10% of 6.3V.  
You could probably use a zener diode too. a 3V one in series would get you within the ballpark, but the wattage you run may make this impractical (read expensive)  
I wouldn't run DC heaters on your power tubes, only the preamp ones. You will probably stay under 1.5A this way, which is easier to design for-- smaller and cheaper components.  
Hope this answers your question,  

Cliff Hey Brian.. Thanks for the info...a... -- 7/24/2000 3:40 AM