Tube DIY Asylum

are you using a regulated AC supply?

68.179.167.19

In Reply to: RE: 300B filament tranny posted by ecir38 on June 02, 2008 at 15:12:45

If not you may be over thinking this. The AC coming into your home will vary quite a bit over time and if not regulated so will your filaments. Even if you get it adjusted to 5V it could be higher or lower shortly after when the line changes. But it can be done for a given line voltage.

Look at it as a series circuit with the Zout of the transformer in series with the filament and the extra R you are adding. If you know the output Voltage at full load and no load you can calculate it.

---Take the difference in the no load and full load voltages and divide by full load current and that gives you the output impedance of the transformer = Zout

---With 5V and a draw of 1.2 A the filament is about 4 ohms

--- multiply Zout by 1.2 amps to get the transformer drop and add the 5V filament drop to that

--- subtract that V from the unloaded secondary V and that is the extra V you need to drop.

--- take that extra V and divide by 1.2A to see what value R you need.

Zout of the transformer is temperature dependent but you'll be real close.

Follow Ups:

RE: are you using a regulated AC supply? - ecir38 15:31:36 06/03/08 (0)

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