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DC heat float with 50V bias tap from mains transformer?


2004-01-04 8:41 pm
I am building an amp with a LM1084 DC heater supply. It is feed by a 12.6V transformer with a center tap.

My main HV transformer has a 50V bias tap. If I put a diode on the 50V bias line and connect it to the CT of my filament transformer with this lift the DC output of my DC heaters ? Or would I do better to just use the normal method with a voltage divider off B+?

Rod Coleman

Paid Member
2004-07-25 6:55 pm
Might work well.

50V ac should yield about 70Vdc which is a good enough choice for most cathode-heater back biasing.

One advantage is that no dc current is required from the winding, so you can use a cheap 1uF 100V MKT [or nice MKP if you want to ride first class] for a filter cap. UF4007 rectifier. Connect the negative of the bias supply to GND/positive to the heaters via a common-mode choke. The choke should have the highest inductance you can find, since there's no current.

The noise from the LM1084 will exceed the raw dc's in this case, probably!

Using a bipolar transistor and a zener [power-follower connexion] would do at least as well.