Tube DIY Asylum: RE: Filament bias for DHTs - how to do it? by Thomas Mayer

Hi Andy,

this is how I do it:

http://img139.imageshack.us/i/filamentbias.jpg/

The resistor is sized such that the voltage drop caused by the filament current provides the desired bias voltage. Actually both the filament and plate current flow through the resistor but usually the palte current is much smaller than filament current and is negligable for the calculation for the resistor.

In case of the 3A5. If you connect the filament halves in parallel and use a 10 Ohm filament resistor, the volateg drop will be 2,2V.

The filament supply voltage supply source needs some means for adjustment, so you can set the right filament voltage. If you want both the filament and bias voltage adjustable, the filament bias resistor needs some means of adjustability too. I don't like a pot at that position, so if I want adjustability of the op point, I make the B+ adjustable instead and set it for the desired current.

a) that's a centre tap example, so what about a normal DHT filament?

No difference

b) if I wanted to raise the cathode voltage above 1.4v where do I put the resistor or resistors in the case of a centre tap - between the 1.4v and the cathode or from the ends of the filaments to earth or what?

filament supply to pin 4 and pins 1 and 7 to the filament bias resistor. A resistor of about 6,3 Ohms will give a voltage drop of 1,4V

c) can I run more than one 3a5 off the same 1.4v DC supply? Is there crosstalk or is this negligible?

If they are separated by chokes as shown in my schematic or current sources, that can be done.

Best regards

Thomas

Filament Bias (Open in New Window)