Filament Bias and supplies

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RhythMick
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Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

#1 Filament Bias and supplies

Post by RhythMick »

In a fit of enthusiasm I decided I'd like to experiment with filament bias. I'd like to make sure I've got my head properly around the supply requirements.

In particular looking at the 01A and 71A valves. Both are

Vf = 5v
If = 0.25A

Starting with the 01A, aiming for Vg = 7.5v and Ia = 3mA, so

Rfil = 7.5v / 0.253 = 29.6. A 30R will do it. It will dissipate nearly 2W, so picking a 10W WW.

I have spares of Andrews marvelous DC boards but I need to get my head around the supply for it. The filament supply will connect across the 01A filament and Rfil in series so with 5v across the filament and 7.5v across Rfil the board would need to output 12.5v. A 12vrms transformer will give 17.5vdc peak so allows for the approx 4v lost in regulation. Seems this might work. Anything I've got wrong?

The 71A with it's higher bias will need a higher voltage input to the board. Say I'm aiming for 20v at the cathode plus 5v Vf, the board would need to output 25vdc and so would need at least 21vrms input. Vfil would be 75R, I'd use 20W.

It's always possible I've completely screwed this up in my head, so posting here for comments and a cross check before I try it out later.
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

#2 Re: Filament Bias and supplies

Post by RhythMick »

Just re-read Andrew's introductory post on the DC boards and the caps would need to be higher voltage than the default. Off to check what I installed.
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

#3 Re: Filament Bias and supplies

Post by RhythMick »

Ok I think I'll...

Remove the caps and diodes from the boards, bridging from inputs to cap +/-
Put the transformer, diodes and caps in the off board PSU box
Use an 01A PSU feeding 2 boards at 0.25A each
Use a 71A PSU feeding 1 boards at 0.25A each

Works out nicely using 50VA toroids 2x12v I have in stock. The 01A can use the secondaries in parallel, the 71A can use them in series.

That's this afternoon's proof of concept then...
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