I've got the Rod Coleman document "FilamentBias_Supply" and notice he recommends a -9v bias for the 4P1L -- it looks like that is implemented via the Coleman Regulators by adding more voltage headroom. And I see that there are filament bias resistors, etc. on the main 4P1L Linestage boards. But I'm confused as to how this "Fil Bias" circuitry fits in, if it does.

 RE: Question About 4P1L Kit Filament Bias PSU Circuit - KevinC 16:16:33 04/02/16 (1)
 RE: Question About 4P1L Kit Filament Bias PSU Circuit - Cappy 16:33:39 04/02/16 (0)

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 RE: Question About 4P1L Kit Filament Bias PSU Circuit - KevinC 16:16:33 04/02/16 (1)
 RE: Question About 4P1L Kit Filament Bias PSU Circuit - Cappy 16:33:39 04/02/16 (0)

//The fil bias pad isn't used for a directly heated filament.//

Ok, makes sense.

What is C2 doing?

//The filament is the cathode, so the value of the cathode resistor and the B+ voltage determine the cathode voltage (and the cathode current). I haven't read that particular document, so I don't know specifically what Rod is referring to, but with the values of B+ and cathode resistor I have used, I get a cathode voltage of about 8V-9V, positive relative to ground.//

Which is exactly what Rod shows in his diagram in that document. Similar cathode resistor values too... But the -9V bias he mentions is confusing. It's just for my intellectual understanding since I'm building away happily anyway...

• Rod Coleman FilamentBias_Supply document