Filament DC bias

Tube DIY Asylum

Do It Yourself (DIY) paradise for tube and SET project builders.

I was on another forum where someone posted a simple 1940s guitar amp that appears to have been modified at some point from cathode to back-bias. This included attaching the 6.3V filament winding center tap to a *negative* voltage node.

Like all of us, I assume, I'm familiar with the idea of attaching the filaments to a positive DC voltage to reduce hum, but I've never heard of attaching them to a negative DC voltage relative to cathode voltage. Someone else on the forum is suggesting that this is the reason for this mod. Is that a valid point or not?

I know that you're supposed to elevate the heaters to the point where no part of the AC waveform is crossing the cathode voltage, but does the reverse apply as well?

If it matters, the input stage is a contact (grid leak) biased 6SJ7 with the second stage a cathode-biased 6SJ7.

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Topic - Filament DC bias - Thermionic27609 08:02:31 05/02/15 (3)

RE: Filament DC bias - Alpha Al 11:56:59 05/02/15 (2)

RE: Filament DC bias - Thermionic27609 18:16:06 05/02/15 (1)

RE: Filament DC bias ... maybe like this... - drummerwill 17:49:29 05/04/15 (0)

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