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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.
Follow Ups:
I've seen it done both ways. Some amps use a voltage divider off the B+ to apply a + voltage to the heaters. Others (that are cathode biased) use a direct connection from the cathode of the power tube(s) which is also a + voltage.
I have also seen some as you describe, that use the - bias supply and apply that to the heaters through a high value resistor, biasing the heaters negative. And these were 40's-50's guitar amps. This seems counterproductive to me, though. I always thought you needed a + voltage to stop the heaters from emitting electrons in tubes with cathodes.
I posed this question in a couple of different places, and I got an answer from someone who claims to have read some early documents on the practice from the 1930s. He also claims to have verified their explanation experimentally.
He says the reason why it works is that heater-cathode resistance is at a minimum with 0V between the heater and cathode, and that as you introduce a voltage difference (positive or negative) between the heater and the cathode, the heater-cathode resistance increases to a maximum plateau at 10-20V difference. The key is keeping the AC waveform out of the region of minimum h-k resistance.
I'm only the messenger here. Does that ring any bells for anyone?
From the 1930's !
Have fun
Willie
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