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Heater Bias with Rod Coleman Fil Regs
Heater Bias with Rod Coleman Fil Regs
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Old 29th September 2021, 03:25 PM   #31
Rod Coleman is offline Rod Coleman  United Kingdom
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Heater Bias with Rod Coleman Fil Regs
IMHO the difference in sound between 300B types is smaller than difference between good and EXCELLENT OTs, anode supply, Filament supply, and driver stage design and power supply.

So if you don't have a Sowter, or Monolithic OT, I would start there. OTs - even high cost ones are good value, and last a lifetime.
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Old 29th September 2021, 05:02 PM   #32
Cagomat is offline Cagomat  Germany
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IMO the most important criteria of amplifier design are:
1.) Power supply
2.) Power supply
3.) Power supply
4.) Driver stage
5.) OPT
6.) Power tube
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Old 1st October 2021, 10:48 AM   #33
goddlediddles is offline goddlediddles  Ireland
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Heater Bias with Rod Coleman Fil Regs
Default How does it work?

Hi Rod. I can't find a comprehensive description of how your filament bias device works. Could you direct me to an explanatory article with a link? Thanks.
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Last edited by goddlediddles; 1st October 2021 at 10:50 AM.
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Old 2nd October 2021, 10:43 AM   #34
Rod Coleman is offline Rod Coleman  United Kingdom
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Heater Bias with Rod Coleman Fil Regs
Filament Bias is a simple technique to apply cathode bias, but with a low-value resistor. The goal for doing it is to have a low enough value of cathode resistor, that the bypass capacitor is no longer needed.

For valves with reasonably low filament current, and low bias voltages, it is a good solution. But for something like a 2A3 (45V bias, 2.5A current: = >110W burned in the resistor) the heating power in the filament bias resistor is far too much.

Before building any stage with filament bias, be sure to check the power burned in that resistor. If the power is too high, grid bias, aka 'fixed bias' is the best solution that allows the bypass capacitor to discarded.

IMPLEMENTATION: My application note for Filament Bias - giving instructions on how to build it for the 26 or 4P1L, including transformer choices and Raw DC design, and a sample preamp circuit are given here:

http://lyrima.co.uk/pdf/FilamentBias_Supply.pdf
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Old 2nd October 2021, 11:40 AM   #35
goddlediddles is offline goddlediddles  Ireland
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Heater Bias with Rod Coleman Fil Regs
Quote:
Originally Posted by Rod Coleman View Post
For valves with reasonably low filament current, and low bias voltages, it is a good solution. But for something like a 2A3 (45V bias, 2.5A current: = >110W burned in the resistor) the heating power in the filament bias resistor is far too much.
Thanks for that. So it looks like the 300B might be out too?
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Old 2nd October 2021, 12:41 PM   #36
Rod Coleman is offline Rod Coleman  United Kingdom
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Heater Bias with Rod Coleman Fil Regs
Yes for 300B, auto-bias or grid bias are the practical choices.
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