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Old 02-14-2018, 04:12 PM
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benman94 benman94 is offline
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Join Date: Jun 2011
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Quote:
Originally Posted by maxhifi View Post
In the context of modern designs, it's possible to use DC on the filaments of tubes like the 2A3, and get rid of that issue. I know some people almost religiously stick to AC heated filaments, but in my 2A3 amp I used DC and am very happy with the result.
The bias is then off somewhat along the length of the filament. If the reference to ground (or the self bias resistor) is halfway between the ends of the filament, then the bias is only shifted by 1.25 volts, which is negligible. If one end of the filament is grounded (or tied to the self bias resistor) now our bias is off at the other end by 2.5 volts, which, with a high perveance type valve, ought to be avoided. Still, for the 2A3 the DC filament situation is manageable.

The situation for the 6A3 and 6B4G is worse. 3.15 volts if the filament is tied to ground or the self bias resistor at the midpoint, a whole 6.3 volts if one end is grounded. This is non-negligable just from looking at the plate curves...

Still, a 2A3 on DC heaters, while more likely to fail prematurely does sound nice.
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