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Old 26th May 2007, 07:27 AM   #1
G is offline G  United States
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Default Question about biasing heater above cathode in a direct coupled circuit.

Hi all. I have a direct coupled version of a 12B4A amplifier that I would like to build. The cathode of the output tube(12B4A) is sitting at 174v above ground. The input tube has the cathode sitting at 6v above ground. I would like to bias the heater of the input tube above the cathode to cut down on hum. Since the cathode of the ouput tube is so far above the heater relative to ground do I need to worry about it producing hum? I suppose I could just use DC on the heaters but I would prefer not to. I think biasing the heaters up to around 50v will be ok but I would like some opinions. Thanks.
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Old 26th May 2007, 11:42 AM   #2
EC8010 is offline EC8010  United Kingdom
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The ideal is to make the heater 40V positive with respect to the cathode. You need two heater supplies; AC will be fine so long as you do it properly. You definitely need to bias the 12B4A heater to > +174V, otherwise you will get noise (even on an insensitive stage) and possibly eventual heater/cathode insulation breakdown.
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