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    Building, troubleshooting and testing of these amplifiers should only be
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Filament Voltage vs. Hum

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Filiment Voltage v.s. Hum

While searching for information of 6A4/6B4G amp designs I came across this statement.

the lower filament voltage means less trouble with residual hum, when AC heating is employed

I would have thought that lower voltage ==> higher current would be worse for hum. I guess it might be different for IDH tubes. I was thinking magnetic induction where current is the issue but I guess the mechanism in DHT is different. Can someone elaborate on the mechanisms for hum production in DH v.s. IDH tubes or point me to a source?

Opinions on the relative merits of the various reasonably priced current production DHTs?

Yes, this is true that the 6B4G has higher hum level than the 2A3. In order to keep the hum to minimum level for 6B4G, it is better to use DC for the filament.

For 2A3, I could get the total hum level at the speaker down to 1.5 to 2.0mV which is very quiet for my 93dB Tannoy speakers.

Joined 2003
6B4G will always generate more hum than a 2A3 because...

Both valves are directly heated. If you assume that the middle of the cathode/filament is at zero volts, one end goes up while the other goes down. You might think that the two effects balance out, but they don't because what you're doing is modifying the grid to cathode voltage, and that varies anode current by a 3/2 power law. Because of that, you impose a 100Hz (120Hz in 60Hz countries) hum on the anode signal. The higher the heater voltage, the higher the variation of Vgk and the more hum you get. Hence, 6B4G (6.3V heater) produces more hum due to this effect than 2A3 (2.5V heater).

Another cause of hum is that the temperature of the filament can start tracking the applied waveform (AC mains). Thinner filaments do this more easily than thick, so the 6B4G is also worse in this respect because it requires a thinner filament for the same heater power.
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