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Some years ago I built a common cathode line-amp around a CCS-loaded Valvo B406 (20-ties)DHT Triode.
Hovever I never got it quiet enough for my taste, despite using battery glow.
Most annoying where the RF-hum just by moving my hand above (not touching) the DHT´s. And also some sort of audiable ringing in the speakers.
The only solution I came up with where to wrap those beautifull DHT´s with a grounded aluminium foil (since the B406 has a 4V/100mA filament, and a 120V/0.008mA = 1.4W disipation the foil did not cause any overheat för the tube).
Does anyone has any similar problems, and came up with any ideas how to solve it. I understand that this problem is very common when using B406.
Follow Ups:
Try running them at 3.5 volt filament and 2mA dissipation.
Other than that, you can try Fancy Shields (something that looks better than aluminum foil.)
Some data:
- Operation Point: 120V/8mA
- Chatode bias (with e-lytic bypass)
- Grid stopper at socket and 100k stepped attenuator at grid
- Anode load: CCS (single IXYS 10M45)
- B+: 150V Well filtered (C-L-C-L-CCS(Single IXYS 10M45)- 150V Cold chatode voltage regulation tube.
- C output cap: 0,470uF with 1M pull down resistor
- Filament Power: 4V (voltage regulated) battery
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