I believe Dynaco even had a case where they ran a 6.3 v tube at 5.7 V. And Steve Bench had a fine discussion on his old web page about running DHT at reduced heater voltage. He found that running a DHT at reduced voltage could result in lower distortion! I believe in some of his test running 5v DHT tubes at about 3.7V produced the lowest distortion. It was several years ago when I read about that and sure wouldn't swear those figures are correct. I don't think his web page is available now.
Did these DHT's feature thoriated tungsten or barium oxide coated cathodes?
Best regards!
An AD1, e.g., which is a DHT, features an oxide coated filament/cathode.
Anyway, there's a great difference between both technologies: A thoriated tungsten cathode may tolerate underheating more easily than an oxide one, as in an underheated operation of the latter one oxygen is released and accumulated to it's suface, a phenomenon which is called oxygen cathode poisoning.
OTOH, overheating it will evaporate elementary barium, thus directly shortening the tube's life.
I think it's the best way to second the tube manufacturers' advice to keep heater voltage (or heater current in series heated tubes) within ±5% of it's nominal value.
Best regards!
Anyway, there's a great difference between both technologies: A thoriated tungsten cathode may tolerate underheating more easily than an oxide one, as in an underheated operation of the latter one oxygen is released and accumulated to it's suface, a phenomenon which is called oxygen cathode poisoning.
OTOH, overheating it will evaporate elementary barium, thus directly shortening the tube's life.
I think it's the best way to second the tube manufacturers' advice to keep heater voltage (or heater current in series heated tubes) within ±5% of it's nominal value.
Best regards!
Diodes shift horizontally volt-ampere characteristics' curve depending on cathode temperature, it is a different story. Also, I saw how EF86 were biased by a single 4M7 resistor in grid leak, with 5.6V filament.
Edit: wow! Thank you for the idea. I am going to adjust bias of 6S17K by adjusting DC filament voltage. ;-)
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woody,
DHT has 2 types of filaments:
Thoriated, i.e. 811, 801, etc. (the Thorium is Infused into the Tungsten)
You can generally easily spot these, because with the Correct filament voltage,
they glow White.
Oxide Coated, i.e. 300B, 45, 2A3, etc. (the tungsten filament wires are Coated)
You can generally easily spot these, because with correct filament voltage,
they run cooler, and usually glow a mild Red.
The 5Y3 is an example of a Coated filament rectifier (no cathode).
DHT has 2 types of filaments:
Thoriated, i.e. 811, 801, etc. (the Thorium is Infused into the Tungsten)
You can generally easily spot these, because with the Correct filament voltage,
they glow White.
Oxide Coated, i.e. 300B, 45, 2A3, etc. (the tungsten filament wires are Coated)
You can generally easily spot these, because with correct filament voltage,
they run cooler, and usually glow a mild Red.
The 5Y3 is an example of a Coated filament rectifier (no cathode).
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