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Filament voltage ?
Filament voltage ?
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Old 17th July 2017, 11:29 AM   #21
woody is offline woody
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Filament voltage ?
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.
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Old 17th July 2017, 11:47 AM   #22
Kay Pirinha is offline Kay Pirinha  Germany
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Quote:
Originally Posted by woody View Post
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!
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Old 17th July 2017, 11:55 AM   #23
woody is offline woody
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Filament voltage ?
DHT stands for directly heated triode which I believe are always thoriated tugnsten .
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Old 17th July 2017, 01:39 PM   #24
Kay Pirinha is offline Kay Pirinha  Germany
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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!
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Old 18th July 2017, 02:26 AM   #25
Wavebourn is offline Wavebourn  United States
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Filament voltage ?
Quote:
Originally Posted by DF96 View Post
Did the Mullard document ever get translated? My interest is not so much audio as running a 6AT6/EBC90 at reduced heater voltage to get better AM results from the diodes. Will this shorten valve life?
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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Last edited by Wavebourn; 18th July 2017 at 02:29 AM.
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Old 18th July 2017, 05:42 PM   #26
6A3sUMMER is offline 6A3sUMMER  United States
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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).

Last edited by 6A3sUMMER; 18th July 2017 at 05:45 PM.
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Old 18th July 2017, 10:37 PM   #27
MelB is offline MelB  Canada
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This is good:
http://www.tubebooks.org/tubedata/HB...f_Cathodes.pdf
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