• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Filament voltage for UX-201a tubes

I've never even seen one, but it's a tube from 1925 meant for battery-powered radio receivers, with a storage battery (presumably a two-cell lead-acid battery) for the filament supply. One would normally connect a rheostat (variable resistor) in series with the filament to control the volume, rather than let it run at the specified maximum filament voltage all the time. With the later oxide-coated tubes, running at much reduced heater voltage would be a way to kill the tube, but for these thoriated-tungsten tubes, it increased their lifetime. Reducing the filament voltage will reduce their transconductance.

http://www.r-type.org/exhib/abo0012.htm
 
Typical 1920's receiver with a 30 ohm rheostat for volume control. You can also switch the number of audio stages.

IMG_20230528_070228.jpg
 
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I've never even seen one, but it's a tube from 1925 meant for battery-powered radio receivers, with a storage battery (presumably a two-cell lead-acid battery) for the filament supply. One would normally connect a rheostat (variable resistor) in series with the filament to control the volume, rather than let it run at the specified maximum filament voltage all the time. With the later oxide-coated tubes, running at much reduced heater voltage would be a way to kill the tube, but for these thoriated-tungsten tubes, it increased their lifetime. Reducing the filament voltage will reduce their transconductance.

http://www.r-type.org/exhib/abo0012.htm
I used 201a before in a preamp with LCL filament supply.

It sounds I can lower the filament voltage a little without worrying -in fact, this will even prolong tube life. This will make tube change in the future easier - no need to tweak filament supply in order to target 5v after tube change.
 
Hi
I have used these fabulous tubes before as AF amplifiers and despite being very old, they are fabulous DHT valves. I used a CCS (240mA - 245mA) to power the filaments. Each valve CCS can float as well. I noticed that Gm begins to drop out below 235mA. I also noticed that the filament voltage did vary a bit between different valves with the same current. I used a large capacitor (1mF) across the filament in conjunction with the CCS to limit dI/dt on power up and power down. My logic here is to lower thermal stress to the filament on power up/down. This may or may not make a difference, others here are more knowledgeable about this, but it is easy to implement with the CCS. The filaments are the most vulnerable part to being damaged. Starving the filament is not good either. I would not operate below 240mA

The datasheet recommendation for B+ is 90V for use as RF amplifier and 120V for use as AF amp. I've found that the sweet spot for these used as AF amp is at 2.5 to 3mA bias, 120V. Hope this info is helpful, good luck with your build.🙂


I have 5 in total. 3 that were made by GE for RCA between '26 to '28 as they have the bakelite base with long brass pins. 1 is made for RCA after '28 with nickel pins, and 1 is brand name Cunningham (made by GE, I think) with nickel pins. All globe type. These will most certainly find other uses for me as they may be one of the best amplifying devices ever created, IMHO.😀

Three ways they can be destroyed.
--Physical breakage. They are fragile, like an old light bulb and must treated as such.
--Overvoltage, arcing out. This is unlikely, as the circuit should not be capable.
--Most likely failure is blowing out the filament.
Otherwise, theoretically these devices don't really have a lifespan limit. They really are fabulous!
 
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My favorite preamp tube is the UX-201A. They are in my DIY preamp, which I built with 26s but switched to 01As. IMO, the ideal way to power the filaments is with Rod Coleman regulators, they sound fantastic and provide a soft start for the filaments as well. Lots of threads about them here on DIYAudio.

26 Preamp and PS.jpg
 
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In terms of sound, my favorites are the RCA UX-201A which IMO have a slightly more euphonic sound than the Cunningham CX-301A. I know they are supposed to be the same tube but I do consistently hear differences (not blinded, though).
 
01A filament voltage should be as close to nominal as possible. This is necessary for optimal filament temperature, which is itself critical for filament regeneration. At lower temperature, thoria diffusion to filament surface slows down, so thorium surface layer is gradually depleted. At higher temperature, surface thorium is evaporated faster than it is replenished from the inside of the filament.

There are several reports (Steve Bench comes to mind) that filament starving improves tube's linearity. Because starved operation causes gradual degradation of emission, tubes should be regenerated from time to time by running filament for several hours at nominal voltage.
 
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