• 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.

01A question

It's transparent, airy, almost as if it isn't there at all. I found it quite magical. Better than a 26 preamp with LL1692 in 2:1. Better than a 4P1L preamp. That's a lot of "better".

But be clear that this is a complex design which needs to be built EXACTLY according to the schematic - absolutely no short cuts. Every component is there for a reason.
 
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The AZ1 mesh is audibly better than the solid plate version, so technically it's the mesh. No idea why.

I think with globes and ST versions it depends a lot on the state of the tube - some of these are almost 100 years old.

Basically the 01A has a thoriated tungsten filament, like the 801 and 10Y and some other highly valued tubes. I'm sure somebody could say why this makes a difference - not sure myself.

Actually it's a bit more complex. You can tell by applying voltage to the filament. Thoriated Tungsten will be much brighter than coated cathode type. Original 201's were Tungsten. These draw 1 amp and light very bright. Later tubes were marked 201A or 01A and used Thoriated Tungsten. These drew .25 amps and were not as bright but still very noticeable. More recent 01A's used coated cathodes. Some of these can barely be noticed in lighted areas. Believe these tubes should have been marked 01AA but not everyone did this. If you find tubes marked 01AA they have coated cathodes but so do later 01A's.

01 = 1 amp filament
01A = 250 ma filament
01B = 125 ma filament
01C = 60 ma filament

More information and pictures here:
http://vinylsavor.blogspot.co.uk/2013/10/tube-of-month-ux201a.html
 
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Don't forget to mention, that 01a is very sensitive to the load.
My breadboarded 201a preamp (SSHV2, 22R filament bias -0.25A Rod Coleman regulator- , cascode CCS load- 5mA-, 0.47uF - 100k load) was able to fall 3dB at 10kHz due to the 5m coax cable (connected to the oscilloscope)!