That's one way for sure, as input capacitance doesn't matter with low impedance source. But I feel this approach is a little awkward. Simplicity is beauty. And who knows what would be the sound of paralleled 01As. 2A3 monoplate vs. bi-plate comes to mind...
A "super 01A" is close to a parallel pair of regular 01As.
All good interstage transformers designed for ~10K tubes have DC limit of 4-8 ma, which fits single 01A.
Last edited:
I feel I'm overlooking something! What exactly is a Super 01A? A selected high performing 01A, a specific brand or variant (Super Airline, Super Silvertone?), or a different tube altogether?
I have only one sample of this tube, Silvertone O-T-3. On the box it says 5 V 0.25 A filament and maximum plate voltage of 150 V. The filament has bright glow of typical 01A. It has Gm of 1,700. The strongest 01As that I have are 1,200.
How high Gm is achieved in O-T-3 I have no idea. The high Gm (1,700) successor of 01A, Type 112, has M filament instead of A filament, which makes 112 akin to a parallel pair of 01As (but with the same grid-to-plate capacitance). Accordingly, the TT filament of Type 112 is 5 V 0.5 A. The TT filament of 112 is more robust and longer lasting. Only limited numbers of 112 have been produced - they switched to the coated-filament Type 112A (5 V 0.25 A).
As a side note, 01As may have coated filaments. Sometimes they are marked 01AA, but other times bear the 01A markings. Their 5 V 0.25 A filaments have dark red glow.
Is there a datasheet available for O-T-3; I couldn't find one online. Just wondering if it's meant to test that strong. I have a globe and a couple of ST shape 01a which test rather high. I figured the filaments hadn't been broken in properly during manufacture and that they would settle down to normal ratings after a period of use. Not used them in circuit.
That is quite possible. TT filament takes a while to equilibrate to its stationary condition under the optimal temperature of 2,000 K. The eq is between thoria migration to the surface, reduction of thoria by carbon to metal thorium on the one hand, and evaporation of thorium from filament surface on the other hand. I've read somewhere that 1 hour burn at nominal filament voltage, without HV applied, is enough to equilibrate 01A filament. Accordingly, I do 1 h burn-in before testing my 01As.
If filament is run at higher than optimal temperature (2,100 K is already too high), surface thorium evaporates faster than it is replenished, and tube's emission decreases pretty fast. At lower temperatures, thorium evaporation slows down, but thoria diffusion slows down even more, so eventually emission decreases, albeit much slower than with overheating. Luckily, in both cases the loss of emission is reversible by 1 h burn at 2,000 K. The filament loses emission permanently after all its carbon is depleted.
I looked hard for O-T-3 data sheet - without success. For several years, I also looked for another tube of this kind, also without success.
Last edited:
I have to correct myself on Type 112 having a TT filament.
I just received two Type 112 globes, one RCA and the other Cunningham. They look exactly same even in minor detail. The filaments glow dark red under specified voltage and current (5 V 0.5 A). The filament is a band, like that of 2A3, and I could see whitish coating under magnifying glass. So, they have coated filaments.
https://nvhrbiblio.nl/biblio/boek/Stokes - 70 Years of radio tubes and valves.pdf
Other innovations during the period were the use of an ‘M’ filament by Perryman in January 1926; the introduction of low-drain tubes, known as type 20IB, by CcCo and Sonatron in 1928, and the use of oxide-coated filaments in 201A type tubes by Arcturus and National Union in 1933.
I wonder if Perryman RH201A after January 1926 have any higher emission?
Other innovations during the period were the use of an ‘M’ filament by Perryman in January 1926; the introduction of low-drain tubes, known as type 20IB, by CcCo and Sonatron in 1928, and the use of oxide-coated filaments in 201A type tubes by Arcturus and National Union in 1933.
I wonder if Perryman RH201A after January 1926 have any higher emission?
Id LOVE to hear other's opinions, I'm still yet to hear an 01A!
I'm still buying a few bits and hoping to start a build soon.
At post #133 bs cites Stokes' "70 years of radio tubes and valves" on page 31 of which is a picture of a Silvertone O-T carton similar to that of your O-T-3. Stokes's picture caption cites the Silvertone O-T as a "bootleg" type made without the benefit of a patent license. Silvertone tubes were sold through Sears & Roebuck (like Silvertone radios) but the (bootleg?) manufacturer(s) are not known. Ditto for Sears and Roebuck's WLS branded tubes. Stokes records that there were many "bootleg" manufacturers, for whom data sheets may have been a low priority.
