Vinnie,
Glad it is sparking interest and really bringing you some joy. Especially awesome when the tubes are usually less than $75 per pair NOS (assuming just RCA or Cunningham).
All of the 01A tubes are basically the same tube (except for really early brass base ones with short UV pins (like the WE 101Ds), they are UV-201As). The 01 (not 01A) would also work in your circuit I am sure as they are the same tube with a 5v, 1A draw, but they also cost about 4 times the price and were last made in 1925 or so. Anyway, the 01A tubes were rebranded and made by about 100 different makers. 01A, 201A, 301A are all the same. UX-201A, CX-301A, OK-201A, etc. All the same tube... Cunningham and RCA are exactly the same in this time frame as the RCA v. Cunningham suit had concluded and RCA was making all Cunningham tubes. Those are the most commone ones. The ones to really look for are the ones with the oddball labels or boxes (if you get boxes). Those are collectible, and so are the ones with pointy tops where the glass is peaked. In any event, watch out buying them. They generally do not sound different than any of the others (except those that are microphoic or noisy and there are no real fancy makers of this tube (there is allegedly a Northern Electric version from Canada, but I have never seen one, and an Arcturus, but it isn't blue so I am not sure it commands a high price), and many of these tubes can be noisy at full filament voltage. Starve them at 4 volts and you should find sonic bliss regardless of the label. All that said, some are occassionally too noisy to be saved.
I have about 30 of them for my DIY linestage project (which should be coming together in time). The other great thing about 01A tubes is that you can regenerate them if they start to get worn out. That discussion is for another time, but man do I love those tubes. They will never be the tube of choice for an output tube because of low power. However, they only spec 1.25 Watts (5v x .25A) of power on the filaments to operate. If you run them at 4 volts, it's 1 Watt. I think that you will barely notice a sonic difference in the tube, but you will notice a lack of noise. Battery tubes can be awesome (and cheap!).
The only two other DHT tubes made that I like for preamp work that your line stage won't take are the 26 and the 10Y. I could be happy forever with 01As though. The 10Y is very different, but it needs 7.5 volts on the filament. The 26 only takes 1.5v (at 1.05A)! Making a linestage that includes all three of those tubes is my goal. That is my project, but I think it will involve a seperate power supply and possibly two different DHT modules. Sigh... I am following your project closely as it is the natural follow through to my DIY project if it fails.
I also love the 4P1L (DHP (direct heated pentode) that can be triode strapped), but it is a loktal socket, wrong voltage (only 2v) and is the most microphonic tube I have ever heard.
Shawn
