WHAT HAS GONE BEFORE
During the past few months I have been hopping from one tube to another trying to find something I would really like. Then one weekend I made up mind mind once and for all. When I started thinking about my power amp I had in mind the 300B. It has gotten a lot of hype the last 2 years and several companies have started manufacturing it again. Looking through some magazines and talking with people I found out there were other tubes out there besides the commercialy overrated designs like the 300B. It seems everyone is building their amplifiers around this tube these days.
MEETING THE 45
Some people I knew brought a few other tubes to my attention. Amongst these were the 45 and 50. The 45 could muster a whopping 2 Watts at 280V/36mA. A little tube with a little power but a big reputation. For my soon to be Lowthers this was reportedly more than enough. With an efficiency of >100dB just a single Watt would bring the sound up to deafening levels as people told me. I had a listening session with these speakers a while back and that would indeed seem to be the case. The 50 was another famous tube reported to have superb sonic qualities, some even prefer it above the 300B. It can produce upto 4.5 Watts around the same settings as a 300B. It is a little more tricky to drive though and it was produced for only a few years so availability will be small.
After tossing and turning for a few nights I decided to go with the 45. It had enough power and was readily available. After making up my mind I went to the drawingboard to make a design for the amplifier. I will be using an interstage transformer with grid biasing, so the output stage is not that hard. Hook the grid up to the IT, the plate to the OT and the cathode to the FT (fillament transformer) since I'm using an AC heater supply. All I have to do is make an adjustable bias supply and get the right voltage onto the plate.
The driver was going to be the 31, yet another DHT. It has a low Rp which makes it very suited for the interstage transformer, although it was meant to be used as an output tube at the time. It's mu is just high enough to be able to drive any of the tubes I was thinking of using as an output tube. Only recently have I heard from several people that this tube is very microphonic, and I am not too keen on this attribute. Special supports and dampening would remove much of this, but it would still be present. One con against a several pro's, so I decided to go with it anyway.
I had already bought a few pairs of 45's for my amp some weeks ago, so I now had spare tubes. What was I going to do with these "regular" 45's now that I was going to use the VT52 in the output stage?! I remembered that Takuma used output tubes like the 300B and 845 in his amps to drive his output stages; this would make the driver stage more linear. That's where I got the brainwave to use my leftover 45's as drivers. They are even better at this job than the 31's I had originally intended (no microfonics). So once again my entire design was turned upside down. Back to the drawing board for the n'th time...
I don't sell any of the tubes listed below! These are just the amounts you would pay for the tubes were you to look around on the WWW...
NOS: tubes are new and in box
NOS ST $40-70
Looking for people who were selling the 50's I found out that prices for a run-of-the-mill tube was around $250 for a NOS tube. This puts it just below the WE300B and VV300B valves. Some used but good tubes can be found for about half that amount if you look for them. Be carefull since prices vary enormously for this tube, upto 100% !!!
NOS ST $175-250
The VT52 is expensive, it hasn't been made this last half century I think. It is not very rare to find this tube though, just hollar and have your second mortgage ready. Some people will try to sell these tubes at $175+, forget it, don't do it!!! You can get these tubes NOS for around $70-100. I've had a number of tubes offered to me around these prices. Beware of the differing brands though! WE type VT52's are reported to have 7V heaters, other brands use 6.3V, so not all brands are interchangable...
SPECS FOR THE DIFFERENT TUBES
"seg" refers to the number of fillament sections, i.e. the shape of the fillament