VT52 #2


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.

Different 45's
A week or so later I found out that there are several different versions to the 45. There is a special version of this tube named accordingly 45 Special, or 45 SPL. It can handle a higher plate voltage, a higher plate dissipation and has a different fillament. There is also a military version of the 45 SPL called the VT-52, this is the same tube but just has a military number. Two other denominators exist for the 45 SPL, namely 2C45 and 38142, although I have never seen these numbers appear in any ad or conversation. The regular 45 also goes by the name CV610, for those who want to know... The expanded specs of the 45SPL/VT52 looked very interesting to me. I would be able to get 3.5 to 4 Watts out of a single tube now, just that little bit of reserve power for my speakers, just in case they would need it. Those 2 extra watts mean a 3dB increase in output ;) It took me about 2 days to convince myself to use these instead of regular 45's considering that prices for a VT52 reflect the doubled output power; double the price...

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...


PRICING

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
Used: tubes test well above minimum

45
Prices depend on condition, brand and the mood of the seller. Try and get tubes from private collectors or other DIY-ers and HAM-ers, they ussualy don't care as much about making a buck...

NOS ST $40-70
NOS Globe $70-100
Used ST $15-40
Used Globe $40-70


50
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
NOS Globe $200-350
Used ST $100-175
Used Globe $100-200


VT-52
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...

NOS $75-125
Used $40-75
NOS WE $150-300 (these are very expensive only for name's sake)
Used WE $75-150


SPECS FOR THE DIFFERENT TUBES

Uh Ih seg Ua Ia Ug Rp S mu Pd Zo Po
45 2.5 1.5 VV 275 36 -56 1700 2050 3.5 10 4600 2.0
VT52 6.3 1.0 VV 300 44 -61 800 ? 3.8 18 2500 4.0
6A3 6.3 1.0 VVV 250 60 -45 800 5250 4.2 15 2500 3.5
50 7.5 1.25 VVVV 450 55 -84 1800 2100 3.8 20 4400 4.6
300B 5.0 1.2 VVVV 350 50 -76 900 5500 3.85 40 5000 6.2

"seg" refers to the number of fillament sections, i.e. the shape of the fillament


SCHEMATICS

 


Signal section

 


B+ supply

 


Bias supply