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...
