A little bit of history...
Some people I knew brought a few other tubes to my attention. Amongst these were the 10, 45 and 50. The 45 could muster a whopping 2 Watts into my speakers. A little tube with a little power but a big reputation it seemed. This should be more than enough for my >100dB Lowthers. With just a single Watt I could bring the sound up to deafening levels, levels at which I normally never listen anyway. The 50 was another famous tube reported to have superb sonic qualities, some even prefer it above the "legendary" 300B. It can produce up to 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 prices will go through the roof. To this day I haven't heard one play yet, so it's sound remains a mystery to me, and 50's costing >$200 each they will stay a mystery as well.
After tossing and turning for a few nights I decided to try the 45. It had enough power and was readily available. After making up my mind I went to the drawing board to make a design for the amplifier. I was going to use an inter-stage transformer and grid biasing, so the output stage was not that complex. Hook the grid up to the IT, the plate to the OT and the cathode to the FT (filament transformer) since I'm using an AC heater supply for the output tube. All I had to do was make an adjustable bias supply and get the right voltage onto the plate.
A week or so later I found out that there are several versions of the 45. There is a special version of this tube named accordingly 45 Special, or 45 SPL. It can handle a somewhat higher plate voltage, a higher plate dissipation and has a different filament. There is also a military version of the 45 SPL called the VT52, this is said to be the same tube but with a military number, something I don't completely agree on yet. Other denominators exist for the 45 SPL; namely 2C45, CV596, 45A and 38142, although I have never seen the first two numbers (2C45 and 45A) myself. The regular 45 also goes by the name CV610 and VT45, for those who want to know... The expanded specs of the 45Special/VT52 looked very interesting to me. I would be able to get 3 to 3.5W 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 over that of the 45 ;)
Check out the VT52 page for more details on this amazing tube...!
My preamp gives a gain of about 8.5 with it's choke load, at 2Vrms input from my CD-player that means it will give approximately 17Vrms on the output. This is a pretty high output, for preamps, they usually only go to 3 or 5V. I only use a CD-player at the moment which allows the driver of the power amp to have a lower mu than would normally be necessary. This resulted in being able to remove an extra gain stage at the front of the power amp.
To drive the VT52 to it's max output I will need 60V (42Vrms) on the grid. Seeing that the preamp already gives 17V output, I could do with a tube with a mu of 3-4. This makes choosing a DHT as a driver simpler since there are more low-mu triodes than good high-mu triodes. I designed my power amp so that it will need about 8V (minimum) on the input. This is about half of what my preamp can deliver, I like to keep some options open for other tubes I might possibly one day use instead of the VT52.
Tubes like the VT25 and some european types such as the RE134 and KC3 had already popped into mind for a driver stage. The KC3 with it's mu of 30 would have been my option if I needed more gain to compensate for a lower input signal. If I were to use the KC3 I could plug the CD directly into the power amp (with a pot) and play it into 3W. Since the VT25 might also make it into my output stage someday it won't be used as a driver. I want to avoid using the same tube twice in the circuits so as not to get the coloration twice, I don't think this is good for the overall balance of the sound.
The tubes that are left after these considerations are the RE134 and KC3. I have three sets of Telefunkens of each type at the moment so either choice would leave me with a good supply of tubes for the next 10 years. Between these two tubes the RE134 is best suited for interstage use because of it's lower Rp. The KC3 has an Rp of 12K, the RE only 5K. The higher current capability of the RE134 (12mA versus 3mA of the KC3) is also a more pleasing factor for use with IT's. So I guess my choice has been made on the driver tube... The circuit I designed can be seen on the "schematics" page. It is a straight forward SE amp, no tricks.
Well... I've bread boarded this thing after receiving all the iron I needed. Again, Bartolucci everywhere, this stuff is so wonderful !!!! The power transformer is a custom wound beast, rated at 400W with all it's windings heavily over specified. I've also had several taps added as to give different voltages, always handy if I am to build another amp using different tubes. Like the OPT's which were wound with several impedance taps, the power transformer is also universal. This set will allow me to experiment with almost every tube out there. Once I'm certain of the tube I want to use I can have the proper OPT or power transformer made.
There were some hum problems in the filament of the RE134, so I changed this to DC heating. Same thing in the B+ supply... hum/fizz on the driver and output tubes. A friend of mine, Erwin Wiesbauer, suggested a while ago that I put a small capacitor (<2uF) on the rectifier output. This meant going from choke input to semi choke input. The small value of the capacitor allows the circuit to keep most of it's choke input characteristics but at the same time smoothes out the ripple enough so that the choke can finish off it's work perfectly. This got rid of the ripple/fiz I heard but didn't really affect the sound as much as I had thought.
The bad news is that this amp won't make it further than the breadboard... Some new ideas came up which I want to try, such as AVVT's new driver tubes, the AV5 and AV8. My new front horns should have no problems with only 1 or 2W output since they will operate over 150Hz only. Along with the new tubes I also want to use an IT between my driver and output tube and use different tubes for the preamp and driver section. My VT52 idea won't be scrapped, at least not the concept, only changed as to allow another setup. The impedances and specs of the AV5 and VT52 are a close match which allows me to use the same type of OPT and supply. This is perfect since I can now switch between VT52 and AV5, if I don't like one I can always plug in the other.
© Jim de Kort