Solitary \Sol"i*ta*ry\, a. [L. solitarius, fr. solus alone: cf. F. solitaire.]
- Living or being by one's self; having no companion present; being without associates; single; alone; lonely.

There has been a lot of interest in my 437A single tube amp. Many people have asked about this "little" tube and what it can do. "Is 0.5-1W really enough?" is the question every one wants to get an answer to. If you run this tube right you can get about 1W out of it. This amp will not drive speakers <95dB so you might as well stop right here if they are less than that. You really need around 100dB to get it playing loud enough to get your family to start complaining.

I've tested the 437A amp on a Lowther PM6A in a Mauhorn cabinet, and it played loud enough, above normal listening levels using a 1.5V output of a DAC. If you can get about 3V output then you can go all the way to the 1W limit. My system puts out 1.2V from the DAC, this is enough to play above normal listening levels. So 95dB would be very "doable" with a 2-3V input signal.

The next question was ussualy if there is a substitue for the very expensive WE437A. Not everybody wants/can shovle over $150 or more for one of these little critters. Here we learn that the cold-war was not for nothing after all. The russians made a similar tube under the designator 6s45p (sorry, no cyrilic keyboard). It's specs are very close to that of the WE437A, so it can be used as such. The good thing is the price and availability, about $25 each. There are cheaper sources for this tube, but it is wise to buy from a good dealer. These kinds of tubes have a very high gm which means that a 1/1000th of a millimeter deviation in the grid results in very large tolerances on the gm and plate current. I have had tubes range from 10mA to 90mA at the same bias setting (150V/-1.5V). Ask for a matched pair people!!! Pay the extra few bucks to make sure you get two similar tubes.

The circuit for the 6S45 is almost identical to that of the 437A. Only the cathode resistor has to be chosen at a different value as the bias is slightly off due to the higher mu (45 vs. 40 of the WE437A). This amp will also use a 3K opt instead of the 5K I used in my 437A amp. They actually came from the 71A projects. The 3K will give a bit more power, just enough to hit the 1W mark.

The bias of the 6S45 in this circuit is quite high using 200V on the plate. If you are not comfortable with this (a.k.a. build at your own risk) then use 175V on the plate and a 92 ohm resistor to get the bias.

Bias at 200V/-2.5V


Jim de Kort