Poweramplifier (discontinued)

This is the story of an Amplifier, not just an Amplifier, it is my Amplifier. The word Power-Amplifier is not used by me at this time, because of the huge power output of 900 Milliwatts.

I used to play with an amplifier built around the 71A as driver and the 45 as output-tube. The 71A had a battery-fed (6V/4Ah with CCS) filament and the 45 was heated AC. Due to my (current) loudspeakers, an open Baffle with a Philis 9710 in it, some hum-residue was heared when putting your ear in the unit. This was not a problem to me, because this is not the way I use to listen to music... So, the 45 is the quiescent output tube, heated on AC, used by me so far.
Some time ago, I could trade some tubes for three pairs of 31's. This is almost the same tube as my 71A driver, although it is less powerconsuming on the filament, so maybe more listening hours for me? I decided to give the 31 a try. Due to my point-to-point wiring a five minutes job (increasing the Cathode-resistor to 2K2 and decrasing the filament voltage to 2V by adjusting the CCS on the Battery). How does the 31 sound, compared to a 71A? First, the 71A is a richer souding tube in the lows, a very warm sound is your deal. The 31 however, is more open on the mids, more present! The highs were about the same... After listening to it for a few days, I decided to put back the 71A, because I thought I missed the lower-end response of the 71A. When done, I realised I had a warmer sound again, but missed the open mids of the 31...
This was an awkward situation, WHAT TO DO? An idea came across my mind: Could it be that 700 Milliwatts would be sufficient to drive my Open Baffles??? Due to my point-to-poit wiring (again) and my custumbuilt universal power transformer it was done after making some calculations of what voltages were needed to drive the "thing". I now opted for a 31 as driver and a 71A as "power-tube". Nice thing to take along was that given the currentdraw on the filament of the 71A, a battery fed filament on the output-tube was possible; hell, I could even skip the bypass capacitor without an evident hum-problem.... I decided to do it all. SO what does it sound like? First, Yes 900 milliwatts (hurray for separate powersupplies for driver and output-tube) are sufficient to drive my Open baffles. And the sound is impressing! It is more open and dynamic as I've ever heared. The 71A is the output tube for me (so far...ahem)! If you don't need the Watts, but can do with lots of Milliwatts, you should give this tube a try! You can look at the schematic here.

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