The 6P7S SE did not come to be as a planned project but rather developed by a lucky sequence of events. In the background of course was the ever present restlessness of the music lover and electronics experimenter.
A surplus reseller catched my eye offering the 6P7S tube for peanuts. Its shape and the data made me curious. I had never heard anything about it before. A search in the web for a SE amplifier schematic based on it was not fruitful. I learned the 807 was a similar tube and both belong to the 6L6 family. Never heard a 6L6 amplifier! Having finished the 2A3 SE there was no real need for another amplifier but I could not let go and ordered four 6P7S tubes.
Still reluctant to embark on a new project I was curious to know if the new tubes were in good shape. The EL84 SE amplifier served as testbed. The tubes were alright but that was not all, the spell of the sound captivated me. It was fluid and clear. "Like spring water" was my first association while listening to some piano music. Checked triode mode and that was it, bewitched! The 6P7S SE was on its way to be...
It sounded nicely on small OPTs and a cathode bias resistor (270 ohms) bypassed with 220uF/40V. Curiosity made me try it with a constant current source for bias and it improved transparency. The OPTs were replaced by bigger ones (from Reinhoefer, originally designed to work with EL84s at 50mA). This improved bass and detail.
The way the 6P7S is setup in this amplifier is working far from its electrical limits. With an 8 ohm speaker loading, the load resistance for the plate is around 10K. Not really optimised for power output, but may be good for linearity?
The experienced eye will probably detect "flaws" in the "design". For being the result of a quick setup (6N3P in SRPP and a similar power supply topology as used in the 2A3 SE) the sound of this amplifier -in my setup- is excellent; smooth, fluid, clear and full of energy. The power transformer used is the NT18 from HB-AmpDesign.
Listening to Art Pepper + Eleven comes close to being there. Bethovens concerto #4 in G major by Glenn Gould and Leonard Bernstein is portrayed with power and grace.
On January 2011 I finished the Aikido driven 6P7S SE Amplifier it sounds cleaner, punchier, more detailed, has a deeper and wider stage portrayal and delivers more emotion.