Paul,
I have found for some time that the sweep tubes in Class A SE, sound more similar than different, to the 45, of all the Fashion triodes. Not being DHT, they do not exhibit the holographic thang that those tubes do, but I could be VERY happy ad infinitum with this amp, with the right speakers. The fact that sweep tubes are out there literally in the millions, and can be picked up for $4-10 ea., makes this a pretty practical set up in the real world of sticking real tubes into amplifiers: there are only two. The 6EM7/13EM7 tubes have far more bass capability than any 2A3, and most 300B's, tho' depending on how well implemented the latter is, the sweeps have a very full and tight sound to them, and Roger's amp is the next step well beyond even the tricked out SEX amp, with Magnequest OPT, because I have heard both, actually I prefer the stock SEX amp, which I also own.
The vertical sweep tubes are interesting inventions, sometimes referred to as "compactrons," I believe this was a tradename of RCA... designed to operate in Class A, in TV, exhibit high resolution characteristics due to the nature of the useage, in a bandwidth in line with the audible one. One half of the tube is ~identical to the 6SL7, for a driver, and the big half is almost identical to a 2A3 for output side. So in one envelope you get pretty much the same result as using an Octal, dual triode 6XX7 driving a 2A3 output tube, which is the nicest combo for 2A3 sweetness anyway... Neat trick, no?? And for 4-10 bucks a tube, with 50% greater output than the average 45 tube, you see why I think these are cool....combine with suitable drivers, such as the Hemp Tones, and you really are getting into some sweet sound. Remember, this is a hand made, Music Reference SET amp with Modjeski circuit for 6EM7, for under a grand. This is something very new in itself!
Because of the interest in open baffle and wideband type speakers, Roger included a second set of inputs with a .01 shunt cap to HP the signal @ 100Hz, which now gives you a pile more smack and headroom, having relieved the OPT, and tubes of the strenuousness of processing LF information. [~40%>100Hz] When used with the 94db BIB's + horn gain, the 6EM7 is all the juice I need for endlessly musical meanderings, in my 15 X 13 music room.
