It's essential that for this kind of operation the tubes have a Vao limit >1600V as plate voltage almost doubles during cutoff.
Also tough on OT primary insulation.
Also tough on OT primary insulation.
The EL34 JJ can work well at the operating point. Forget Russian and Chinese ones. The JJ can take 800V on the plate without troubles. Just buy pairs of truly tested and selected tubes. I have tried myself and it worked well with 400V regulated for the screen grid and 7 dB cathode feedback. I designed myself the 11K OPT with what I had (aka EI120 core laminations with 40 mm x 60 mm cross-section and it worked flawlessly down to 30Hz).
I do not consider the comparison to PA amplifiers to be a fair one for the simple reason that a HiFi amp is not a heavy duty amplifier and will hardly output more than few watts most of the time. From the Philips graphs it can be seen that the EL34s reach their max recommended dissipation when the output power is around 20W. In home environment that will unlikely be a long term power demand. Comparison to guitar amps doesn't stand at all as in this case tubes are out under much more stress even when they operate at more standard levels, IME.
Having said this, I have also tried the other variants and personally I do not find any practical advantage of the 100W version in comparison to the 60-75W that can be obtained with just 420-500V, respectively (both with 400V regulated on g2 and about 7 dB cathode feedback as well). They are much more convenient and sound a bit better. For the 60W variant I was able to use the EI-96 32x50 for the 5K OPT and squeeze the full power down to 30Hz. A considerable saving in weight and size. As I said, the lower power variant sounded a bit better and that's how everyone who listened to them was able to distinguish but not for the power. 100W in comparison to 60W seems a big difference but in practice is unlikely going to be noticed. No one did in my case. The speakers used were the old good Mission M73 that have a real efficiency around 86 dB/1W.
I do not consider the comparison to PA amplifiers to be a fair one for the simple reason that a HiFi amp is not a heavy duty amplifier and will hardly output more than few watts most of the time. From the Philips graphs it can be seen that the EL34s reach their max recommended dissipation when the output power is around 20W. In home environment that will unlikely be a long term power demand. Comparison to guitar amps doesn't stand at all as in this case tubes are out under much more stress even when they operate at more standard levels, IME.
Having said this, I have also tried the other variants and personally I do not find any practical advantage of the 100W version in comparison to the 60-75W that can be obtained with just 420-500V, respectively (both with 400V regulated on g2 and about 7 dB cathode feedback as well). They are much more convenient and sound a bit better. For the 60W variant I was able to use the EI-96 32x50 for the 5K OPT and squeeze the full power down to 30Hz. A considerable saving in weight and size. As I said, the lower power variant sounded a bit better and that's how everyone who listened to them was able to distinguish but not for the power. 100W in comparison to 60W seems a big difference but in practice is unlikely going to be noticed. No one did in my case. The speakers used were the old good Mission M73 that have a real efficiency around 86 dB/1W.
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