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RE: You don't need the second half

That is the 1947 Lipman-Robin amp. A 6SF5 is like a 6F5, with a mu of 100, which is also much like a 12AX7. or, more correctly, half of a 12AX7. The $2.00 ( in 1947 ) Lipman-Robin book entitled " Practical Amplifier Designs" and subtitled "45 Proven Circuits for the Technician and Experimentor", describes this circuit you show us, and has a parts list. If anyone wants this data, contact me.

In Japan in the 1960s, 1970s, people like Isamu Asano had "their" versions of the same Lipman-Robin amp, as did Nobu Shishido who unfortunately, paralleled both sections of the 12AX7, an inherently inferior approach. ( Skews the music on a mu of 100 tube ). Maybe we didn't really get Nobu's "best" DC circuit, just "one published for the public " ??

Readers should realize, a indirectly heated mu of 100-DHT finals amp of this type, ( two stage and DC coupled )done properly, is about as good as it gets on efficient speakers.

Andy Evans in the UK has a small listening area and does NOT use efficient speakers, so, he builds "current" amps for his ten inch 92 dB (?) driver, not voltage-swinging amps.

Also, most experienced people I have met in audio, and on this Forum, do not know how to effectively bypass an Rk!! Dennis Fraker knows how !!

On efficient speakers, Andy's double 4P1L amp will get eaten by any well made 1/2 12AX7-DHT Finals amp. The low overall gain of Andy's amp, will make the efficient speaker play "dead and lifeless" at lower listening levels, no fun, and the amp will only come somewhat alive when pushed hard. Remember, its a current amp, not a voltage-swinging amp. Now on Andy's less efficient speakers, its hard to say if the opposite would occur.

Poor Andy, has never had the pleasure of hearing a recent pair of Serious Stereo amps, with a INdirectly heated mu of 100 input tube, and a 2A3 output tube. The Serious Stereo amp, particularly recent production, that only Dennis Fraker properly pulls off, is still my favorite for efficient speakers ( 98 dB and above ) in THIS world.

Have fun,

Jeff Medwin

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