I very much take your point on the Concertina, Trelja, but here's the rub: the Concertina, or split-load style of phase splitter & driver, really is not properly used "naked" driving a Class AB output stage, or with any sort of output stage where you are going to be drawing much grid current. This is because the frequency balance between the two phases -- which interacts in various complex ways with other characteristics of performance -- depends crucially on the plate circuit and that of the cathode in the Concertina seeing EQUAL IMPEDANCES at all times.
Contrary to audiophile mythology, in a circuit where the two loads are effectively kept close in value and stable, the output impedances of the splitter, top & bottom, hence the "drive" characteristics of the two halves, are VIRTUALLY IDENTICAL. Not so in a Stereo 70 trying to push out nearly 35 watts into most real world speaker loads. How many "respectful" ST-70 mods retaining this splitter arrangement, but operating in pure Class A, where neither OP tube cuts off, have you seen -- preferably with grid chokes, etc, to handle the grid current? None, I bet. This is why D.T.N. Williamson added a LTP driver stage after the phase converter in his classic design... at the expense, because of multiple poles in the now more complex circuit, of marginal stability vis-a-vis the global loop feedback -- which itself interacts with complex, dynamically changing speaker loads. D.T.N. got it pretty much right -- others didn't.
Thus, some Williamson copycat amps of the '50's, being adulterated by cheaper, slightly wonky or inconsistent output transformers and such, could be -- and STILL ARE -- very dicey performers. The Heathkit W4-M is an example. It will work fine with a LTP converter and a simpler front end circuit.
Their are ways to stick some band-aids on these problems, but you know, why not get it right?
By contrast, a little EL-84/7189 amp with Concertina, running Class A or close, at 10-12 watts out, can sound excellent in ways a stock Stereo 70 never can. So, I have the doggone cutest little refurbished Fisher SA-16 on the way to me now to add a little reinforcement to my point! And I have, I guess, a very nice ST-70 refurb with the stock front end for sale (in theory).
Contrary to audiophile mythology, in a circuit where the two loads are effectively kept close in value and stable, the output impedances of the splitter, top & bottom, hence the "drive" characteristics of the two halves, are VIRTUALLY IDENTICAL. Not so in a Stereo 70 trying to push out nearly 35 watts into most real world speaker loads. How many "respectful" ST-70 mods retaining this splitter arrangement, but operating in pure Class A, where neither OP tube cuts off, have you seen -- preferably with grid chokes, etc, to handle the grid current? None, I bet. This is why D.T.N. Williamson added a LTP driver stage after the phase converter in his classic design... at the expense, because of multiple poles in the now more complex circuit, of marginal stability vis-a-vis the global loop feedback -- which itself interacts with complex, dynamically changing speaker loads. D.T.N. got it pretty much right -- others didn't.
Thus, some Williamson copycat amps of the '50's, being adulterated by cheaper, slightly wonky or inconsistent output transformers and such, could be -- and STILL ARE -- very dicey performers. The Heathkit W4-M is an example. It will work fine with a LTP converter and a simpler front end circuit.
Their are ways to stick some band-aids on these problems, but you know, why not get it right?
By contrast, a little EL-84/7189 amp with Concertina, running Class A or close, at 10-12 watts out, can sound excellent in ways a stock Stereo 70 never can. So, I have the doggone cutest little refurbished Fisher SA-16 on the way to me now to add a little reinforcement to my point! And I have, I guess, a very nice ST-70 refurb with the stock front end for sale (in theory).