power amp design -- Lafayette LA-224

The previous post basically concludes the (initial) design of the preamp.  Now on to the power amp.  As mentioned earlier, I may eventually develop a range of power amp choices for this amp, but my preferred configuration (and the one I plan to build first) is (2) EL84 per channel.

I've been looking at the circuits of various guitar amps and hifi amps that use push-pull EL84 outputs, to see what I can copy.  One that I especially like, for several reasons, is the Lafayette LA-224.  I have an LA-224A which I've restored: it's our main stereo amp in the living room.  I love the sound, especially with vinyl, though I have to admit the distortion seems pretty high.  I think EL84s are better for guitar amps than for hifis!

I found a schematic online for the LA-224B (various scans of the same original image, at various internet sites); I believe the main difference between the "A" and the "B" is a change in external cosmetic appearance, but maybe there were other changes.  I notice, e.g., that the "B" apparently has dual-ganged pots for bass and treble, i.e., a single knob adjusts L & R together, for each; the "A" has concentric pot shafts so that you can adjust left and right separately (which is usually more annoyance than convenience: I approve of their change here; unfortunately, the cosmetic changes were not an improvement, the "A" looks much better IMO).

This is my re-drawing of just the power portion (one channel):

This combination of tube-rectified power supply, cathode-biased EL84s, and negative feedback, is the basic formula that I'm looking for, so I figure I may as well start from a known-working circuit design.

The Eico HF-81 was another possible template to copy, but that amp has R-C stuff in its feedback loop.  The LA-224 has a nice, simple negative feedback loop (the 7k Ohm and 200 Ohm resistors); it will be easy to add the presence and resonance controls into this circuit.  With the HF-81, it seems to already have frequency-shaping in the NFB loop, and... I dunno, I like it simple.

Really, the only subtlety in the LA-224 schematic above, is the R-C circuit in between the two 12AX7 stages.  From what I can tell, this is a lowpass filter with a very high cutoff frequency, around 2 MHz.  It reduces the gain well above the audio band, presumably to prevent oscillation.  (Maybe the R-C components in the HF-81 NFB loop accomplish this same frequency-compensation task; regardless, I like that the LA-224 doesn't have 'em there.)