Dear friend Laylow,
My first impression was a light-hearted one: Has this designer won a box-full of 22nF capacitors somewhere - because everything is 22nF!
But you would want a more serious pronouncement. The nicest thing I can say about this design is that it is an admirable example of fitting everything in so that it ju-u-u-st struggles up to its expectations (as in an exam design). But as a comfortable 90W valve design, also coping with normal mains variations and component spreads etc, this is not the topology that I would prefer, and many others would agree with me. Too many things barely fit in. (Are there distortion figures available somewhere? My PC would not open the reference given by you; I only have the schematic appearing above.)
Omitting elucidation with actual arithmetic for now, the ECC83 is a poor choice for both an input stage as well as the concertina phase splitter. Both triodes just about make it, with uncomfortable distortion possibilities at maxima. The grid resistance for two EL34s (about 350K.ohms for fixed bias) is on the maximum spec of 700K per valve ... and so forth.
.... implying that there is altogether a better topology for this sort of thing. It will need an extra valve (not just one slaving away for all main amplifier driving tasks). That then opens a selection of possibilities. Just one - since Leak amplifiers have been discussed in another thread - would be his topology of an input pentode followed by a LTP (long-tailed pair or Schmitt) phase inverter. The Williamson topology offers another far better option.
To conclude for now then, in my opinion not this circuit if you desire a reliable output of close on 100W. Power saving 4 x 6L6GCs will be comfortably higher rated than 4 x EL34 at about the same cost, plus ... but as said. Other suggestions might come along of a different valve or component value for this or that stage; just remember that power amplifier design is hardly ever so straightforward as to just use a different valvecomponent here and there Q.E.D. It needs to be designed as a whole.
Thus for now I am sadly leaving you without a viable 'alternative-in-ten-easy-steps'. Other options from the internet can be examined. For myself I can test-hookup something of such a nature (I have such basic facilities), but I would need a week or two, present schedule being what it is.
But there are other knowledgable members here ...
