Thanks to help from wd40 I seem to be getting somewhere, the front end, LTP PS and driver circuit is working well and could drive a six inch nail through a brick wall, metaphoricaly speaking. I'm getting 80w which looks flat from 100hz to 20khz with about 5% THD, which is mostly 2nd harmonic from what I can see. with a 1khz square wave there is a spike on the front edge.
With that out the way for now ( I'll need to go back and tweak it for lower distortion perhaps etc) when the OP clips, it's followed by Xover distortion and the OP power drops. It's not the PSU, that's rock steady. Am using a TEK 120-0140-00 power tfmr ( see -
http://www.pmillett.com/Tek_xfmrs.htm for full info) using taps 5-10 113v @ .5A and 20-21 180v @ 320mA paralleled with 3 x 220u 500v caps on each winding. I've noticed that I get different clipping and Xover distortion with different grid resistors etc.
I've looked at the Marshall VBA400, the Ampeg SVT 400 and a few other schematics to see how they configure the OP stage. There seems to be two ways the negative bias has been connected and the grid R's are terminated. Most have the 2 (or 6) grid resistors of each side connected with the neg bias also connected at this point. The other way is to split the grid R ('s) connect one end to ground with the neg bias connected at the junction of the two R's. I'd appreciate comments on the two approach's.
The other differences I've noted are some (most) have a grid R on each grid, same goes for coupling caps, other's notably the GEC 400w has two coupling caps and two grid R's, very simple. Lastly, most use a 220/270 scr grid stopper.
I'm a bit stuck here as I can't find any info regarding the design of multiple valve OP stages, for instance I presume a resistor is put on each grid, then that means they are in parallel looking from the driver's, does it also follow that the coupling caps are in parallel too?
TFL (thanks for looking) any comment's advice welcome, Andy.