... I was taking a closer look at some Traynor schematics the other 
day and noticed a few interesting things about the YBA-4 topology ...

 (i) the input gain stages have decoupled cathode circuits, and neither 
have any cathode bypass capacitance - in fact there is no cathode 
bypassing in any gain stage in the amp ... this is unusual for a guitar 
preamp ...

 (ii) the topology is based on the 5F-A (58/59 Tweed Bassman/Twin) Fender
 circuit but not as closely as the early YBA-1's . there is what seems to
 be a slightly lossy RC mid-cut circuit between the second and third gain
 stage and the tweed-like tone stack is coming off the plate of the third
 stage and not the cathode ...the YBA-4 is same as '72 YBA-1 except for a
 choke shunt in the RC network of that YBA-1 ... in the YBA-4 and later
 YBA-1 amps the third gain stage is biased differently to accomodate 
plate coupling of tone stack ...

 (iii)the phase driver circuit has a 47k grid suppressor tied to the grid
 ...the 5F-A circuit is known for overdriving the output stage easily and
 this will cut OD down somewhat . the same thing occurs in a Jan'67 YBA-3
 with 100k, Jan'69 YBA-1A mkII with 47k,June'72 version of the YBA-1 with
 47k - none in earlier YBA-1 versions as I can tell ...

 I think the phase-driver grid block might solve a problem I have with my
 Tweeded Twin ... thanx Pete !