Bias question - what's going on here? 1979 Traynor YBA-1

[plumcrazyfx]

I want to add a bias pot and figured I'd just follow what was done in my 73 Bass Master.  But alas, the 79 doesn't have a dedicated bias tap as far as I can tell.  I tried to draw a schematic of it (attached) to help work through the process.  Here are some questions: where is the best place to put the bias pot and what are the best values?  What is that .047uf in there for?  Since I'm in here, should I do additional changes (reconfigure the bias, replace diodes, change the 68K to something higher, etc)?  Thanks.

[sluckey]

Here's the schematic for you amp. Look at page 6 and/or 7.

     https://el34world.com/charts/Schematics/files/Traynor/Traynor_yba_1_yba_4_bass_master.pdf

I would replace R31 47K with a 50K pot and 27K resistor connected in series.  Connect the wiper to one of the outside terminals of the pot. This should be the only change you make. Well, you can change the filter cap (not the .047µF) if you think it needs replacing, but don't change anything else.

Here's that same bias circuit with an explanation of how it works. Look on pages 5 and 6.

     http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

[plumcrazyfx]

Dang you guys are good.