Below are the instructions I received from Kegger at AudioKarma on an alternative assembly and installation of Roy Mottram's VTA driver board upgrade for the ST70. This is how he (Kegger) has assembled them for a few of his own amps and some for other people.
The idea is to optimize the circuit for low-mu tubes like the 12BH7. Also included is the method for removing Negative Feedback if one so desire.
NOTE: I have assembled, but not yet installed my own (due to lack of experience). I only know how to work from directions, but can't really troubleshoot or tweak, so without further assistance from Kegger (apparently his new job has him very busy), this one's for the experienced DIYer.
I have tried to clean up his text. Further corrections are welcome and encouraged.
"OK, All parts are labeled the same as the original schem so you can follow them.
The basis is changing all plate loads to 33k, which allows more voltage to the tubes, and with a bit of tweaking to the bias of each front end tube by lowering the bias resistors, raises their current. This also optimizes the circuit for 20ish mu tubes like 12au7 and 12bh7.
Then we are removing the bypass caps on the driver tubes and I remove the loop feedback, decrease the size of the bias resistors feeding the grids of the output tubes, and adjust the onboard RC networks for proper board voltages.
I use all Carbon film resistors except R17 and R18 where I use metal film on most builds (carbon comp plate loads are very nice as well). Mixing in a few PIO caps doesn't hurt either.
Change R13, R15 and R14, R16 to 3K (1/2watt will work but 1watt would
be better). Set's board voltage.
Change R11, R19, R21 and R12, R20, R22 to (33K) (2watt for R11 and R12, at least 1watt for others)
Change R9 and R10 to 820ohm (1/2watt is fine)
Remove C9 and C10
Change R17 and R18 to higher wattage rating (1watt will work 2watt is
better)
Change R27 and R28 to 390ohm (1/2watt is fine)
Change R29, R31 and R30, R32 to 120K (1/2watt is fine)
If leaving the loop feedback from the output trannie R7, C11 then move it to the 8ohm tap (if on 16) Otherwise, I leave out [R7, C11] and [R8, C18] (ed. Does he mean C12?) altogether and don't connect the loop feedback here. The loop feedback would be the only controversial removal but I've ran it both ways and prefer without.
Checking voltages: Plate voltage on front tube with 12au7 shold be in the range of 170v and phase splitter running a 12bh7 should be around 200v, if off by quite a bit you will either want adjust the last RC network in your power supply to get you roughly 375 going to the board or you can adjust R13, R15 and R14, R16. (Easier to adjust the power supply voltage in my opinion)
I also have two different local feedback "optional" mods one can experiment with, I like them both (one is a bit tricky). Obviously I upgrade the supply as well.
I'm not posting the actual schem on the board as I'm not sure how it should be, but no problem talking about the actual part #'s and or values, so ask away if you need/want to.
I'm going to do a thread on the VTA/70 upgrades here soon as well.
Hope I covered it all!"
Further corrections, revisions, concerns, and personal experiences with this are encouraged, so let me know what you think.
Best,
Brinkman
EDIT: After visiting the tubes4hifi website a couple nights ago (12/11/07), I noticed Roy Mottram offers a new low-gain version of his VTA ST-70 board. It is similar to the above tweak in that it is built around the 12AU7 or 12BH7 medium-gain triodes. No mention of options for removing loop feedback, so the adventurous are welcome to share their thoughts on this.