Hey everyone.
I know some people are interested in using combinations of the various after-market ST70 upgrades. Seeing as how both the SDS Capacitor/Power Supply board and VTA Driver are popular choices (and that installation directions for both assume they're being installed in an otherwise stock build), I thought I'd share how to utilize these boards in the same build.
The first thing to consider is whether or not you're going to use the the bias supply on the VTA board or the bias supply on the SDS Cap board. Since either way can make use of the bias potentiometer feature of the VTA board, there really is no better choice. However, if you plan on auditioning various drivers in your ST-70, it's probably best to use the SDS bias supply since not all driver PCBs include one.
If you choose to use the SDS Bias supply:
The following can be omitted from the VTA assembly: D1, C17 & C18, and R39. Otherwise, assemble it per the instructions.
No parts are omitted from the SDS assembly; once it is assembled, wire up the SDS per the instruction sheet, with the following exceptions:
Nothing is wired to eyelet "L"
Eyelet "R" on the SDS board is wired to the "B-" eyelet on the VTA board. If there is no "B-" eyelet on the VTA board, wire it to the right eyelet formerly reserved for R39 on the VTA PCB.
Eyelet "19" is wired to the eyelet marked "B+" on the VTA board.
Nothing is wired to Eyelet "20"
The VTA board can now be wired and installed as instructed, ignoring all references to the quad cap (replaced), red/black wire (already in use), and bias supply (redundant).
If you choose to use the VTA Bias supply:
The following can be omitted from the SDS assembly: D1, C9 & C10, and R11 & R12. Otherwise, assemble it per the instructions.
No parts are omitted from the VTA assembly. Go ahead and assemble it, but do not install or wire it.
Once the VTA board is assembled, set it aside and begin installing and wiring up the SDS board as instructed with the following exceptions:
Nothing is wired to eyelet "L"
Nothing is wired to eyelet "R"
Nothing is wired to eyelet "Bias"
Eyelet "19" is now wired to the eyelet marked "B+" on the VTA PCB
Nothing is wired to eyelet "20"
The VTA board can now be wired and installed as instructed, ignoring all references to the quad cap which has effectively been replaced.
Powering Up:
Follow the VTA board instructions in regard to initial power up and B+. If your B+ is too low (rising to 320VDC and settling well below the specified 400-440VDC), you may have to jumper either R9 or R10 (or both) on the SDS board make the B- settle between 400-440VDC.
The B- (bias supply) should measure around -50 to -55VDC where R39 meets the bias supply on the VTA board.
Additionally, you may want to retain the pair of 10K bias pots in the event you choose to revert the ST-70 to stock or come across a driver board that does not feature on-board biasing of the output tubes. Furthermore, the quad cap's sole purpose is now aesthetic, though one could install a Triode/Ultralinear switch in it's stead.
Good luck.
I know some people are interested in using combinations of the various after-market ST70 upgrades. Seeing as how both the SDS Capacitor/Power Supply board and VTA Driver are popular choices (and that installation directions for both assume they're being installed in an otherwise stock build), I thought I'd share how to utilize these boards in the same build.
The first thing to consider is whether or not you're going to use the the bias supply on the VTA board or the bias supply on the SDS Cap board. Since either way can make use of the bias potentiometer feature of the VTA board, there really is no better choice. However, if you plan on auditioning various drivers in your ST-70, it's probably best to use the SDS bias supply since not all driver PCBs include one.
If you choose to use the SDS Bias supply:
The following can be omitted from the VTA assembly: D1, C17 & C18, and R39. Otherwise, assemble it per the instructions.
No parts are omitted from the SDS assembly; once it is assembled, wire up the SDS per the instruction sheet, with the following exceptions:
Nothing is wired to eyelet "L"
Eyelet "R" on the SDS board is wired to the "B-" eyelet on the VTA board. If there is no "B-" eyelet on the VTA board, wire it to the right eyelet formerly reserved for R39 on the VTA PCB.
Eyelet "19" is wired to the eyelet marked "B+" on the VTA board.
Nothing is wired to Eyelet "20"
The VTA board can now be wired and installed as instructed, ignoring all references to the quad cap (replaced), red/black wire (already in use), and bias supply (redundant).
If you choose to use the VTA Bias supply:
The following can be omitted from the SDS assembly: D1, C9 & C10, and R11 & R12. Otherwise, assemble it per the instructions.
No parts are omitted from the VTA assembly. Go ahead and assemble it, but do not install or wire it.
Once the VTA board is assembled, set it aside and begin installing and wiring up the SDS board as instructed with the following exceptions:
Nothing is wired to eyelet "L"
Nothing is wired to eyelet "R"
Nothing is wired to eyelet "Bias"
Eyelet "19" is now wired to the eyelet marked "B+" on the VTA PCB
Nothing is wired to eyelet "20"
The VTA board can now be wired and installed as instructed, ignoring all references to the quad cap which has effectively been replaced.
Powering Up:
Follow the VTA board instructions in regard to initial power up and B+. If your B+ is too low (rising to 320VDC and settling well below the specified 400-440VDC), you may have to jumper either R9 or R10 (or both) on the SDS board make the B- settle between 400-440VDC.
The B- (bias supply) should measure around -50 to -55VDC where R39 meets the bias supply on the VTA board.
Additionally, you may want to retain the pair of 10K bias pots in the event you choose to revert the ST-70 to stock or come across a driver board that does not feature on-board biasing of the output tubes. Furthermore, the quad cap's sole purpose is now aesthetic, though one could install a Triode/Ultralinear switch in it's stead.
Good luck.