Hooking Up A VTA Driver Board With An SDS Cap. Board

Hi guys, new guy here. I've been trying to figure out how to hook up the VTA board on a pair of Mark III's, modified by a previous owner (Triode Electronics driver board and SDS cap. board). I guess the best way to go about this is to describe what I have. On the cap board the first section is connected to one side of the choke and pin 8 of the rectifier. Second section connects to other side of the choke and red output transformer wire (?), also one side of a large resistor (probably 2 watt). Section 3 connects to the other side of the resistor and also eyelet 5 on the Triode board. Section 4 connects to eyelet 6 on the Triode board. The original bias pot is connected to a black wire going to a terminal strip on one side and a red wire connected to the middle going to the Triode board. Do I just connect the black wire to the bias eyelet on the VTA board? Also a wire connected to eyelet 7 on the Triode board connects to one side of a large resistor along with a yellow cloth wire with the other side of the resistor connecting to the neg. speaker post. I hope I am clear with my description, just trying to avoid white smoke. Thanks for any help, camaroguy.

You can always undo the undocumented mod and revert it to stock MkIII . What i understand the
mod consisted in replacing one set of caps with another set.

Documentation is the key, whatever mod you do, keep a record of it and pass to the next owner!

Hi guys, after a lot of searching around I think I have everything figured out except for two ground wires. The ground eyelet next to the B+ and the one next to the NFB eyelet. What should I do there, just run wires to the chassis or something else? Thanks for any info., camaroguy.

This info might be helpful
(excerpted from Triode electronics site.)


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), here's 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.

Hi guys, thanks for the advice but I should have mentioned this is a pair of Mark III's I'm working on. I'm sorry for any confusion. I'm almost there, I still need to figure out the previous owner's bias circuit. When I get back home in a couple of hours I'll describe it in detail, because I'm lost with what he did. camaroguy

Hi guys, I'll never buy another modified piece of equipment without the circuit drawn up and explained. Here goes, the red/black transformer wire connects to a diode which connects to a 47uf 100v cap. and a 1k resistor. The other end of the resistor connects to one terminal of a three terminal strip, which also connects to a wire that connects to the original bias pot. The other leg of the 47uf cap. connects to the middle terminal along with an orange cap. marked 203k500v and also a red/yellow cloth wire. A wire goes from the middle terminal to the neg. speaker post. The other leg of the orange cap. connects to the third terminal along with a green/yellow cloth wire. I know the red/black transformer wire connects to the bias eyelet on the VTA board but I don't know what I have to do with the rest of the circuit. Thanks for any help, camaroguy.

just connect the red/black wire to the bias input on the VTA MK3 board, and you can throw the rest of all those parts away, not needed.
BadDog, thanks for the very thorough and correct info, even though it's for the ST70 the info applies exactly the same to the MK3.

Ok guys, I think I have it. I should be finished by the end of the weekend. Car shows are right around the corner here in New York, so I have to get the electronics projects off of the dining room table. Thanks, camaroguy.

Well after spending the better part of the day putting the driver boards together, I discover they won't fit with the SDS cap boards. I need about another 3/8 inch so the caps. on the two boards will clear each other. If someone did this mod. I would sure like to know how they did it. The SDS boards I have are about 15 years old, I'll call them Monday and ask if they changed the mounting of the boards over the years. If not I guess I'm going to have to replace the cap. boards with something else. Well I can rest for Easter now, camaroguy.

You could also think outside of the box.

The way I see it, I have two choices. Relocate the mounting bolts for the SDS cap. boards and also relocate the fuse holders or go with a modern quad cap. Being the SDS boards are about 15 years old I'm leaning toward quad caps, plus it would clean up the inside of the amps. Any suggestions out there? I see Dynakit Parts sells several different types. Thanks for any help, camaroguy.

Hi -- I have used the Dynakitparts 80-40-30-20 Quad multiple times since it is rated at 550/600 volts which gives you some margin for today's wall voltage of 121+ volts.
Stew

stewdan wrote:Hi -- I have used the Dynakitparts 80-40-30-20 Quad multiple times since it is rated at 550/600 volts which gives you some margin for today's wall voltage of 121+ volts.
Stew

Hi, I'm going to call them tomorrow. I think that is the best way to go. Should only take a couple of days for shipping, Dynakitparts is only about 50 or 60 miles from me. Thanks, camaroguy.

camaroguy wrote:Well after spending the better part of the day putting the driver boards together, I discover they won't fit with the SDS cap boards. I need about another 3/8 inch so the caps. on the two boards will clear each other. If someone did this mod. I would sure like to know how they did it. The SDS boards I have are about 15 years old, I'll call them Monday and ask if they changed the mounting of the boards over the years. If not I guess I'm going to have to replace the cap. boards with something else. Well I can rest for Easter now, camaroguy.

This is a timely thread as I'm just getting ready to upgrade my MK III's in a similar fashion. I just installed the SDS power supply last year, so don't want to abandon it. I'm wondering if my "new" SDS board is the same size as your 15 year old one? Is the new VTA driver board larger than the original Dynaco driver? I plan on completely disassembling my MK IIIs, repaint my transformer covers, replace the tube sockets with new, install new speaker binding post (this could be tight with my SDS cap board), install the bias test socket, and triode/pentode switch. I hope everything fits. I might have questions on wiring when the time comes. One more question; are any of the caps supplied with the VTA driver board redundant with the SDS cap board? I assumed the SDS board just replaces the original quad cap?

Hi Glacier John --- In answer to your questions:

(1) The caps on the Mk III VTA Board are NOT redundant to the caps on the SDS Cap Board and YES, the SDS Board's Capacitors just replace the 30-20-20-20 MF capacitance of the original Quad Cap.

(2) The size of the VTA Mk III PCB is the same size as the Original Dynaco PCB.  I believe Roy Mottram says to mount the VTA PCB below the top of the amp(from the underside).  Because it was so congested under there, I took the lazy man's approach and mounted my VTA Boards on top of the Chassis using 3/4'' or 1'' threaded stand-offs.

(3) I have multiple SDS Cap Boards (which I purchased from Triode in the late 90's and early 2000's) in 4 pairs of Mk IIIs and they all attach to two of the Output Transformer bolts, one located by the speaker connection strip and the other to the bolt on the other side of the transformer.  Note: the 8 caps on the circuit board will face the bottom of the Mk III.  I measured the height of the caps above the PCB and they are 1" tall which leaves some space between the cap and the bottom cover plate of the amp. I suggest putting a layer of PVC electrical tape on the bottom cover plate in the area of the caps, so that the tops of the caps (which are 'hot') do not make contact with the bottom cover. Another capacitor covering possibilty are circular paper price tags which glue to each cap top. (My preference is the Vinyl or PVC electrical tape).

I hope that I answered your questions and was not confusing.
Stew

Thanks Stew, that is exactly what I was thinking. I did the SDS mod last year and it seems to clear ok, I didn't add tape to insulate the caps, but it wouldn't hurt to add that insurance against grounding.

Hi Guys, sorry for being away from the forum. I appreciate the info. you guys have given. I have one amp together, but I'm getting a high bias voltage (about 1.5 volts with the pots turned down). Everything seems to be working properly, bias pots adjust the bias voltage and the voltage on pin 5 of the output tubes. I'm going to look for a bad solder joint tonight but I would appreciate any advice. I'm getting about 38 volts on pin 5 with the pots turned down all the way and about 56 volts half way (both tubes). As for the SDS board not fitting, the VTA board has components on the edge where the Triode board overlapped the SDS board. That's the fitting problem. I went with the quad cap from Dynakitparts after speaking with Kevin (nice guy). Thanks, camaroguy.

I'm stumped! I checked the voltage at the bias eyelet (55.5v AC), the other side of the diode (77.1v DC) and the other side of the 1K resistor (75.6v DC). After looking around those numbers seem to be correct. Where should I look next? Thanks for any info., camaroguy.

camaroguy wrote:Hi Guys, sorry for being away from the forum. I appreciate the info. you guys have given. I have one amp together, but I'm getting a high bias voltage (about 1.5 volts with the pots turned down). Everything seems to be working properly, bias pots adjust the bias voltage and the voltage on pin 5 of the output tubes. I'm going to look for a bad solder joint tonight but I would appreciate any advice. I'm getting about 38 volts on pin 5  with the pots turned down all the way and about 56 volts half way (both tubes). As for the SDS board not fitting, the VTA board has components on the edge where the Triode board overlapped the SDS board. That's the fitting problem. I went with the quad cap from Dynakitparts after speaking with Kevin (nice guy). Thanks, camaroguy.

Upon looking at pictures of the VTA driver board next to my upside down amp I can see the conflict you are talking about. The VTA board has a couple caps that near the edge of the board that look like they would hit the edge of the SDS board. Having spent the time and money to build the boards last Spring, and buying into their necessity, I don't want to abandon them for the new quad cap option. It looks to me like the Triode Posidion board might fit better. Any other ideas, I plan on ordering this stuff tomorrow.