SDS Board With VTA Board Mk 3

Does anyone know how you would go about hooking up the SDS board with the VTA driver board in a MK 3 amp? Would you simple not use the last cap section on the SDS board or tie the last two sections together?

As an FYI, the SDS cap boards I bought recently for my MkIII's are incorrectly marked. It's not really an issue if you use all the same value capacitors. But I used two smaller µF caps in the first section, so as not to stress the 5AR4. If I'd followed the silk screened numbers on the board when installing the caps, instead of looking at the traces underneath, it would have been wrong.

The wiring of the SDS Cap Board to a Mark III with the VTA Board is as follows:

Connection points to the cap board are silk screened on the board for the "4 sections" that basically parrot the Dynaco Quad Cap. I used what Bob Latino suggested and kept the GZ34 Rectifier Tube, so no diodes went to the Cap Board.

(1) connect the Red/Black wire from the Mark3 Bias line to the BIAS Eyelet of the VTA driver board.

(2) connect Section 1 of the Cap Board to pin #8 of the GZ34 and one side of the choke.

(3) connect Section 2 of the Cap Board to the other side of the choke and the red tap wire from the output transformer.

(4) connect Section 3 of the Cap Board to one end of a resistor, 2200 ohm 2 or 3 watt. And run a jumper wire from Section 3 back to Section 2.

(5 connect Section 4 of the Cap Board to the other end of the resistor, and run a wire from section 4 to the B+ eyelet of the Mark 3 VTA board.

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So, between sections 2 and 3 of the Cap Board you run a jumper wire

And between sections 3 and 4 you run a 2200 ohm 2 or 3 Watt resistor.
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The Stock Mark III had 2 wires coming from the Stock Quad Cap to the Dynaco PCB --- solder points 5 and 6.

The VTA board only has 1 wire coming to the PCB from the Quad Cap, hence the jumpered sections of the Cap Board.

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Hope this info helps. If you have questions, just ask.

Thanks all. I saw this trend and tried the above but the B+ was way too high. I'll try a larger resistor value.

I put a 6.8K resistor there and I'm getting 455 volts...too high? How are you able to use a 2.2K?

Hi -- Saw your question about the 2200 Ohm Resistor and the Voltage.

I need to dig out some of my build notes from my Mk3-VTA projects before I can answer definitively.

Found them ---- I have a pair of MK3-VTA(using Octal 6SN7 Tubes) that use
the SDS Cap boards wired as I said above. Wire Jumper between Sections 2 and 3 and a 2200 Ohm resistor between Sections 3 and 4, and the output of Section 4 feeding the VTA B+.

When I checked the voltages on mark Mk3's, this is what I saw:

GZ34 Pin #2 and Ground 550 vDC
GZ34 Pin #8 and Ground 550 vDC

GZ34 Pin #4 and Ground 410 vDC
GZ34 Pin #6 and Ground 410 vDC

6550 Pins #2 & #7 6.2 vAC and 6.3 vAC

6550 Pins #3 & Ground 550 vDC (both tubes)
6550 Pins #4 & Ground 550 vDC (both tubes)

Red-Black Bias Wire and Ground = -55.2 vDC

On the VTA Board ---

B+ was 455 vDC
at C14 (+ side) - 309 vDC
at C16 (+ side) - 391 vDC

I know that schematic says the B+ voltage is about 400 vDC, but I think that 400 reading (theoretically) is a function of what you are getting out the the Rectifier Tube and how high your line voltage is. I am at 120/121 vAC

Do your voltage readings resemble mine? Are you getting any audio output?

Roy, any comments?

Let us know.

The amp works but I was wondering how your voltage is the same as mine when I'm using a 6.8K resistor to the B+ and you're only using a 2.2K. You have the 6.8k resistor on the SDS board jumped out...correct?

Hi --

No -- I am using a 2200 Ohm Resistor (Red-Red-Red Bands)

What are your GZ34 voltages? Also, which GZ34 Tube are you using?

Mine are:

Output -
GZ34 Pin #2 and Ground 550 vDC
GZ34 Pin #8 and Ground 550 vDC

Input -
GZ34 Pin #4 and Ground 410 vDC
GZ34 Pin #6 and Ground 410 vDC

Stew

2.2K is 2200 ohm. I swapped rectifier tube and voltage only changes by 2 or 3 volts. I have the same readings as you on the rect pins. When the amp settles down I have 450 B+. Don't know how tubes4hifi gets 400-440v?

Hi --

Take a look at the "Voltage Check Point Chart" in the original Dynaco Mk3 Manual for the 1950's/1960's amplifier. The manual is available at the Tubes4Hifi.com website and also elsewhere on the Web.

GZ34 Pins #4 & ground or Pins #6 & ground ---> 430 vAC
GZ34 Pins #2 & ground or Pins #8 & ground ---> 490 vDC

Those vintage readings were based on line voltage that was 110 vAC

Today, we have 120-122 vAC, so it is not inconceivable that today's readings on GZ34 Pins #2 & ground or Pins #8 & ground would be over 525 vDC.

Are you using a "modern" (made in China or Russia or Eastern Europe) type GZ34 or a vintage made in "USA" or "GT. Britain" or "Western Europe" type GZ34? I ask, because I typically get much high voltages on Pins #8 and #2 with vintage type GZ34's.

Maybe Roy can provide some info on this topic???

So, how do your Mk3-VTA's sound??????

Please let us know.

Stew

stewdan wrote:Hi --

Those vintage readings were based on line voltage that was 110 vAC

Hi Stew,

Curiosity got the best of me, so I checked the Mk-III assembly manual Roy has posted on his Tubes4hifi site. The original Dynaco schematic for the Mk-III, and the even earlier Mk-II both indicate the incoming AC voltage at 117V, so the DC specified in the manuals (at least the two Roy posted) would be based on 117 VAC. Your point is well taken that today's AC voltages average three to five volts higher than Dynaco's reference.

Thank you for posting your voltage results. They will prove to be very helpful when I get my SDS'd Mk-III's up and running.

Hi --

I built my original Dynakits back in 1965 and from what I remember of that era, my AC Line Voltages were in the 110 vAC range.

I also checked all the manuals that I downloaded from the tubes4hifi website and they show the 117 vAC that you mentioned? The manual for the Mark 6 which came out in the Mid 1960's or later shows incoming AC Voltage of 120 volts and the Mk2's which came out in the late 1950's shows 117 vAC? I wonder if they were edited before being posted???

There is a newer version of the Mk3 Manual at www.theplanet.org which in the assembly pictorals shows the selenium rectifier as a 1N400n type diode and on the schematic shows the incoming AC Voltage as 120 volts and uses KT88 type tubes instead of 6550's

Somewhere in my home I have my original manuals for the PAS-3X and the Stereo 70 and FM-3 that I built, but can't find them now. Maybe they will cast a different light on the AC Voltage? I wonder what they show?

I will let you know what they say when I find them.

Stew

Hi Again --

I remembered that there had been previous discussion on the Forum about the "change in line voltage over the years", so I did a search for "AC line voltage".

Please check the following forum link:

https://dynacotubeaudio.forumotion.com/t1386-line-voltage?highlight=ac+line+voltage


you will probably have to paste the link pieces together if it does not make into html link when posted.

Stew

Thaks for all the input guys...

Still don't know how Roy gets 400 - 440 B+ based on todays voltages? This is what his VTA board instructions indicate.

When I built my Dynakitparts MKIII I ran into the same issue you are wrestling with now such as the 400VDC requirement for the VTA board.
The position I took on resolving this was that the 400 VDC value, the VTA board expected to see from the MKIII rectifier/power supply, was independent of the value of incoming AC line voltage. This therefore necessitated changing the value of R44 from 6.8K to 12K so that the 500VDC before R44 was reduced to approx. 410 VDC to the VTA board. My incoming AC line is 121 VAC. These resister and VDC values quoted are from the documentation supplied by Roy for his Hi Gain VTA board for the MKIIIs.
Given the tolerances in components one experiences, some tweaking may be required to meet the voltage values published in the circuit schematics. For this application, a 5% drift from spec. is acceptable for me and has worked well.
I do not use the SDS board.
Hope this helps.

OK...I removed the 6.8K resistor on the SDS board, jumped the last two sections together and installed a 10K resistor on the last section connecting to B+. Now I have 410 volts!
Stewdan...you must have ADDED 2.2K to the 6.8K that was on the SDS board. When you stated that you ran a jumper between section 2 & 3 on the SDS that would take out the resistor. It sounded to me like you were replacing the 6.8K with the 2.2K. I hope this helps and clarifies things for anyone using the VTA and SDS with MK3. Thanks again all!