Can someone review my MK III voltages?

I've just completed the rebuild of a pair of MKIII's. I installed the VTA octal driver board and the original quad cap was replaced with the SDS (Sheldon Stokes) cap board. I also replaced the original hook up wire, input jack, and speaker terminals. The amps power up nicely and hold bias. My readings from 122 vAC line voltage are:

VTA boards:
- B+: 463 vDC
- C14: 315 vDC
- C16: 398 vDC
- Red/Black bias: 57 vAC
- Bias diode out: 79 vDC

Rectifier amp1:
- Pin 2 and 8: 590 vDC
- Pin 4 and 6: 437 vAC

Rectifier amp2:
- Pin 2 and 8: 548 vDC
- Pin 4 and 6: 430 vAC

6550 amp1
- Pin 2 and 7: 6.85 vAC
- Pin 3 and 4: 589 vDC
- Pin 5 and 6: -59 vDC

6550 amp2
- Pin 2 and 7: 6.85 vAC
- Pin 3 and 4: 535 vDC
- Pin 5 and 6: -69 vDC

Anyone see any problems there? Should the amps be used with a variac to lower the line voltage? The rectifier tubes are old stock 5AR4's.
In system, the amps sound very good.

Thanks for any ideas or suggestions,
Ed

EWALAA wrote:I've just completed the rebuild of a pair of MKIII's. I installed the VTA octal driver board and the original quad cap was replaced with the SDS (Sheldon Stokes) cap board. I also replaced the original hook up wire, input jack, and speaker terminals. The amps power up nicely and hold bias. My readings from 122 vAC line voltage are:

VTA boards:
- B+: 463 vDC
- C14: 315 vDC
- C16: 398 vDC
- Red/Black bias: 57 vAC
- Bias diode out: 79 vDC

Rectifier amp1:
- Pin 2 and 8: 590 vDC
- Pin 4 and 6: 437 vAC

Rectifier amp2:
- Pin 2 and 8: 548 vDC
- Pin 4 and 6: 430 vAC

6550 amp1
- Pin 2 and 7: 6.85 vAC
- Pin 3 and 4: 589 vDC
- Pin 5 and 6: -59 vDC

6550 amp2
- Pin 2 and 7: 6.85 vAC
- Pin 3 and 4: 535 vDC
- Pin 5 and 6: -69 vDC

Anyone see any problems there? Should the amps be used with a variac to lower the line voltage? The rectifier tubes are old stock 5AR4's.
In system, the amps sound very good.

Thanks for any ideas or suggestions,
Ed

Voltages are a bit ( 10%) on the high side.
A voltage drop ( either a 120/6.3V transformer connected as drop or a variac) would be of value.

The power dropper is described alsewhere but is basically a 6.3V ( or 5V ) transformer where the secondary is connected in series with the primary , this will either raise or drop
the AC 6.3V depending on phasing.

See https://sound-au.com/articles/buck-xfmr.htm for a better description.

Thanks. There’s a lot of interesting information on that site.

I find more of a problem with your other voltage readings than the heater voltage. First we need to determine a few things. It sounds like these are vintage MKIII with the only changes being the VTA front end board and a cap board. I no nothing of the VTA boards so others will have to comment on those voltage readings.

With house incoming voltage at 122 VAC your readings should be higher than in the Dynaco manual where they start at 117 VAC input. Your bias supply voltage seem about right at 57 VAC and -79 VDC after the diode. Now both amps have pin 4/6 at 430 and 437 VAC. Pretty good and reasonably close. However amp 1 has 590 VDC rectifier out and amp 2 has 548 VDC out. That is not close to matching so lets keep looking.

Amp 1 output tubes has a -59 VDC tube bias reading which is about right. Amp 2 has -69VDC. That seems way high. You say the amps bias just fine and sound good. Have you measured both of the 11.2 ohm resisters that you test the biaset across? Are they both 11.2 ohm? Once tested what VDC settings did you use to set the bias? I assume they are both set the same. Please check.

Once that all seems right then my issue is this. Amp 1 at -59 VDC bias should be setting the current flow thru the output tubes to draw down the B+ voltage but yours is at 590VDC. Very high, nearly max and not what I would expect. You replaced the original cap with a new system but the original was only rated at 525 VDC. Amp 2 has the opposite problem. At -69 VDC of bias it should be almost cutting the output tubes off and driving your B+ to maximum. The B+ is 535VDC so that might be OK – maybe but -69 VDC seems way to high. I would guess that both would have about -59 VDC bias with B+ at 490/510 VDC. All with about 100 mAmps of total current thru the 11.2 resistor for a reading of about 1.12 VDC. If you set the biaset so the reading is higher (1.3 or 1.4 VDC) then I would expect the B+ to be slighty lower and the tubes bias voltage to be reading in the low -60/62 VDC range.

I question the bias resistor values and wonder what setting you used to set the bias. Both of my MK IIIs have voltages that may not be exactly equal to each other but are pretty consistent between amps. I also have a spare MK III and it has almost the same VDC/VAC values as my other two. Another concern is if there are other modifications that you do not know about or you did not state to begin with.

I am very interested in hearing more of the story after you respond. Interesting numbers for sure.

I’ll check the readings again and report back.

Updated readings with line voltage of 123.1 vAC are below. The variance between the two amps is now much less. My plan is to construct a voltage buck to reduce the line down to the 115-118v range. I'm not sure why my first readings were so high on amp 1.

VTA board for amp 1 and 2:
- B+: 415 and 428vDC for amp 1 / amp 2
- Red/Black bias: 57.5 and 56.9 vAC for amp 1 / amp 2


Rectifier amp1:
- Pin 2 and 8: 545 vDC
- Pin 4 and 6: 432 vAC

Rectifier amp2:
- Pin 2 and 8: 553 vDC
- Pin 4 and 6: 434 vAC

6550 amp1
- Pin 2 and 7: 6.75 vAC
- Pin 3 and 4: 537 vDC
- Pin 5 and 6: -66.8 vDC

6550 amp2
- Pin 2 and 7: 6.75 vAC
- Pin 3 and 4: 541 vDC
- Pin 5 and 6: -70.5 vDC

Much closer readings between amps. Much better. Your overall B+ voltage is still pretty high. The negative bias voltage on each output tube is as well. They are consistent with each other. I still wonder what the Biaset voltage is that you use. And the actual resistance of the 11.2 ohm sensing resistor. Did you measure those? What voltage did you set the Biaset for?

My MK III have the original 11.2 ohm resistors and they measure pretty close. All new resistors other than that one and all new capacitor. Stock circuit. I set the Biaset for 1.1 to 1.2VDC. They sound and work great. I have also tested them with an HP 8903 THD analyzer and the results were better than I thought. Thru the mid band and 10 watts of output they measure .07 %THD. Hopefully after your rebuild you can get some good, similar results.

The VTA driver board replaces and upgrades the original bias circuit. It has two bias pots and replaces the 11.2 ohm resistor with a 10 ohm for each output tube. Here's the description, as quoted from Roy:

"The VTA driver PCB has one bias pot for each output tube which will allow you to balance the currents in each output tube individually. This way you can measure the voltage across each resistor and therefore the current thru each output tube individually. With the 10 ohm resistors in place the proper bias adjustment voltage reading is now 0.50 vDC."  

Ok so .5 VDC on each resistor using the four adjustment pots yields 50 mA per output tube. I would think you would have different readings on pin 5/6 for each of the four tubes. Not just one reading per pair. If the tubes were perfectly matched they would have close readings though.

With 100 mA per pair of output tubes your load is similar to my MK IIIs. Your SDS cap board does have slightly higher total capacitance though. That will help raise your B+ but it still seems higher than I thought it would.

As long as everything is working right your values are more consistent than you originally published and you should be good long term.

I'm not sure why those first readings were so out of line. My guess is that I recorded them right at start up and the voltages hadn't settled. They were made just as I had completed the assembly and turn on of the first amp. I was trying to catch any catastrophic build errors.

Thanks for your help. I still plan to lower the AC line voltage.