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Old 1st March 2009, 01:49 PM   #11
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Further on the subject of the MK VI I recently purchased a pair which cosmetically was pretty beat up. They have good transformers and they came with Sonic Frontier upgrade boards which are exactly the same circuit as the MK VI Dynaco boards with upgraded parts.

I took one so far and did the following to it.

1. New PS caps in the second bank & bypassed with film caps.
2. Separate Bias supply control for each output tube.
3. 10 ohm resistor in each cathode to measure current via voltage.
4. Separate coupling cap for each output tube.
5. Separate grid resistors for each output tube and lowered to 39K from 47K.
6. 0.5A fuse in the CT of the PT.
7. Installed Sonic Frontier upgrade board.
8. Removed bias control from front panel.

Did a listen test this morning and all sounds real good. As I have found with all Dynaco units the voltages on the tubes even with new or as new tested tubes never seams to match the spec's in the manuals. Also there is always this "feedback" connection from one side of the OT UL tap to the driver board on all Dynaco amps with such OT. In the MK VI it goes to pin 5 of the driver board as shown in the attached schematic.

Can anyone give the real value of this connection?
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Old 1st March 2009, 02:37 PM   #12
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That feedback cap from the UL tap compensates for the fundamental primary imbalance in the (rather cheaply made) output transformers. You can adjust its value for a given feedback scheme by feeding in a 1kHz square wave, monitoring the output, and adjusting for equal ringing on negative and positive halves of the square wave (I usually use a ceramic trimmer cap in this position). Then adjust the RC network on the global loop to get the overall ringing minimized.
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Old 1st March 2009, 02:43 PM   #13
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thx SY

One other question. Normally I would not be worried about this. It seems that not matter what set of tubes (8417) I put in I still get a different current reading on the side which uses this UL tap as feedback.

This can happen with a new set or a good used set. It happens on the rebuilt amp as well as the stock amp.

Any thoughts as to why?

Also since the rebuild amp has separate current adjust would you balance as normal here?

Thx again
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Old 1st March 2009, 02:52 PM   #14
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I think separate current adjust for each side (at minimum) or each tube (optimum) is an excellent idea. As for your imbalance, let me guess that the side with the feedback cap runs hotter?
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Old 1st March 2009, 02:54 PM   #15
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Yes it does.

Again adjust to equal balance for each tube regardless yes?
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Old 1st March 2009, 03:05 PM   #16
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Yes, adjust for equal currents.

And for 5 points here in the bonus round, why would the side that needs the feedback cap be expected to run a little bit hotter?
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Old 1st March 2009, 03:18 PM   #17
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I wish I knew SY its the tube amp mystery of the day.

Care to inform?
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Old 1st March 2009, 03:21 PM   #18
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Hint: disconnect the output transformer and measure the DC resistance between center tap and each end.

(I was tempted to merely answer, " You have been in Afghanistan, I perceive.")
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Old 1st March 2009, 03:26 PM   #19
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understand now, thought about that but just seemed to obvious for the way you set up the question
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Old 1st March 2009, 03:51 PM   #20
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"Subtlety" is not my middle name.
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