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For those searching the archives

Posted by 1973shovel on July 28, 2021 at 08:20:31:
I thought I'd give further information on the 680 ohm resistor from the 16 ohm tap to ground in the MK-II and MK-III amps for those, like myself, who still refurbish and use old Dynaco equipment.

I always questioned the purpose of the 680 ohm resistor, wondering why the ST-70 or MK-IV didn't require it. Some speculate speaker protection, and the late Bill Thomas (RIP, Bill) thought it a voltage divider for the feedback network. I ran the question by Dave Gillespie creator of EFB and writer of ST-70 baseline testing. Here, below the line, is his reply to me, used with his permission.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

"Through experiments and testing I did many, many years ago - when I too was questioning the reason for this resistor - the reason became quite evident when I was so heavily involved in the study of NFB and applying it in a stable way globally around vacuum tube amplifier designs. The resistor serves one purpose: It helps to maintain HF stability when the amplifier is either unloaded, or has only a capacitive load connected to the output. But here's the real point of understanding: The resistor is required because of the unique characteristics of specifically the Dynaco A-430 and A-431 output transformers. The HF stability of the MK II and MK III amplifiers that use these transformers is certainly good, but Hafler was aware of their limitations in this regard as well. By the time it came to producing the A-470 transformers for the ST-70 and MK IV amplifiers however, he had is para-winding design down pat, such that these latter two amplifiers using the A-470 display absolute stability into ANY kind of load, and guess what? Don't need any add-on load elements (i.e., the 680 Ohm resistor) to maintain it, either.

If Hafler was anything, he and Laurent were creatures of habit when it came to designing their vacuum tube equipment, which is why all their designs were so similar. And why not? When something works, you stick with it.

But notice that the ST-70 and MK IV amplifiers use no such resistor strapped across the output - which surely would be there if for the reasons the other have cited, but theirs are not the reason. The resistor is in place to help maintain a minimal load on the amplifier PURELY for the benefit of maintaining the HF stability of the amplifier under certain load (or lack of it) conditions. It can be shown mathematically to have absolutely nothing to do with the NFB voltage divider ladder, and is much too high a value to provide any OPT or output tube protection if the amplifier is operated without a load. That protection is primarily provided as an added bonus by the presence of a global NFB loop placed around the amplifier in the first place.

Dave Gillespie"