MK3 question

Last year I rescued a basket case original ST70. I chose the VTA driver kit as a more modern update. I loved it enough to embark on a MK3 clone build pieced together by several vendor's replacement items, but centered around the VTA drivers.  Everything used for the MK3s was new except for original output iron. Scored a nice pair of OPTs that started me on this project.

The amps have been up and running for months and sound wonderful. There is one nagging question left unanswered. I asked on another forum of knowledgeable tube folks and have tried to reason it out myself. I don't feel it has been completely addressed.

On most MK3 (re)builds and pretty much all original MK3s there is a 680 ohm resistor between the common and 16 ohm speaker tap. The assumption of many is this is a no-load protection for the transformers. I suppose it could be, as some custom builds that have the resistor installed have it between the common and 8 ohm tap, as the 16 ohm tap goes right to the NFB lug on the driver. It is drawn on the original MK3 schematics at the 16 ohm tap, and then the 16 goes to NFB.

My original thought was it was part of the NFB circuit and that by installing the VTA driver it may not be needed and longer. There is plausibility in the no load protection, except if that was the case, why doesn't the ST70 also have it?

In any case, I always wonder about stuff that can't be readily explained. I completed square wave tests at 20, 1K and 10K with and without the resistor and found no difference with it installed between common and 8 or 16 ohms. I decided to not install it in my amps.

Does anyone here actually know why that resistor is in the original schematic?  I'd love to have this answered!

My MK 3 clones.

I will certainly protect the amp from a no-load condition. As it connects between 16 ohm tap and ground it has nothing to do with feedback, it's only a "last resource load".

It could be smaller.

And yes it's missing in ST70. ( not in mine :-) It does no harm, it only comes in action at "no load"
like a lightning arrester.

Thanks.

No load protection does seem to be the prevailing wisdom. I think the fact the it is left off the ST70, which I am sure has the most sales of their tube designs, is what throughs me off. You'd think it would be universal. I am pretty safe from a no load situation in my set up, but I'll probably add them next time I'm in there, or before selling if I ever do that.

I like the bias test points in the quad cap hole. Very nice.

EWALAA wrote:I like the bias test points in the quad cap hole. Very nice.

Thanks. Did the same with the ST70 in place of the front octal sockets. Neater looking IMHO.

The MK3 chassis I used are the ones sold by Triode. The spot where the quad cap would normally go is cut for a standard octal socket. You'd need to buy one of those twist lock cap adapters to fit one in that spot or a cap clamp would work too, I suppose. It is the reason I chose that chassis for the project. I planed on the cap board mounted inside with the external test points from the start.

SS904 wrote: There is plausibility in the no load protection, except if that was the case, why doesn't the ST70 also have it?

Does anyone here actually know why that resistor is in the original schematic?  I'd love to have this answered!

I, like so many others, used to wonder about that MK-III 680 ohm resistor, and I read all the same speculation that you did. I was in contact with Dave Gillespie (inventor of the SCA-35/ST-35 EFB (bias) circuit, and author of of the thorough "Dynaco ST-70 Base Line Testing" article), asking him something about his EFB. The subject of MK-IIIs came up, so I asked him about the 680 ohm resistor. Below is his quoted reply to me, reprinted with his permission. I hope you find his response as informative and helpful as I do. He came to his conclusion by testing, rather than speculation. When I rebuild my pair of MK-III amps, I'm going to include the resistors.


"As for the 680Ω resistor, the idea of it being part of the NFB ladder circuit is just silly. I’m sorry, but that idea cannot be supported from any angle of circuit analysis. Neither does it have anything to do with providing protection for the output transformer or tubes should the secondary become unloaded, either.

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 others 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"

Interesting and timely thread....I like to switch speakers out at will and I was VERY concerned about disconnecting speakers from my VTA ST120 while it was operating. I was told by many with much more tube knowledge than me that running one of these amplifiers with no load was not good for the transformers. I recently added a couple 10w/100ohm resistors. I was also told I didn't need to add them to both 4ohm and 8 ohm taps, just the 4ohm taps.....I sleep MUCH better now!

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1976 Mk VI has 620Ω  R122 between 16Ω tap and 0vA.