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The K&K Audio ST70 Boards: Part 3 of 3

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Publisher’s note: Holmes returns on the K&K Audio ST70 modification. Read Part 1 and Part 2.


Pound for Pound…..

A few years ago, I started writing about the K&K mods to the ST70.  I went over some various steps I take when building/rebuilding anything, tweaks like rubber shock-mounts, and so on. Here is the finale to the story.

When sports people talk about athletes, they often refer to a particular athlete being, “pound for pound, the best…”. I don’t think the ST70, in original trim, was ever pound-for-pound a contender for best anything, except for bargain. To save a few bucks, literally, Dynaco went with a triode-pentode circuit, so that voltage gain and phase splitting could be accomplished by one tube. The circuit, for lack of a better word, sucks. The split-load phase inverter doesn’t have sufficient drive to push the output circuit to full power with low distortion. Those 7199 tubes, contrary to what some people believe, were not better than the other triode-pentodes available, but did give Dynaco the ability to sell a less common tube type, with a much higher profit margin.

All that was necessary to make the ST70 a much better amp was an additional tube, namely 3 tubes on the input, instead of 2, probably a single 12ax7 or 12at7, shared for both channels, driving a pair of 12au7. The result would’ve been similar to the much better sounding Eico HF89, perhaps the best amp of its type.

The K&K board gets rid of the horrible stock ST70 triode/pentode driver circuit completely. The phase splitting is accomplished by a Lundahl input transformer that also gives you the option of changing the gain of the amplifier. The board also includes modern high performance diodes that are quiet with fast settling time, has the filtering caps on the board instead of old fashioned inferior “can caps” on the chassis and a time-delay circuit to allow the output tubes to warm up before applying plate voltage to prevent cathode stripping. This delay function was one of the reasons the original ST70 used the 5AR4/GZ34, an indirectly heated rectifier that had a built-in delay (a 5U4 put out full voltage in a few seconds, where the 5AR4 took 13~15 seconds to reach full voltage, preventing cathode stripping of the tubes).

Recently, my friend Marion, unhappy with the sound he was getting, asked me about the K&K ST70, and I told him it’s the best deal in audio, but not easy to assemble. Marion, not knowing how to use a VOM, not to mention an inability to read schematics or solder, asked if I would assemble one for him. I said sure.

Since my first K&K ST70 project, K&K Head Man, Kevin Carter, has upgraded the board to support the KT120 output tube, meaning the board allows you to use everything from a 6L6GC/KT66, up to the massive and wonderful KT120. With adapters, you can use even curiouser and curiouser tubes. That’s tremendous flexibility. It means hundreds of combinations of manufacturer and tube type are possible for those who obsess over tube rolling. I do have a personal favorite output tube for the application, but when using Kevin’s mods, even a plain old GE 6550 will sound great. Inherent to the circuit is its ability to make parts “disappear,” meaning, when properly constructed, the “sound” of a particular type of resistor, or tube, or capacitor, is less obvious. He does this by forcing the amp to be highly symmetrical, the push-and-pull being very nearly identical, and without resorting to feedback.

Back to tubes. If you want to fork over the money for real KT66, you could change the idle current so that you are able to enjoy them, without destroying them. Western Electric 350B would be a stretch, since they have a relatively low maximum plate voltage, but who knows? If you limit the total dissipation, which is plate voltage times the idling current, they might last longer than Methuselah.


Options Options Options

I suggest you go to K&K’s website, which is informative. Looking at your starting point, do you have an old ST70 to modify? Do you want to do as much with stock parts as possible? Do you want an all-new amp, new stainless steel chassis, new hardware, new transformers, etc? Do you want a low-power amp with maximum purity? Do you want the most power possible from the design? Are your speakers very efficient, or no so efficient? The multitude of starting points, parts choices, configurations, and possible endpoints makes it impossible to give a thorough overview of what is possible with the K&K ST70 mods.

You can outfit the amp with a number of options. My friend, Marion, chose to purchase the regulator boards for the input tubes, with two additional small printed circuit boards that are under the chassis and supply the input tubes. Officially called the “Shunt Regulator Kit for K&K Audio ST-70”, at $79 each, it is a good investment. The board acts as a constant current source, which “fixes” the mu and transconductance of the tube, meaning the tube performs at its peak. Further, the regulator boards provide nearly perfect DC, so the noise floor is a googolplex better than the truly pathetic Dynaco circuit. So, less noise, and much less distortion. Better and better (way better). Is there a best? No! And anyone that tells you there is a “best” should be avoided. They’re like the guys that need “growing room”, or propose “5-year-plans.”

Next, Marion opted for the Lundahl UL output transformers, instead of the Dynaco design, which are said to be better than the well respected original iron. I’ll get to these in a moment.

To support the use of the much bigger KT120 tube, it is mandatory to upgrade the power transformer. The stock tube was the EL34, which doesn’t use as much filament current, and could not dissipate as much power as a KT120. There are a couple different options from different outlets, and the one that K&K offers seems as good as anything. So, Marion purchased the K&K Audio SST-70 for $170. Really, you shouldn’t scrimp on power transformers. Poorly spec’d power transformers are poor regulators of your power supply; they “droop” when used in Class AB amps. Because the K&K mod forces the tubes to run class A, the load on the power transformer is relatively constant. Even so, don’t use a stock power transformer, unless you are limiting yourself to EL34/6L6GC.

Since Marion didn’t have a donor ST70, he had to purchase a new chassis, which is thicker gauge stainless steel versus the original carbon steel, plus all the bits and pieces. Lots of bits and lots of pieces.

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