bnc31 wrote:The PAS preamp has the stock circuit boards, but, they are new replacements from dynakit.com. Robert

Are you referring to Dynakitparts.com? I thought they only had circuit boards for Dynaco power amps, not preamps. Have I missed something on their website?

Charlie[/quote]

I apologize for the bad sentence structure. What I have from Dynakitparts.com is the driver board for the ST-70. The PAS replacement boards were purchased through ebay.They could be Roy Mottram's boards, I do not remember.

Robert,

Thanks for your reply and no apology is necessary. I just wanted to be sure I hadn't missed something. Actually, I "scratch" built my PAS preamp back in the 90's from a NOS chassis/cover/transformer, purchased from Sound Values and circuit boards/selector switch/Noble pots, purchased from Frank Van Alstine. Still works great, especially with the new ST-35 amplifier I just built, primarily from Dynakitparts except for Triode Electronics transformers. BTW: I replaced a SET (2A3) amplifier with the ST-35. I am driving a pair of "one way" folded horn loudspeakers, am very pleased with the results, and it is unlikely that I will return the SET to service.

Charlie

ST70_Junkie wrote: FYI, my ST-70 currently has the stock driver board.

Both the tubes4hifi board (12au7 model) and K&K Audio boards run, or can be modified to run, without global NFB. And both would still sound different from each other. Just as you rightly point out that low THD is exploited in the marketplace, so too is the superstition about NFB "sucking the soul" out of an amp.

The stock ST70 has gobs of global NFB and is possibly the most wooly, veiled, congested, lazy thing I've ever heard. And the K&K driver, which has none, is the least "tubey" sounding of any I've auditioned. My point is that driver design can make or break a topology regardless of class of operation, output tube choice and configuration or the use of global NFB.

A driver design which inarguably failed to utilize the strengths of a given designer output transformer in a SET amp would be rejected without a second thought. That fact that it is permissible in the case of a PP amplifier such as an ST-70 boggles my mind.

bnc31 wrote:Robert,

BTW: I replaced a SET (2A3) amplifier with the ST-35. I am driving a pair of "one way" folded horn loudspeakers, am very pleased with the results, and it is unlikely that I will return the SET to service.

Charlie

Charlie,

I too enjoyed the sound of the ST-70 on the LaScalas, very much. I also enjoy the sound of the SET amp driving them. I guess I get to enjoy both. Oh well, it is a burden I will have to bare...

Robert

Brinkman wrote:
Both the tubes4hifi board (12au7 model) and K&K Audio boards run, or can be modified to run, without global NFB. And both would still sound different from each other. Just as you rightly point out that low THD is exploited in the marketplace, so too is the superstition about NFB "sucking the soul" out of an amp.

Well I have not heard that description of NFB. I just like the "minimalist" approach. I am also the same way about motorcycles, when it comes to pure enjoyment of riding them.

What would be involved in running the ST-70 without NFB, and it still remain a stable running amp?


Robert

ST70_Junkie wrote:
What would be involved in running the ST-70 without NFB, and it still remain a stable running amp?

A great person to ask would be Kegger at Audiokarma. He emailed me directions on how to modify the VTA driver to run open loop (no global NFB). It is his preferred mode of operation. You can always decrease the NFB by simply replacing the NFB connection from the 16-ohm tap to the 8-ohn tap. When I was using the VTA driver, I never attempted to remove NFB completely.

Shannon at diytube forums designed a driver for the ST70. In the instruction manual to it, open-loop measurements are included so I'm sure he could explain how to modify his driver for that mode of operation.

The K&K Audio driver doesn't use NFB in its stock configuration. You can ask any questions at the K&K Forum at Audio Asylum. This is the board I'm currently running and yes, it is stable.

However, a great sounding driver that uses NFB isn't going to necessarily sound better without it. There's lots of topologies out there and if any "one way" was the best that's how all amplifiers would be designed. NFB helps simplify the design of a wide-bandwidth driver circuit but is not a prerequisite for it.

I know the story's been told a million times, but the original ST-70 circuit was designed simply to be inexpensive and functional. The power supply was minimal. The rectifier was being pushed to it's limits. The irony of all this is that it allowed A LOT of ST-70 amps to be sold back in the day. Had it not been a success, we wouldn't be here discussing all the different options of how to go about modifying it.
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I wanted to add that in my last post I asserted my opinion in regards to the ST-70's stock driver. Later in the evening I realized that I may have disrespected Kevin at dynakitparts.com by speaking unfavorably about a product he sells. Kevin stands behind the products he sells, is a pleasure to do business with, and is more than generous with his time when it comes to troubleshooting. He sells detailed reproductions of original dynakits and his transformers are top-notch. It was not my intention to offend.