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Follow-up on new build ST-70 with the 6NS7 tube

A couple of weeks ago I posted about my new ST-70- based amp from Chris Keller in NC and I asked how much it might benefit from 6NS7s instead of 12AU7s. I learned from inmate airtime in a prior post about the ability to replace just the center 12AU7 with an adapter. So I did that earlier this week.

My impressions are consistent with the consensus among those of you who have done this before, or replaced the driver board, or simply heard a lot of both tube types in different applications. It's a better sounding tube for this application. The amp now sounds bigger, clearer and more dynamic. The bass is stronger. I'd say the changes are beyond subtle but not revolutionary. And I hear no downsides. I lost no high end extension and there is no his or hum or buzz or other artifacts that I can hear. It's just a better sounding amp.

It doesn't even look bad. It's kind of cool. Although it does look a bit like it's ether very happy to see me or flipping me off, what with the smaller tubes flanking it on each side. Har Dee Har Har.

All of that said, I don't hate the 12AU7s. The amp went from sounding really, really good to really, really good+ so I'm not ready to join the !2AU7s Are Garbage Society as far as their impact on SQ. The amp sounded great with them even if not quite as clear, bold and dynamic. And I've noticed that warm up is more vital for the 6NS7s. When the amp is cold, there is a thinness that goes away over half and hour or so, and I didn't notice that nearly as much with the 12AU7s.

As a follow up comment and question, my preamp is an all 12AU7 piece (Rogue Audio RP-5). I'd be interested in knowing how much that preamp might be holding back the improvement potential. I love the RP-5, including its built in phono stage. That preamp is flawless operationally, silent, and sounds really nice. But I cannot deny being intrigued by the SP14 preamp from VAT/Tubes4HiFi.

As a related question, in what applications does the 12AU7 most impact performance and least impact performance?

Thanks for this most enlightening and interesting (at least to me) discourse.

P.S. I chose an NOS Sylvania "Chrome Dome," which I bought from Brent Jessee. And both Chris, the builder, and Roy from VTA, the driver board supplier, were super helpful in thinking this through. Roy actually recommended the tube type and the adapter I ended up buying.










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Edits: 04/01/23 04/01/23
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Topic - Follow-up on new build ST-70 with the 6NS7 tube - JoshT 09:14:38 04/01/23 (20)
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