Speaking about input tubes, keep an eye out for Philips/Pope/Adzam (holland) 6sn7gt. Nothing has come close to it in my system, nothing... Watch the auction site sometimes you may find one that slips through the cracks, I paid $50 for mine :)

I got those from Jim Mc Shane a small tube dealer in Chicago many years ago. He tests tubes very thoroughly at real world voltages and culls the ones that don't cut it. Any tube you get from him will be electrically sound. He's not selling these any more because there were too many duds in the lots he was getting. That does not mean there are no good ones left though.

Those CV181's are very nice. I have one in the amp and a UK version in the preamp. :thumbsup:

Were you running a real CV181 or one of the newer fake ones which are 6SN7s in disguise? A good 6SN7 nonetheless.

The 'fake' ones. They are good tubes and surprisingly better than many, but not all, old production in these amps depending on the rest of the chain. They take a LONG time to settle in so patience is required. The 'UK' version, not so long.

Funny that they felt they needed to market a very good 6SN7GT as a CV181, which is not a direct equivalent of a 6SN7. You'd think they might have labeled it as a VT-231 if anything. I don't like it when you can't trust the tube type.

I've settled on the 6SN7 input tube for my amp, as the 6SL7 that came stock, as some others I have, seemed to be slightly less transparent, other things being equal, although the latter provide more gain. Yet, I bet there must be a few 6SL7's out there that may even improve upon some of the not so great 6SN7's.

Between those two, in a triangulation, I believe lies the CV181 in terms of electric characteristics. I wonder if someone can shed some light into the perceived sound tradeoffs involved as a statistical group (i.e., generalize based on hands-on sampling). This may prove interesting because it would narrow down the potential trial-and-error experimentation into a "more likely to succeed" group (one of the 3 involved) ... In fact, I share FloridaBoy's view that input tubes may have a greater impact in the final sound over other alternatives, including rectifiers, especially if one settles the latter on one of the sure winners, like the Sophia Electric 274B mesh plate.

Maybe FloridaBoy or some of our other top tube-rollers?

Note: You can consider those "fake" CV181 alternatives also in this proposed tradeoff summary.

BTW, here's a link on a 7 year-old article reviewing the ubiquitous Shuguang Treasure CV181Z (interesting read): Two drawbacks with this tube are its taxing on the trannies and its price, but the reviewer is fascinated by it! The trannies may loose control of its regulation due to excessive current ... We may want to ask Dennis.

http://www.tnt-audio.com/accessories...s_cv181_e.html

Update: We can safely use the "fake" Shuguang Treasure CV181Z as it has a heater/filament current exactly the same as the 6SN7: 0.6 Amps (600 mA). No wonder it is a 6SN7 in disguise! All other (true) CV181's have a heater/filament current at least 50% higher at between 0.95 and 0.9 mA, seriously taxing the trannies:

forum.audiogon.com/discussions/shuguang-treasure-tubes

https://frank.pocnet.net/other/Servi.../cv3/CV181.pdf

The higher MU (higher gain) 6SL7 draws a heater/filament current 50% less than the 6SN7, at 0.3 Amps or 300 mA:

http://www.radiostation.ru/tubes/6SL7GT-GE.pdf

Thus, a good 6SL7 is healthier for the trannies and allows the latter to better control current in the same circuit topology. What may be good 6SL7's out there which do not reduce transparency (i.e., resolution)? This is one of the three directions in the triangulation cited earlier.

This is a link to a "premium" 6SL7 example, the 5691:

vacuumtubes.com/6sl7.html

My amp came with a 6SN7 as the stock driver tube. I've now tested 8 of them and, to my surprise, the two which I felt sounded the best were two vintage USA Westinghouse brand. Upon some research, I found that Westinghouse ran their own tube manufacturing plants, so these are likely originals.

But they sound great. Can't hear any noise from them at all, I had to toss two of my tubes for being too noisy. Well defined deep bass, clear highs, great vocals. Perhaps if I had a really good 6SN7 the Westinghouses would sound deficient.

I have no plans to try a 6SL7, even though I have 4 of them. Usually these two tubes don't substitute all that well for each other. If a circuit is optimized for one, then they usually don't sound as good with the other.

Dennis sent my KT-88 amp with the 6SL7, although I believe most people use 6SN7 for better sonics. I believe the amp handles both types without problems and the former is less demanding on the trannies and improves their control at half the 6SN7's heater current.

Yours came with a 6SL7? The last 15 or so amps that Dennis has old on eBay have been KT88 with 6SN7. He does switch things up.