Originally Posted by NightFlight
Possibly too late now, it takes nearly no time to solder in the components and it would be better to break-in while you work around the house.
On a happy note, my BH S.E.X brain surgery was a success! At least after two hours of troubleshooting lol. Never solder at 02:00am in a tight rats-nest with poor lighting and several beverages! At least no light-show to accompany the fanfare.
Early impressions are very good. I previously characterized the S.E,X (2.0) as very airy and spacious with a decent soundstage and great detail - surprisingly so for tubes. Where it lacked was some bass punch.
The old girl now has 2.1 iron and plate chokes installed to bring it up to current standard. I changed out the coupling and parafeed capacitors for some Mundorf Supremes. Both of these upgrades are expected to need a fairly long settling period, and I'm in the first two hours or so. I also removed the 120ohm jack resistors for a little more current / less attenuation on power-hungry cans.
According to Doc, the upgraded iron is supposed to improve bass linearity by an octave or so and increase the overall dynamics. I can clearly hear that - both bass extension and definition are better, and the amp seems a little more SS-sounding in terms of not rounding edges. There's a hint of harshness surrounding some of the transients which I trust will smooth out.
The Mundorfs are said to be "highly detailed yet musical".
Through the 650's right now the detail-retrieval is way up over what it was already - with more dynamics and punch without killing the 650 sound. The soundstage is a little more closed-in than it was - likely as the bass warmed up a bit. The whole thing got more Crack-like but with better detail than what I get from even more lively tubes like the Cifte 12AU7/5998 combo I enjoy so much. I may have to experiment with the output resistance a little as the gain is very high so volume-control within reasonable ranges is more restricted, with the benefit of more overall current / voltage swing. The objective there was getting more power and punch out to my LCD2.2c's and HE-400's - ditching the resistors did that in spades without noticeably increasing the noise-floor.
Hoping the rougher edges smooth out a bit - almost a certainty with these particular mods - gotta let them cook. All-in-all a success from the get-go. It's like a more refined Crack. Detail, great power and punch, and a dead-black background even using the 32ohm taps and no output attenuators. Win!