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I finally got around to installing C4S boards in my 6 month-old Seduction and S.E.X. amp, and what a difference the two make over the already-refined sonics of the stock builds! The Seduction boards went in first, and resulted in the reported higher gain with the 6922 tubes and a ~5mV cartridge (AT120E w/microline stylus). Through the S.E.X. amp, the gain results in at least 1/4 of a turn on the pot of extra headroom. Also, the Seduction seems to have a somewhat darker, blacker background and more stable bass. Midrange also seems to pop out or "float" on certain recordings even more than usual, which I attribute to the quieter background. This is especially evident for things like chorused guitars or woodwinds like saxophones and bassoons.
My soldering station conveniently died when I tried to turn it on the next day to do the S.E.X. boards. Once I bought a new iron and popped the boards in the S.E.X., similar changes were evident. I've not yet updated the parafeed or coupling caps, as I may wait until ordering the MQ iron update and do it all at once. However, even without the latter, the amp is definitely improved - even cleaner and smoother sounding, as well as a bit louder. Bass has definitely improved in the lower octaves. Most impressively, percussive sounds are so much more "certain" and have a "snap" to them that wasn't there in the stock amp. Sounds (especially attacks) that previously would push the S.E.X. to its limit now are handled with extra headroom. I think some may mistake this for a more "solid-state" type of sound, but the warmth and presence in the midrange, as well as the lack of hardness/glare in the upper registers clearly identifies as tube.
The changes were definitely worth the $120 for the set of boards. I would recommend that anybody who still has stock amps perform this modification - it's so good, it makes me wonder why the kits don't come this way to begin with (higher cost, I'm assuming).
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There is also a "fear" in some that a tube amp with some solid state components is no longer a tube amp. Consequently, the SEX, Seduction, and Foreplay have minimal solid state parts (just diodes), but you would be surprised at how many folks won't even consider a tube pre with solid state rectification.
"it makes me wonder why the kits don't come this way to begin with (higher cost, I'm assuming)"
Thanks for the kind words! Part of the reason is cost as you guessed, but a big part of it is also how would you know what an improvement the C4S upgrades make in the kits if you don't build it without them first? We feel very strongly that the process of building our kits should somehow expand the builder's knowledge of what circuit changes do for the sound.
I totally see your point - part of the fun of the Bottlehead experience is the journey we all take in getting to the destination of perfect sound. I also learned a bit of theory by studying the C4S circuits and trying to figure out what they do (and why they do it better than a simple plate resistor). If everybody else's experiences hold true, I can expect yet another layer of improvement when I upgrade the caps and output irons/plate chokes!
Slightly off topic, but related: once I get the MQ irons in there, I may want to play around with 6EM7's in the circuit. Do the C4S boards need to be adjusted for a different bias point? Also, several have described the 6EM7 as sounding "different" or even inferior in the stock S.E.X., but "good" with the C4S and MQ upgrade. Does "good" mean similar to the sound of the 6DN7 in the stock amp, before C4S/MQ/cap upgrades? Or a different sonic signature completely? I like the sound of the 6DN7's in there now, but wouldn't mind and extra dB or two to help with my Klipsch bookshelf speakers. (I'm assuming any benefit will indeed be in the range of 1.5 or 2dB in gain over the stock circuit - I've heard people say that the 6EM7 *may* put out 3Wpc in the S.E.X., but that extra watt is still one watt short of a doubling of the original output (2W). 4W would be 3dB louder than 2W, so I'm expecting that 3W would be only 1.5dB over 2. Correct?)
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