Well, I've been busy lately, evaluating different combinations with my Modwright Transporter -> Modwright LS 36.5 -> McCormack DNA-500 -> SP Tech Revelation MR1 MK III Ultimates. Mainly I've been eval'ing balanced interconnect between the pre and the amp and have disocvered a great (but fairly pricey) XLR set by Synergistic Research, the Tesla Accelerators (active coupling, etc.). Anyway...they are ruthlessly revealing, yet not at all bright or boomy at the extremes. They sound like no cable at all.
Why am I mentioning this? Cuz once I found these I realized it was a good time to call Brendan at Tube World and try a couple sessions of tube rolling for my Transporter. I currently have a USA Sylvania 5U4G rectifier in there, and some Sovtek 6N1P's (which I keep liking better than other 6CG7's or 6H30's I've tried). Dan and Brendan have both recently become enamored with the 5BQ7 signal tubes. Moreover, when I told Brendan that I was still breaking in the SP Tech's he mentioned a nice big GZ32 rectifier bottle to round things out, and suspected it goes even lower than the 5U4G. BTW, Brendan is a great source of history, experience and guidance. He really knows his stuff. So, since the 5BQ7's wer so cheap ($30 pair) and the GZ32 sounded interesting an not outrageously expensive ($100) I ordered them.
I first tried the GZ32 in place of the 5U4G, and let it run for several hours (it's not an instant hot kind of tube and Brendan recommended patience). I did not initially replace anything else, so as to keep a singular variable. If there is anything about the SP Tech Mundorf cap break-in (a 600 hour exercise) it is that they can be a bit forward during this process (like row one for quite awhile). However, their ability to handle detail and dynamic swings are priceless, and I wanted to make sure I didn't lose those qualities with any new glass.
To my surprise, the sound qualities of the GZ32 were just a little too rounded for my taste, at least with the 6N1P's. The detail was still there, and if I hadn't heard the SP Tech's incredible dynamism earlier I likely would have been pleased with the slight softening (I use Brendan's "roundedness" comment cuz that's a good descriptor). It just lost some air, some attack and some "live" feeling. Instead of the players being in the room they sounded like they were great recordings; nice, but not enough.
I clearly didn't give the GZ32/6N1P combo enough time, and will go back to it later, but I was itching to hear if the swap of the 6BQ7's would be the right synergy. Gave them a couple hours to settle in. So, synergy? Well, in a word, no. The promise of deeper, more extended bass from the rectifier now came partly true. The bass output was increased but at the expense of tightness and musicality. The soundstage seemed slightly smeared too, as if the lower registers were out of phase.
It was time to swap out the rectifier and try the more aggressive 5U4G. Voila. The Cowboy Junkies 1986 Whites Off Earth Now, a collection of blues covers, has been reissued by Mofi and has an incredible you-are-there feeling. It was recorded with a single Calrec Ambisonic microphone, and the details and air are palpable (and that's only the rebook layer; the SACD is slightly better still). On Crossroads, the bass guitar is much more real with this 5U4G/6BQ7 combo than any previously. The attack and decay of the recording is portrayed beautifully, with just enough sizzle but no harshness (Mundorf cap breakin qualities discounted). This is the combo I'm staying with for awhile. The 6BQ7's bring a better tonal quality and a slight increase in gain, but with little or no downsides.
I will keep (actually I don't even know Brendan's return policy frankly) the GZ32 rectifier and bring it back out later, when the Revelations are mature. Additionally, it might be cool to try it (with lid off, it's 5-6 inches tall) in the LS 36.5. Anyway, it certainly does some things very well, and for those of you with digititis, it is a beautifully smooth rounded painting, very analog, across the entire fr. Hell, I may use it when listening to difficult recordings....ah, the sickness we call tube rolling.