Hi Ecclectique--most interesting. Sometimes it seems like half of high fidelity audio can be attributed to half a dozen extremely impressive individuals. I was cued into the 6922/6BQ7 sub in Roger Modjeski's "Manufacturer's Comment" to the 2002 Stereophile review (http://www.stereophile.com/amplificationreviews/560/index8.html). The relevant context is this excerpt:
I take good note that a 6922/6BQ7 is *not* a general substitution.
There is something a little more interesting going on here. Pre-amps like the Supratek that use 6SN7s and 6L6s are fertile platforms to rolling; there is no question that NOS tubes elevate the performance. But the KT88 is a desert for rolling; there is the GEC/Genelex and Tung Sol 6550--which can put you back $1K+ for the quad--or about half a dozen current productions that are more similar than different (JJ, Electro-Harmonix, Svetlana, Sovtek, Shuguang, etc.). So Roger's approach somewhat anticipated--in philosophy, but not in implementation--that of the H-cat; i.e., superb reproduction occurs (some would say demands) exacting control of balance and phase. This minimizes left/right and time smear, and results in the correct delivery of those higher-order harmonics and slight time delays that differentiate middle-C on a cello, from that on a high-school violin, from that of a Stradivarius. This is not what we do explicitly when we roll NOS, but perhaps some would say that implicitly this is what is happening, and that this is what separates the WE's and GEC's from the Sovteks.
Through my research, I determined that the much-maligned Chinese 6550 was capable of the highest peak-cathode currents of any 6550 available, and about 1.5 times higher than the venerated Genelex KT88. Once the grid-leakage problem was solved through the driver design, it was full speed ahead with the Chinese tubes. Granted, I have to toss quite a few, but far fewer now that my driver circuit can tolerate grid leakage. As to rolling the driver tube, if you like the 6922 or its higher-current 6H30 version, go ahead, the circuit won't mind—though in this application I prefer the 6BQ7 for its ruggedness, less delicate grid, and higher current. Just be sure the DC is matched in the two sections, as it is directly coupled to the output grids. We want to keep the bias balance within 10% in the output tubes.
I take good note that a 6922/6BQ7 is *not* a general substitution.
There is something a little more interesting going on here. Pre-amps like the Supratek that use 6SN7s and 6L6s are fertile platforms to rolling; there is no question that NOS tubes elevate the performance. But the KT88 is a desert for rolling; there is the GEC/Genelex and Tung Sol 6550--which can put you back $1K+ for the quad--or about half a dozen current productions that are more similar than different (JJ, Electro-Harmonix, Svetlana, Sovtek, Shuguang, etc.). So Roger's approach somewhat anticipated--in philosophy, but not in implementation--that of the H-cat; i.e., superb reproduction occurs (some would say demands) exacting control of balance and phase. This minimizes left/right and time smear, and results in the correct delivery of those higher-order harmonics and slight time delays that differentiate middle-C on a cello, from that on a high-school violin, from that of a Stradivarius. This is not what we do explicitly when we roll NOS, but perhaps some would say that implicitly this is what is happening, and that this is what separates the WE's and GEC's from the Sovteks.