Hi Sandy.
Your hypothesis ignores the fact that Hypercat still produces very high stain when used with the Potassium carbonate B solution of Pyrocat HD. I would guess that the pH of the Hypercat A/ Pyrocat B solution would be lower than the pH of Pyrocat-HD. If the difference in the pH levels of the working solutions of Hypercat and Pyrocat HD was responsible for Hypercat's tolerance of its much greater ascorbic acid content, I would expect to see much reduced stain formation when using a working solution made up of Hypercat A/Pyrocat B. It seems clear to me that the presence of sodium metabisulfite reduces the pH of a working solution of Pyrocat HD, and the addition of ascorbic acid would reduce it further, which, beyond a certain point, could impact stain formation. According to your own tests, that point is fairly low. I would expect the same, or similar result by adding sodium metabisulfite to Hypercat. If I wanted to increase the energy of hypercat, I would substitute some of the ascorbic acid for sodium sulfite, which would activate the catechol/phenidone pair, but Hypercat is energetic enough as-is, and the addition of sulfite would require much higher mixing temps, and generally compromise the formula, I think. Adding sulfite to the working solution wouldn't work very well, because the ascorbic acid is scaled to reduce general stain to zero, and the addition of even a small anount of sulfite would impact stain formation, just as the addition of a smal anount of ascorbic acid to Pyrocat HD impacts stain formation.
For all their similarities, it is the differences between Pyrocat HD and Hypercat that fascinate me. Pyrocat HD depends on the catalytic action of the sulfite ion on the catechol/phenidone pair for its activity, while Hypercat relies more heavily on the ascorbic acid/phenidone pair, lacking any form of sulfite to activate the phenidone/catechol pair. I suspect that like it's close cousin, phenidone/hydroquinone, the developer product of the phenidone/catechol pair is itself a developer, possibly more active than the catechol itself, while the product of the ascorbic acid/phenidone pair is weakly acid and retards development. The choice of restrainer/antifoggant is also interesting. Pyrocat HD uses KBr, and Hypercat, BZT. Thes agents also work in completely different ways. KBr retards development, and reduces contrast and film speed, while BZT works much more selectively, having no effect on the progress of development, which might be why Pyrocat HD needs 60% more phenidone than Hypercat uses. Then there's the choice of alkali. Since both developers use separate alkali solutions, it's very easy to isolate the effect of the alkali in testing, by substitution. I should rephrase, it's easy to test the obvious effects of the alkali, such as activity and stain formation, by substitution. I believe that the choice of alkali also has an important effect on the formation of adjacency effects, but while that seems obvious enough, it's very difficult to quantify, and even more difficult to ascribe the difference to the choice of alkali. The conventional wisdom has always been that a strong solution of a weak alkali is to be preferred to a weak solution of a strong alkali, for robust and constant working properties, but the formation of edge effects depends on the local exhaustion of the developer, which is much enhanced by a weak solution of strong alkali. It's all very fascinating.
Back to the subject, it seems that if one was interested in adding ascorbic acid to Pyrocat HD, one should consider substituting sodium sulfite for a part, or all of the sodium metabisulfite in the Pyrocat HD formula. This substitution would negate the pH reduction effect of the addition of ascorbic acid with its attendant reduction in stain formation, while maximizing the energy of the developer. If the reason for specifying sodium metabisulfite over sodium sulfite is to decrease the pH of the working solution, it seems to me that the same thing could be accomplished by the scaling of the alkali, as Steve Sherman suggests above. I think that the high phenidone content of Pyrocat HD is a prime contributor to fog, and only needed to offset the effect of KBr, which is added to control fog. A real catch 22.
Jay