I'm on Tom's (N6AJR) side on this one. One of the overlooked aspects of changing out 811As with 572Bs, is which ones you choose.
The Svetlana is NOT equal to the Taylor 572B or the Amperex 572B. If you doubt this premise, look here: http://www.rfparts.com/572b.html
. What they say is not advertising hype.
While the amplification factor slightly favors the 572Bs, they do in fact have a more plate and inter-electrode capacitance. In some designs, this can (and does) cause loading problems.
The Svetlana tubes are almost the exact size of the 811A, but the Taylor and Amperex 572B are longer, and cannot be used in some amps as a result.
The main reason most folks opt for the 572Bs is because they over drive the 811As, thus drastically shortening their life. The Ameritron 811, and 811H have have maximum drives levels as outlined in their respective manuals (65 watts for the 811H, 50 watts for the 811). I venture to say, 95% of the amateurs who own these amps over drive them. What's more, they take several minutes to tune them up (it shouldn't take longer than 15 seconds), fail to use the ALC as Ameritron instructs, and thus shorten tube life by several magnitudes.
If you just HAVE to HAVE more power, buy a bigger amp! Over driving a small one only produces more splatter and distortion products; all for just a couple of dB increase in signal strength.