Short answer-- The 6550s run in Tetrode. Check out Frank McIntosh's design notes in 1948 Journal of the Audio Engineering Society for details. The McIntosh amp was a revolutionary design that has two primary windings in the opt. transformer instead of one like most amps; signal is taken from both the plate AND the cathode. This is very cool because this tremendous additional coupling in the primary makes it unnecessary to use matched pairs of output tubes. Unfortunately, it also requires HUGE amounts of current drive from the front end, and the front end tends to clip well before the output stage would run out of power-- the dual 5U4 rectifiers, class AB2 operation and HUGE opt xformer will deliver over 125 watts when driven by a driver transformer like the McIntosh 50W2, an incredible amp that made 50 watts of power from a pair of 6L6G tubes at only 400 volts..... the same tubes in the Heathkit W4 make only 20 watts at the same voltage. The MC60 has no bias adjustment; it runs in basically class AB2 (grid current flows at high power levels) and the tubes draw very little current at idle. Unfortunately, the negative bias is taken from the 6550 output tubes plate supply-- the main DC voltage in the amp. This puts much ripple and power supply artifacts from the output stage in the very worst place for any such noise and AC hash/artifacts-- the control grids of the output tubes. A great mod for the '60s is to install a very small transformer inside the chassis that develops -60 volts or so and to apply that voltage at the existing point where the bias is applid to the control grids.
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