I can see a small amount of contact bias being generated by the 6H6, 6AL5, etc. But that would be easy to compensate with divider from the cathode bias of the output tubes or any small positive source. I guess they may have been looking for an easy fix to get the S-meter to zero. This is all very interesting but why not just solder in a Germanium 1N64 or a NTE109 or Silicone NTE177 or even a NTE519 with 4 ns recovery time. I pulled the 6AL5 out of my HQ170 and went to diodes. Saving 150ma of heater current WEEEE. I ask this sincerely because I really don't know. What would be the disadvantage of using small signal solid state diodes as long as the IF frequency isn't too high. John
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