[Arran]

Mike;
  I don't think that the diode is really necessary either, I think it's theoretically more of a problem running the 6Q7 without any bias on the grid then fractions of a volt too much, most have a resistor between one end of the bias cell and ground as well I think. It's not uncommon to see 12SQ7s, 12AT6s, etc., run with no external at all, just a 10 meg resistor from the grid to ground, and the cathode tied to ground as well, millions of AC/DC sets were built that way. For years I was a bad boy and ran a Deforest Crosley "Elgar" without the bias cells, someone had soldered in some AAA cells under the chassis, which had failed and leaked, which were removed, and in my ignorance I shorted out the bias cells and ran it that way. Maybe the set would have worked or sounded better with them installed but it didn't kill the 6Q7M tube, schematic here:

https://pacifictv.ca/schematics/rogers13-71data.pdf

Regards
Arran