[morzh]

Yes, 500 Ohm vs 300 Ohm is way out of tolerance, but if anything, it should result in 6F6 conducting less current and so not lowering B+ as much as it should. Which is opposite to B+ being lower than it should be.

Though not necessarily related to what we are troubleshooting,

1. Do you know for a fact both your 5Y4 rect tubes are good? Or at least one of them?
2. What is your AC IN voltage? (I cannot find what they did their measurements at, usually 115 AC). Also do check with voltmeter when set the volts by Variac: if not dititally displayed, the Volts on top of the Variac are for reference only and do not really reflect the real values.
3. Unless you use the old style meter with 1,000 Ohm/V impedance, the real volts you should be seeing on the DMM should be higher than those in the chart. So, if you use a DMM, yours are even lower than expected if referenced against the older measurements equipment which were used to make that chart.