Evening Les,
I would check what voltages that are on the center tap of the power transformer when it works and when it doesn’t.
This radio raises the center tap above zero volts to generate the negative bias voltage for the output tubes in particular.
Perhaps something is amiss there as well.
As well, a type 80 tube is the same as a 280 according to the powers that be all knowing in this sort of thing.
Good Luck with it.
John G. VO1 CAT