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need to see a schematic

If you have two stages of RC coupling, you run a high risk of an
unstable amp that can dive into an oscillation mode.

Further, you need decoupling between each amplifier stage otherwise
the the big current drawing stage upstream will modulate the B+
on the driver stage. To do this, you slap an RC network into the B+
rail. The R's drop the voltage to a needed voltage for that stage and
the C's provide a buffer (energy storage).

Your schematic should include that of the bias supply. Using a variac and a 530V transformer is an expensive way to do it.

A cheap bias supply can be made from a 120V : 120V isolation transformer. Use a bridge rectifier and a small 250V cap
followed by a simple RC coupling network. You can then use either
an 0A2 or 0B2 gas regulator to create a regulated -150V or -105V
supply. That regulated reference can then be dialed down with a
20K pot. Put a 100uF 100V cap at the pot wiper to provide a nice low
AC impedance. Cheap, compact and very reliable.

Play safe with that 750V. Dont work on it if you are tired and
frustrated. It's always best to be fresh, and alert. ;)

-- Jim

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