Reviving an old full wave rectifying valve

[Herald1360]

At this point I put in parallel with each diode a 10 Nanofarad capacitor between anode and cathode of the diode each side. That is also 0.01 Microfarad if you wish. This reduces the high voltage at the cathode somewhat through capacitive and inductive reactance.

This may have a side effect which reduces cathode high voltage peaks a bit but that's not really what it's there for.

What it does is to kill any RF damped oscillations in the rectifier circuit stray inductance/capacitance excited by the very fast turn off of the reverse recovery current pulse which happens every time the voltage on a slow silicon rectifier goes into reverse polarity. It does this by effectively tuning this resonance to a much lower frequency where the Q of the stray inductance is negligible. These RF damped oscillations with a prf of 100Hz can radiate a lot of buzzy hash noise if they're bad. In modern kit more than enough to fail EMC emissions tests.

There may be some slight reduction in peak voltage at the cathode from the superimposed RF train being damped down.