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Having secured a 8873 tube in good conditions I decided to restore the SB-230 with that tube but
I wanted to modify the circuit in order to have a true grounded grid. This helps a lot
to improve the circuit stability and dispenses with the necessity of finding feed-thru
capacitors, which in the original design were tiny and delicate components.
To have a true grounded grid requires the negative side of the HV supply to float at
-8.2 V (the bias Zener diode voltage). The Zener then needs to be insulated from the chassis,
a simple task by using the usual hardware for mounting TO-3 power transistors.
Some modifications are needed to the plate and grid current metering circuits.
These were anyway required since the original S1 switch was broken and I had to
replace it with a simple 2-pole six-positions rotary switch.
The original SB-230 had a PTT control that is no longer suited to
modern solid-state exciters. I therefore inserted a small logic circuit to
control a small relay thus avoiding the presence of -120 V on the relay
input jack. A PC board accomodates the relay interface. A second relay
is used to switch off the tube in case of grid overcurrent. A pushbutton
switch on the back panel is used to reset the overcurrent trip.