For Henry RF amplifier Ground-Grid configurations, zener D1 provides 10V to the center tap of the filament transformer to limit the idle current of the tube(s) to approximately 125ma when the high ohm value BIAS resistor is in series (see R18 in schematic below). When the T/R relay shorts across the BIAS resistor, removing the resistor out of circuit, the tube(s) will go into conductance.
Original 10V zener with parallel 220 ohm 2 watt equalizer resistor
The 10 volt zener bias diode D1 is a very common failure with Henry RF amplifiers. It faults when a tube shorts, HV voltage surge/spike or when the input is over driven. Normally the zener will open and all BIAS current will be carried through the resistor which is in parallel to the zener (see R17 in schematic below). One of the symptoms of a bad zener is high unstable grid current and/or the HV overload protect relay will drop out when keyed.
To test the zener, you must remove one end of the zener out-of-circuit. You can use a simple OHM meter to check the resistance. A good zener should read infinity resistance in one polarity direction and approximately 5 to 20 ohms resistance in the other polarity direction. If shorted it reads the same resistance in both polarity directions, and if open it reads infinity in both polarity directions.
If you have a bad bias zener, you can replace it with either a "1N3309 10V 50watt" "1N3310 11V 50watt" or "1N3311 12V 50watt zener." These zeners can be easily obtained on ebay or Newark for the amount of $20 USD.
Using a 11 or 12 volt zener will set the bias 1 or 2 volts higher, resulting in the tubes running a bit cooler on idle and will require a few more watts of drive to over-come the higher bias cut-off threshold. Its not recommended going any higher then 12 volts bias, or linearity will be lost and distortion will occur.
When replacing the zener, observe polarity, make sure you provide adequate heat-sinking and use heat transfer grease. If required, insulate the zener off chassis ground. Consult the schematic for details.
D1 Protection Modification Recommendation:
Add a 1.0A 120V (6x32mm) fuse in series with either D1 anode or the BIAS line to protect the zener. It is a lot easier and less expensive to replace a blown fuse then the zener. I drilled a 1/2 inch hole (15mm) in the rear of the chassis near the location of the zener and installed a chassis/panel mount (15x55mm) fuse holder.
