Critical Meter Test Tuning Ameritron Amplifiers

The grid current is the single most authentic and important tuning indicator in any grounded grid amplifier.

In the 1980s and earlier FCC rules required watching power input, this mandated we watch plate current. Additionally, our transmitters were generally grid-driven tetrode or pentode stages with only the plate current monitored. Older hams are just trained to "dip the plate" even though that is passe'.

In grounded grid AB2 amplifiers, we should really focus on grid current. No matter what any instruction, video, or Elmer tells us the grid current is key to proper tuning.

In the spring of 1983, the FCC changed rules. Instead of the plate input power of 1000 watts, amateurs could run 1500 watts PEP output. This meant we had to know the peak envelope power output to the antenna, which is forward peak power minus reflected peak power.

Because of this, all amplifiers I designed had a dedicated grid current meter. I installed a negative rail clamping diode to protect the meters. That diode conducts and sometimes shorts if the tube or anything on the high-voltage lead from the power supply to the tube should arc to the chassis. If you have a shorted rail clamp (aka meter protection diode), it is likely from a tube arc. It could also be from any arc to chassis in the HV system.

This video shows how to safely test the meter protection diode without removing the amplifier cover.

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