The AM Forum

For years, even dating back before I had an oscilloscope to monitor modulation, I used an 866A as a negative peak overmodulation flasher.  Using a 2.5 volt filament transformer insulated for high voltage, run the midtap of the filament winding directly to the modulated HV lead to the final.  Ground the 866A plate.  As long as the plate of the final remains at positive potential with respect to ground or at zero volts, the 866 will not conduct.  As soon as the plate is driven negative (the condition with negative peak overmodulation) the 866 conducts with the characteristic blue glow.

I put mine inside a box with the interior painted flat black, with a window for viewing the plate area of the tube.  It made a very effective overmodulation indicator.

You can bias the plate of the 866A positive to get it to flash at less than 100% modulation.  For example, your final runs 2000 volts on the plate.  Bias the 866A plate 200 volts positive with a small power supply rated at a few milliamps, and it will flash at 90% negative modulation.

A preferred variation is to place a power resistor equal to your modulating impedance (plate volts ÷ plate current in amps) in series with the 866A, preferably from the filament winding midtap to the modulated HV lead, and you have modulation transformer protection for any negative peak that exceeds 100%.  Normally, whenever a plate modulated rig is overmodulated in the negative direction, the modulation transformer operates without a load for the duration of the overmodulated peak, a potentially fatal condition for the transformer, particularly if some kind of unexpected transient occurs during the period while the final amplifier tube plate is negative.  With the diode wired as previously described above, instead of seeing an open circuit at >100% modulation in the negative direction, the transformer abruptly sees a dead short, which could conceivable generate a transient that could still arc over the modulation transformer primary. With the resistor in series with the diode, the modulation transformer sees normal load impedance throughout the audio cycle, even while the final amplifier plate is being driven negative.  Since this peak occurs over a very small portion of the audio cycle, a small resistor, 50 watts or less, is sufficient to handle a full kilowatt plate modulated.  Of course, the 866A still flashes, allowing the tube to serve double duty as transformer protector and overmodulation indicator as originally described.