[Boatanchors] Replacing Meter Shunt Johnson Valiant

[Rodger Singley]

For shunts I order replacements from Mouser rather than fooling with trying to cut a wire to length.  It is very common in kit built Johnson transmitters to find final and modulator plate current shunts that are far from the desired value because the builder was expected to cut very small pieces of nichrome wire and end up with the exact very short length AFTER tinning; fortunately most of these were cut too long so the meter reads on the safe side (high) and this is a common cause of low power output from a Valiant.  5% resistors are fine; the original Johnson meter movement wasn’t a high precision build and it certainly hasn’t gotten better in 50 or 60 years of usage.

Early Valiant builds also had an incorrect metering scheme  where the final plate current meter also read the VR tube current; Johnson changed this scheme and advised users to either modify their transmitter or alternatively permitted loading to 30 mils higher than what the manual states for AM mode.

Another issue with very early builds was the filamentary cathode leads from the LV transformer to the 866A rectifiers used standard hookup wire.  This wire wasn’t rated for the HV impressed upon this circuit and would short to chassis taking out the LV transformer in the process and potentially also damaging the HV supply. If this wiring is original either replace it with proper HV wire or sleeve it.

Early builds also used a single 500 pf plate coupling/DC blocking capacitor which is too small for 160 and the low end of 80 resulting in poor efficiency.  Later models had either two 1,000 or 1,200 pf caps in parallel.  If you have low output on 160 only then check to see if this has been changed.  It was corrected on later schematics.

Later Valiant production and early Valiant transmitters modified to work with the Johnson 6N2 transmitter changed the bias circuit slightly so that adjustable pots now feed the final (for SSB bias) and modulator bias circuits so proper idling current is set by a simple turn of a pot rather than sliding taps on the bias resistor to set bias voltage.  Either system works well but if your manual doesn’t match the actual bias circuit this is the reason.  The fine wire of the bias bleeder resistor is easily damaged and these parts are somewhat expensive now so when setting voltage via the bleeder method make sure to completely loosen the tap before trying to slide it.

Rodger WQ9E

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