The AM Forum

Hi Folks,

I'm muting my NC-173 using the AC jumper plug. Pins 1 and 4 are paralleled across the send / receive switch and are connected to the NC contacts on a dowkey on the Viking II. When flipping the plate on to transmit the NC173 S meter briefly jumps up to the 20 to 30db over mark and then drops to zero as B+ is cut with a thump. Seems to be working correctly to me but is there anything to worry about? Should I install some overload protection on the NC173 like back to back diodes?

Thanks 73
Kevin

I ran my Viking-II and 75A-3 for many years in the early '50s using a Dow Key antenna relay and experienced no problems with receiver input stage overload. However, I think the model of relay I used provided a short to ground on the receive port when in transmit mode. Regardless of that, vacuum tubes are not susciptable to instant destruction by an excursion beyond many maximum ratings in the manner of solid state devices, so stray RF on the grid will not ordinarily cause degradation of tube performance. The grid will simply develop self-bias that will keep the tube cutoff and free of damage.

I later used an NC-173 with a Johnson Valiant and Matchbox. The T/R relay in the Matchbox shorts the receive line when transmitting to prevent high level RF from reaching the receiver. During that era I also used my home brew 60W mobile Tx and Rx with antenna and power switching by an ordinary open contact relay with no short on receive line during transmit, and no detrimantal affect on receiver performance. That receiver used a 6DC6 preamp copy of the 75A-3 input amp.

I would definitely not use back-to-back diodes to limit RF level on a tube-type receiver; they are unnecessary and too likely to generate intermodulatrion during normal reception from strong signals elsewhere on the band.

Bottom line: Go ahead and use your Dow Key relay and sleep soundly at night!

Bob - NE

Perfect! Thanks Bob, I appreciate it.