The 12BE6's in your Swan500C are pentagrid converters employed as mixers in both the receive and transmit modes. These tubes are more prone to becoming defective as opposed to having a long life and slowly weakening. The noise appears when the vacuum space barriers between grid elements start breaking down with high resistance shorts. The tube will still work with this type of short; but, not distortion- free; and, can damage other circuit components. Usually R702 22K 1W resistor burns through causing too high a potential on the grid of pin #6. R702 is on the top 10 list for components, other than tubes, showing the highest failure rate. I always replace that one with a 2 watt resistor. If the exact value cannot be found - 20 to 25K is fine. - - K4BOV
