af amp hum

[kb3ouk]

the other day i turned on my old crosley radio for the first time in a few weeks and noticed a 60 cycle hum on the audio that wasnt there before. messed around today with it some and discovered that when both output tubes are in it(6k6gt) the hum is there but if i yank one of the tubes out the hum disappears. it doesnt matter which socket the single tube is in but as long as there is only one tube in the af output there is no hum. when i put the second tube back the hum comes back with it. I tried swapping about a half dozen tubes around through it with the same results:any one tube by itself will not hum but any combonation of two tubes will make it hum.

[W1RKW]

Just a guess but maybe the filter cap is passing 60cps more so than with one tube ie. less load on the PS.

[Jim, W5JO]

Is the cathode bypass cap for the final amp tubes good?

[Rob K2CU]

Do you still get basic Received audio with one tube in place? If you are just listening to an idle condition, it is possible that with only one tube, you are saturating the primary since there is no balance and the low level hum does not get through. Is the HV half wave or full wave rectification? If full wave, then any power supply hum would be 120 Hz. If the hum is from power supply ripple, then removing the load of one tube would probably only reduce the ripple in half at best since there are other loads on the supply. In any case, the drop in hum voltage would be only a couple of dB and not noticeable. Could this be hum that is normally there but usually masked by receiver noise, but for now the front end is dead or quiet and you hear the hum come through......just some ramblings