[Amps] RF on audio using 374A

kc0hfl at cox.net kc0hfl at cox.net
Tue Sep 13 07:16:13 PDT 2011

 I was told I have RF on my audio when using my 374A so I did the usual checked antenna for common mode tried all the tricks 1:1 balun Ugly balun even swapped out antennas after a week of messing with this I swapped in different radio still RF then swapped in my SB220 RF on audio gone. I guess antenna was fine so the Alpha went to the bench changed out bypass caps on the filament supply looked for loose screws changed bypass caps on the B+ lead plate choke looks good Parasitic suppressors which are original look good still have RF with the 374A? THis amp has been converted to the 3cx800's running a pair fed with a seperate filament transformer mounted on the rear of the amp both the leads are a short run with a sheild grounded at both ends even bypassed at both ends with .01 ceramic disc caps beating my head against the wall any ideas?

Current set up Radio Flex 5000 driving the 374A into a MBV tuner into a single 75M dipole remember no RF with the Heathkit only with the Alpha both tested at 1KW also tested with the amps grounded made no difference 


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