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Your problem sounds like filament to cathode leakage in one of the common receive and transmit audio tubes. The tube I suspect is V12, 12AT7.

V12A is used in the receiver as the first audio amplifier stage. In transmit V12A is used as the microphone amplifier. If hum is produced in V12 because of heater to cathode leakage then hum will be present in the receiver's audio and heard in the transmit audio. I recommend replacing V12 (12AT7).

Most common inexpensive tube testers cannot detect cathode to filament leakage that's causing hum. A tube that tests good in a tube tester may not always work, or it may work improperly, in the circuit it's used in. This is especially true with tube used in RF circuits.

73s
Mike

[QUOTE="falcon2448, post: 334680, member: 22958"] Your problem sounds like filament to cathode leakage in one of the common receive and transmit audio tubes. The tube I suspect is V12, 12AT7. V12A is used in the receiver as the first audio amplifier stage. In transmit V12A is used as the microphone amplifier. If hum is produced in V12 because of heater to cathode leakage then hum will be present in the receiver's audio and heard in the transmit audio. I recommend replacing V12 (12AT7). Most common inexpensive tube testers cannot detect cathode to filament leakage that's causing hum. A tube that tests good in a tube tester may not always work, or it may work improperly, in the circuit it's used in. This is especially true with tube used in RF circuits. 73s Mike [/QUOTE]

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