An old post, but will comment anyway.? I was never able to find an Invader to buy, but did buy some Hallicrafters stuff (HT-44) from the same period and had lots of 60 Hz hum on the transmit audio.? In my case, most of the hum was due to the fact that they chose to bundle wires carrying the line voltage to a front panel switch with everything else including shielded audio lines going to/from the front panel mic gain control.? I ran new wires outside the bundle for the line voltage and installed new shielded cable, also outside the bundle.? Almost all the hum disappeared.??
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While restoring an RME-6900 receiver recently, the first thing I noticed was that there was ZERO background hum.? The RME engineers had followed the advice in the RCA tube manual to a Tee.? Instead of using the chassis ground as the return for the filaments, they ran twisted pair to every tube socket with no ground involved.? In addition, and here's one I've never seen before but is in the RCA tube manual, they applied +80 VDC to the filament string.? According to the manual, this is an effective way to reduce filament hum in receiving tubes.? No reason that wouldn't work I guess with the audio string tubes in a transmitter.??
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73, Floyd - K8AC
Floyd - K8AC