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You just want the tube shield not to be grounded, if you can keep it up a bit from the chassis, that is good, I think I used the cardboard to keep a strip of metal from touching the tube shield. The goal is to couple the signal thru the glass of the osc tube, the shield therefore must not come into contact with the chassis which would ground out the signal.
The hardest part of working IF for me is correct termination and cabling from gens to test equipment. You want short leads, and correct termination blocks (at least when looking at band pass from sweep gens).
I think the tuner may have a defined test point that may work better than the tube shield, check the riders. I do think its not as critical when just doing a single freq peak (unlike the sweep gen and scope when you are trying to analyse a trace).
and using the long fibreglass rod helps, just moving around the set can damp out signals and change tuning, you notice that as I peak up the last coil near the tuner.
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