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Just a SWAG, but it fits under the damper tube and connects a cap between the cathode and plate (pins 3 and 5 respectively, typical of 6AX4 etc.). The cap would be ceramic, 150pf or so, at around 4KV rated.
What this does is boost the sweep width slightly while reducing the HV, further allowing the pic to 'bloom' slightly. Plate current of the HO tube is also reduced slightly. Old techies would sometimes do this as a "last resort" when fighting a problem of insufficient width. They would try cap values like 82, 100 and 150pf to see which gives the best overall compromise.
Old color techies would sometimes do it as a last resort with problems of high HO tube current. But the caps would be 6KV rated. Bill(oc)
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