Sparton 16A211 Width Problem

[old_coot88]

Originally Posted by Penthode

What makes me think it may not be the HOT is that the horizontal output tubes are running cool.

That would be my prognosis as well (assuming the fly itself is running cool and emitting no telltale whiffs of internal arcing).
Now that you've got near-normal width, I was gonna suggest the old damper shunt cap trick, but see another poster has already mentioned it. The cap should be disc ceramic, rated at 6KV minimum. Value can be anywhere from 68 to 150 mmf (pf). We used to keep several values on hand for this very purpose (higher value gives more width). The cap goes directly between cathode and plate of the damper tube.
But there's a trade-off as in the old saying "there is no free lunch". The HV drops a bit with each increase in width, as does the HO tube current. So the mod is only useful as a last resort in cases of intransigent marginal width such as this set appears to have.

The same trick can be used on B&W sets with the cap rated at 4KV minimum.

[Mal Fuller]

Thank you old coot88 for providing "the rest of the story." Does the Sparton use a shunt regulator for HV regulation? If it does, would the "damper cap" have any real effect on 2nd anode voltage?
Dropping the two horizontal output tubes' current can't be a bad thing.