Hi Jeff,
Why do you say there's 6.2 megohms limiting the cathode current?
I only see about 1.83Meg (R810, R839, R829, the - guessed - dynamic impedance of GVS4, R838 and R837), and still, that already includes the 1 megohm at the anode and assumes the intensity control at minimum intensity (maximum resistance).
The only bunch of resistors that are in that ballpark of 6 megohm, but it's nore than 6.2Megs are all the focus divider string... But they are not in series with the cathode, they're actually loading the cathode voltage (drawing current from the cathode to ground) and therefore, even contributing to lowering the series resistance seen by the cathode current.
So, either I`m looking at the wrong schematic (I`m looking at TekWiki) or I`m completely wrong about it (or you...).
Now, talking about weak CRT, there are usually some other signs accompanying weak emissions..
1. Double peaking... Do the trace intensity increases, then decrease, than start to increase again, while the focus gets much worse, as you turn the intensity knob CW, and the trace "blooms".
2. Improper astigmatism... Astigmatism setting can't be left alone, but need to be continuously adjusted, in concert with focus, and you need to re-touch focus every time you change intensity?
I have myself a 464 (a storage scope) which shows all the signs above, and of course, it's dim, which is less noticeable at the slower sweep speeds, but is barely usable at the fastest speeds, never mention if I engage x10.
To your relief though, I can say that increasing the heater voltage as you're thinking is much less scary then one might think.
On my 464, I did try rejuvenation of the CRT and, as part of the procedure I needed to increase the heater voltage...
Usually where an over voltage of 30% is called for, I got up to as much as 50% over voltage, without burning it.
So, if your CRT is really weak, you should really try rejuvenating it (there are MANY successful stories, search the web), and if it still doesn't improve, then boosting the heater by some 30% should help make matters better, and will not ruin the CRT (well, at this point one must reckon that it's as ruined as it can be).
Rgrds,
Fabio
Fabio Trevisan