Some further followup on this set -- The CRT red gun has always been fairly weak on this set. Seems it died a natural death with a worn-out CRT after years of usage way back when. No problem with loss of vacuum on this CRT; just not much brightness from the red gun. You could get an OK picture on it in a dark room, but it's no fun to watch in such a state.
Adding a brightener helped a lot, but I still noticed another problem: The DC level of the CRT grids is set by the DC restoration circuitry and brightness control. This works fine, unless you try to push the signal to the point that the grid goes positive with respect to cathode. Then grid current flows, and the DC level of the grid gets pushed down as in any grid-leak scheme.
For example, on bright scenes, the DC level gets pushed down, which lowers the red level not just on the bright parts of the picture, but everywhere. So bright pictures were very weak on red.
I've done some experimenting and found a way to boost the red grid signal, allowing it to go positive and draw grid current without any effect on the bias. I added a cathode follower as shown below. It is capable of delivering enough current to the grid to hold the DC level and appropriate brightness.
The CRT cathodes sit at around 125 VDC in this set, and with this cathode follower in circuit, I can drive the red grid well above that while holding the black level at around 85 volts where things look good.
Now the set looks good in a moderately lighted room -- something I've never really had before. Much more fun to watch than before.
Anyone else tried this method to squeeze a bit more life out of a CRT that has seen better days? If this were not a 15GP22, I'd give it a good whack with a rejuvinator, but that seems far too risky with such a valuable CRT.