Tom, thank you for your continuing detailed notes.

Well, this set is basically done. To cap things off, here are some shots of the set in its current state. Thanks to the kindness of some VK members, I have some of the front panel controls now. If anyone has the rest of a channel selector knob they can spare, I can put it to good use!

I've been enjoying watching it in the shop for the past week or so, and this evening I'll be moving it into the house so it can have a better climate controlled environment and stay cleaner (as well as being easier to enjoy watching).

And a few more pictures.

Thanks for all the help, everyone!

Tom

Beautiful! I should take some daylight pics of mine. Second thought, my back would protest so upstairs it will stay!

Tom, Fantastic job. Looks like your just missing the plastic channel # insert on the channel selector knob. Hopefully someone out there can supply you w/one.

-Steve D.

Absolutely gorgeous! Congrats, Tom!

-Clark

Someday I hope 3D printer and some sort of 3D scanner will be able to replicate these parts. Cause I need one of those for my 21CT55.

Dave

Tom,

Here's a 1-1 scale scan of the channel #'s paper insert. Not sure it's of any use to you, but good for reference.

-Steve D.

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.

http://www.videokarma.org/attachment...1&d=1517469546

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.

Thank you for doing this interesting research and for sharing your success, Tom.

Grid current is not recommended according to the data sheet, though I suppose you have nothing to lose.

http://scottbecker.net/tube/sheets/093/1/15GP22.pdf

Thanks for posting the data sheet. That's one of the reasons why I posted this -- to find out to what extent operating with positive peak voltages on the red grid may cause some damage. I see on the data sheet that the max rating for grid voltage is +2 volts. Is there an additional reference on there specifically about grid current? I may have missed it.

One thing I noticed in the circuitry is that while the green and blue cathodes are fed by a fairly stiff voltage divider, the red cathode has an extra 2.7K cathode resistor. This reduces the gain of the red and reduces the positive grid voltage with respect to cathode. Shorting this resistor results in a pretty nonlinear response, where the red beam gets too bright on positive peaks and gets completely out of balance with the other colors. I tried reducing the value of that resistor and came away with the impression that it's best left alone. Intriguing, however, to think about how the red signal could be further boosted in some useful manner that gets rid of the nonlinearity. Maybe some kind of active feedback on the beam current? With a little help from a modern op amp that would be possible to do.

If any of you knows anything further about the risks of boosting the red in this way, let me know.

As miniman82 points out, I don't have so much to lose here, but I also don't want to completely kill the red very quickly by trying something completely crazy.

I didn’t look, but from experience reading lots of data sheets I can tell you that going beyond 0 on the grid will typically result in grid current being drawn. For most output power pentodes this isn’t problem occasionally, my concern here would be overheating the grid cup and warping it. That could cause a permanent short and damage the gun.

My instinct tells me that shouldn't be a problem. The grid goes at most 10-20 volts positive with respect to the cathode (I think it is in fact only a few volts because of the cathode resistor), so the power dissipated in the grid will be tiny. Probably gets way more heat from the nearby cathode than from the actual power dissipated in the grid. I was more worried about whether this will cause some kind of cathode stripping or something like that.

Running it into the positive grid region is probably along the lines of a mild clean and balance on a rejuvenator. Odds are it won't hold it's emission for long...It is probably either going get brighter as the cathode cleans or darker as the remaining cathode gets used up. If brightness improves with use you may want to revert it to stock.