12VP4 rebuild

[jeyurkon]

I've been looking for a 12VP4 for a long time and one finally arrived in the form of the 1-125 Sylvania set I got from rca2000. However, the CRT was very weak and wouldn't wake up.

I decided to have Scotty rebuild it and just barely made it under the wire. I think it was one of the last 3 CRTs he rebuilt.

The gun, being a triode, was not one that Scotty had in stock so it had to be rebuilt also. It presented a challenge for Southwest it turns out. Bob Killburn said that the cathode fell apart, that they had no technical information on the cathode/grid spacing like they do for all other tubes, and that they had no way to measure this critical distance which affects the cut off voltage. It also used an unusually long cathode.

I had three options. Forget having it rebuilt. Have the cathode assembly rebuilt, but since they had no similar parts it would take at least a month and drive up the cost. Or, let them shorten the grid and weld in one of their standard cathode/grid assemblies.

I chose the latter, and while it was supposed to take two weeks they did it in one.

Scotty said the rebuild and activation went well, but he had no way to test it. I installed it in my 1-108 chassis to test it. I was treated to a bright raster, washed out image, and wasn't able to adjust the brightness.

Well, being a triode pin 10 isn't used but was needed by Southwest to support the grid. The standard bases that Scotty uses has pin 10 and pin 6. While he didn't solder pin 10, the wire was still there making contact to the pin. So, pin 10 and pin 2 were common.

I had modified the 1-108 chassis to work with a 12LP4 by providing 300V to the screen, which is pin 10. I didn't expect the 12VP4 to have pin 10 connected to anything and Scotty and Bob didn't expect I would try to test it in a set I had modified to work with a 12LP4. This was the cause of the brightness/contrast problem and was easily solved by cutting off pin 10.

I thought I understood what Bob did. But, I was told that the CRT neck would be shortened due to the need to cut the grid in half to use a shorter cathode. The neck is the same size and the grid appears to be the same length. I can see where the original cathode was removed.

Whatever he did, the CRT works quite well. For the attached photo I had just fired it up after cutting pin 10 and made no adjustments other than centering the raster a bit. I didn't adjust the focus.

After some subsequent adjustments the image is nearly as good as with a 12LP4. The difference is barely perceptible.

I knew that there was a pin hole sized burn in the center of the CRT when I sent it in for rebuilding. I was aghast when I saw a quarter sized blue stain/burn around this when I got the first raster. However, after running the CRT for a few minutes the blue spot faded to barely noticeable. My 10MP4 had a similar spot that behaved the same way, though it's not centered. Anyone know why it would improve? Is it really a burn?

Now I have to decide whether to use it in the 1-128 Sylvania console I just finished restoring. This has the 1-108 chassis I used to the the 12VP4. This would make it more original and with the 12VP4 it requires less drive on the horizontal and vertical, it also requires the horizontal centering pot be set need the midpoint while the 12LP4A requires it to be at an extreme end.

The 12LP4A would give a slightly better image. Though it's hard to see the difference.

If I use the 12VP4 in the 1-128 I would then have to add a similar screen bias circuit to the 1-125 Sylvania. However there is lots of room in this set to do that. By screen bias I mean grid 2.

I plan on watching the 1-128 console a lot. This is my childhood set. I would operate the 1-125 only occasionally. I have a straight gun 12LP4A, no aluminum, that might be good for the 1-125 since I wouldn't operate it enough to worry about ion burn. The 12VP4 would also stress the horizontal and vertical output sections a little less in the 1-128 that I plan to use a lot.

Both the straight gun and slant gun 12LP4As were rebuilt by Scotty.

If I use the 12VP4 in the 1-125 then I wouldn't have to modify the chassis to use it.

What would you guys do if you were in this situation?

John

[bandersen]

Congrats on getting the 12VP rebuilt

I think I would go with the straight gun it the 1-125 at least for a while to see if any ion-burn shows up. (I need to re-read your thread to refresh my memory on how that CRT came to be)

As for the other set, I'm pondering a similar situation myself. Use a freshly rebuilt CRT or NOS CRT in a set that gets played a lot. Or drive weaker / more common CRTs into the ground before swapping them out

[jeyurkon]

Well, I've been using the 12VP4 in the 1-128 Sylvania for awhile now. I can't see much difference in performance betwee it and the 12LP4A. The 12VP4 has more consistent focus across the face of the CRT. Of course the chassis was designed for it.

The 12VP4 seems brighter, but is bothered more by room lighting. This is probably the difference between a CRT with a grey faceplate and one with a clear face plate. The transmission for the A version is 64%. So it cuts down the brightness a bit, but the light refected off of the phosphor should be down to 40% compared to the non-A version yielding better contrast.

I didn't realize this until now. I'm not sure what I had thought the grey filter was for.

John