17LP4 CRT ( Need one... )

[Eric H]

Originally Posted by grimer

BAd news,this tube I have here shows no sign of life at all
But I suppose it was worth a try
Pat

Is the heater lit?

Possibly re soldering the pins would help?

[grimer]

Yup,the filament lites up.I let the tube warm up for 20 minutes,even cranking
up the heater to 8 volts did not help.

[bandersen]

I had the same problem with a 16KP4 recently - good filament, but 0 emmision. It turned out to be an open cathode. I was able to re-weld it by using the technqiue described over here: http://www.videokarma.org/showthread.php?t=248067
Just a thought.

[Eric H]

Magnetic focus uses an external magnet assembly, either electric or mechanical to focus the tube, electrostatic focus uses voltage to focus it internally inside the gun, that's probably what the missing pin is for.

You might get lucky and have it work without the magnet assembly but probably not.

[jr_tech]

Originally Posted by Eric H

You might get lucky and have it work without the magnet assembly but probably not.

But it might be possible to add a magnetic focus coil or permanent magnet assembly salvaged from another set to adjust focus on this tube. Years ago, I used an aftermarket add-on permanent magnet focus assembly called a Quam "focalizer" (or something close to that) to focus a crt in a set where the focus coil had failed... something like that might work.
jr

[RitchieMars]

Right, I was thinking that. I wonder if this worked like it does on an electrostatic speaker where the magnetic field is created by current flowing through a coil, which would means the coil would have to be tapped into a circuit somewhere. Obviously, a permanent magnet would be much easier. I'll look into this a bit...

I'm still unsure about this type of tube and I wish I could find some specifics on it. I'd hate to pass up a NOS tube. In my opinion, changing to a magnetic focus would be an improvement over the electrostatic if it were possible. With the electrostatic type, I understand that the focus degrades over the life of the tube.

[jr_tech]

Originally Posted by RitchieMars

Right, I was thinking that. I wonder if this worked like it does on an electrostatic speaker where the magnetic field is created by current flowing through a coil, which would means the coil would have to be tapped into a circuit somewhere. Obviously, a permanent magnet would be much easier. I'll look into this a bit...

Here are a couple of pix of a permanent magnet focus assembly (mounted between the yoke and the ion trap) in my 1953 Philco... it would take a fair amount of mechanical modification to support such an assembly in your set, but it might be worth a try if you can't find the correct CRT. The focus adjust is the little brass knob on the flex shaft, shown on the second picture. The little lever, with the wing-nut lock at the top of the assembly is the position adjuster.
The Philco uses a 17YP4, which also has a cylindrical face.
jr

[bgadow]

I have at least one spare PM focus assembly, if you needed it. I agree that it could be difficult to fit, depending on how the yoke is arranged.

[leadlike]

I might have a spare assembly as well. It is a large ring magnet, so you don't need to make an electrical connection to it. It came out of a tv that I got a new crt for, and I had to change it from magnetic focus to electrostatic, which is much easier to do (just rip out the old m-focus hardware). If you need it, I'll see if I still have it.

[RitchieMars]

For the time being, I guess I have time to decide which route I want to go. It would suck if I had the tube and magnetic focus assembly shipped to me only to find out that neither will actually work out as planned. I believe there's a good chance that it would, but as I yet I still can't find the specifics on this 17CQP4. I've only read about the 17QP4, and I have no idea what the "C" stands for.

It should be a little while before I get the set recapped and ready to test again, and hopefully my CRT will be adequate for those purposes. I'm rather afraid to jump at such a tempting but risky chance when for all I know, a good 17LP4 might turn up somewhere the day I'm sitting here scratching my head over how to install this magnetic focus assembly. That would be just my luck!

Just curious, what would be the best route to safely ship an 18-pound CRT this size and what would it cost?

[Eric H]

Are you sure it's not a 17CGP4? I ask because I don't see a CQP4 listed here:

http://tubedata.milbert.com/sheets14.html

The CGP4 is quite similar to the LP4 but is two inches shorter.

[RitchieMars]

Nah, I've seen mention of Sylvania tubes such as the 21CQP4. Oddly enough, that particular tube uses electrostatic focus. The 17CGP4 there doesn't look like a cylindrical face plate, but it's also an electrostatic focus. It would be so nice if that "C" stood for an electrostatic version of the 17QP4. I'd be pretty sure it would work if that was the case. I'll check into this, but I rather doubt that's what it means.

[julianburke]

I'll look tomorrow as I have several NOS 17" tubes. That number sounds like one I have.

[RitchieMars]

Alrighty, let me know what you've got. A proper replacement would make life so much easier than the modifications I'd considering. The following should be drop-in replacements:

17LP4
17VP4
17LP4A
17VP4B

There may be more, but these are the most closely related.

[tvrepairtech]

I have a 17LP4 that is NOS rebuilt by a rebuilder that has been out of business for several years.
I can let you have it for $50 plus shipping.