7GP4 vs 7JP4

[benman94]

I'm currently considering a deal for a set that used a 7GP4 CRT originally, but now has a 7JP4 replacement in it. This particular model set can only produce about 3 kV EHT. Is there any issue using a 7JP4 as a substitute for a 7GP4? Has anyone here ever done this with good results?

[vts1134]

I've never heard of a 7GP4. What kind of set uses that CRT?

[benman94]

The Belmont 21A21 and the Philco 48-700.

[baursam]

Some of the older Philco 48-700s did, then they switched to 7JP4s Don't know how many they made before the switch

[baursam]

Here is the factory manual for the 48-700 go to the last or second last page and it will tell how to achieve best results based on which crt you are using. This on the assumption, that you are talking about the 48-700.

http://www.earlytelevision.org/pdf/p...ory_manual.pdf

[rca2000]

The one in my avatar--on top of the 2000

[benman94]

The set I'm looking at is the Belmont 21A21. The later 22A21 and 22AX21 both use the later 7JP4 with 5kV on the CRT. I still may be able to use that Philco data to figure out what mods are necessary to get a 7JP4 working satisfactorily in the Belmont.

[rca2000]

Does that one have line-powered HV like my Philco 48-700,, or is if RF osc HV like the philco 50-701, and most other E-D sets?

[benman94]

RF oscillator: a 6V6 driving a coil, rectified by an 8016.

[tvdude1]

I have a few spare 7gp4 CRTs . I have swapped CRTs do see much difference in picture quality

[baursam]

Good luck in acquiring the set!! Def on my short list!

[EdKozk2]

Originally Posted by benman94

I'm currently considering a deal for a set that used a 7GP4 CRT originally, but now has a 7JP4 replacement in it. This particular model set can only produce about 3 kV EHT. Is there any issue using a 7JP4 as a substitute for a 7GP4? Has anyone here ever done this with good results?

Hi,
The 7GP4 was really the forerunner of the new and improved 7JP4. The 7GP4 maximum anode voltage was only 4000 volts. They used this tube in some late 40's Sylvannia Oscilloscopes.
Substituting a 7JP4 for a 7GP4 should be no problem. Just don't go the other
way around, the voltages would be too high.
If you make changes to a set designed for 3 to 4 KV you may have to increase the working voltage of the high voltage capacitors. You may also have to add more resistance to the high voltage bleeder string.
Here's the data on a 7GP4: http://frank.pocnet.net/sheets/049/7/7GP4.pdf
Ed

[timmy]

Ok here it is hope you get it,

[EdKozk2]

Originally Posted by timmy

Ok here it is hope you get it,

Timmy,
What model set is in your picture? The knobs on the set in your picture are
what I need for my Garod 1244G set.
Ed

[timmy]

Well it's a long story so maybe ask Baursam he would have the answer you are looking for, sorry.