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Old 16th March 2010, 04:47 PM   #1
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Default Wire construction for capped tube plates

Hello all --

Not sure if this has already been discussed. I am using power tubes that use a cap on the top for the plate. What is the best way to build the wires for this purpose? Should I use shielded wire? What type of insulation is best? cotton? teflon? How about gauge? Stranded or solid?? Thx much!
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Old 16th March 2010, 04:56 PM   #2
kevinkr is offline kevinkr  United States
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Well insulated (proper voltage rating for application) to eliminate the possibility of electrical shock due to inadvertant contact. Flexible (stranded) IMO because you don't want the wire to break with repeated removal and installation of the plate cap. I like teflon insulation for its heat resistance and dielectric qualities. Not shielded generally because of the effect of unwanted capacitance in the plate circuit. Gauge is generally not critical..
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Old 16th March 2010, 05:01 PM   #3
TheGimp is offline TheGimp  United States
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High voltage rated should go without saying, but I'll mention it any way. Rubber gromet at the feed through. Cage over the tubes when done.

I'd be inclined to use 22AWG or 24AWG multi strand 1KV or better wire unless it were some monster amp. If you have a topcap, you are running real HV so watch for corona.
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Old 16th March 2010, 05:12 PM   #4
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so watch for corona.
Hmm .. not sure I know about this ... can you elaborate a little? Does this happen because of the cap? The tube I am using is a GU-32 (832). It doesn't really have a cap really -- more like a stout wire coming out of the top. Where does the corona form?

Looking at a plate voltage around 250-300V if that helps.

I have some high voltage silver teflon wire I think I will use (22ga). Thx Kevin. For the terminator I have a nice silvered wire "stud" that has compression "fingers" on one end and a hole for the other to attach the wire. I have been able to form the compression fingers to make a solid connection around the plate. Was planning multiple layers of shrink wrap on top unless this "corona effect" blows that idea up.
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Old 16th March 2010, 05:24 PM   #5
kevinkr is offline kevinkr  United States
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Corona is not a problem at the low plate voltages you are talking about - starts to be a concern around 700 - 800V or so.
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Old 16th March 2010, 05:28 PM   #6
TheGimp is offline TheGimp  United States
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300V should not be a real big issue. Use 600V rated wire. I was thinking of 500-800V plate supplies.

Corona occurs mostly when there is a sharp point or perturbance in an otherwise smooth surface with a high charge. It is a self discharge to the air. It is more likely to occur with HF present (I remember a lot of corona around the flyback supplies on TVs running 17KHz?? or somewhere close to that with 22KV supplies). We used to use Corona Dope on the solder joints.

Amazon.com: GC Electronics 10-4702: GC Corona Dope, 2-oz. Bottle: Home Improvement

Heat shrink tubing should be good at 300V, especially with multiple layers.

A ceramic cap is nice if you can find one to fit.

Ach! You beat me to it while I was looking for Corona Dope.
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Old 16th March 2010, 05:39 PM   #7
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Default Corona dope

Thx Geek -- looks like cheap entertainment (whats one more noxious fume in the lab anyway right? ). Think I will get some unless it deadens signal or something.

There are ceramic caps for this tube I guess -but they are crazy $$$$. Was thinking of finding a ceramic jewelry bead of some type and slipping it over the warm shrink wrap for looks.
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Last edited by moonbird; 16th March 2010 at 05:42 PM.
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Old 16th March 2010, 06:16 PM   #8
db! is offline db!  Canada
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I have made plate caps for my 832 by cutting apart a female spade connector and soldering it inside of a ceramic tube cap. It's not the most elegant of solutions but it works well and looks pretty good (as long as you don't look under the cap).
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Old 16th March 2010, 06:28 PM   #9
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Quote:
Originally Posted by moonbird View Post
Hello all --

Not sure if this has already been discussed. I am using power tubes that use a cap on the top for the plate. What is the best way to build the wires for this purpose? Should I use shielded wire? What type of insulation is best? cotton? teflon? How about gauge? Stranded or solid?? Thx much!
Here's my favorite method. I use test probe wire, because it's tough, flexible, and rated for pretty much any voltage I want to use. Through the chassis I use a "snub bushing" which is a 2 piece grommet that goes around the wire and pushes into a hole in the chassis, compressing and locking the wire in place.

Cheers,

Michael
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Old 16th March 2010, 06:42 PM   #10
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I second the use of Meter probe wire as a good solution.

Shoog
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