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Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Steve McConner wrote ...
Is that spot silver? If so, I'd say it was the getter, as per our discussions.
Hello, This tube actually does have some silvering, but near the bottom on the glass. This spot does look like a hot spot had formed. Anyone know if this is bad? No holes, so I imagine its good. I wont be able to test it out for a week or so while I get the rest of the tesla coil built. Matt
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
My guess is that the markings are an artefact of the manufacturing process.
If I am wrong, and the rectangular marks on the anode are the result of over-heating, I would hesitate to spend any time, effort, and money on them. Valves which have over-heated for any length of time will often, but not always, have gone 'soft.'
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
All the 833a tubes I have seen have had those horizontal lines on the plate. They seem too uniform to be damage marks. What I am concerned with is the small shiny area of discoloration. Any quick way to test out a vacuum tube? Thanks everyone! Matt
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
EEYORE wrote ...
Any quick way to test out a vacuum tube?
You can test filament continuity with an ohm meter.
You can make a basic breadboard point-to-point test circuit with nails knocked into a piece of wood.
Connect the filament to a 10V supply, and determine that it draws about 10A.
According to the graphs in the RCA data sheet, if you now put a reduced voltage of 1kV on the anode and leave the control grid 'floating' at 0V, then the valve should draw about 150mA anode current. Depending on the valve's history, I'd expect anything between 125 and 175mA for a viable valve.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Hello, The filament is good. So you are saying that I could apply 1kv or so to the plate, leave the grid disconnected, and see that 125mA or so is drawn? Thank you, Matt
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
EEYORE wrote ...
Hello, The filament is good. So you are saying that I could apply 1kv or so to the plate, leave the grid disconnected, and see that 125mA or so is drawn?
Yes.
If you look at the Average Plate Characteristics graph in the RCA 833A data sheet, and follow the curve for zero grid volts, you will be able to test its functioning at other voltages that might be easier for you to provide, though as you will notice, the graph is very cramped for anode voltages less than 1kV.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Hello, Thank you for all the wonderful tips! I am using a level shifter for my MOT supply. Can I assume DC on my DMM (good for up to 1kV) is accurate enough to shoot for while looking for 125mA or so of current draw?
My first tube with the strange plate blemish did 100mA with "960v" on the plate using my DMM...Seems to be a little under the weather? I am warming up tube number two...
EDIT: So I tested number two that came with the other tube that I just tested and its only doing about 60mA at "1kv"....It does look to be used a good bit, so maybe it has seen its days. I tested my last 833a which I got from a different seller. It looks to be a much older version of the RCA 833a tubes I just got in the mail. I tested it out and got 100mA at 1kv just like tube 1. Perhaps they are okay if both are drawing the same current for a given plate voltage? Matt
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
EEYORE wrote ...
Hello, Thank you for all the wonderful tips! I am using a level shifter for my MOT supply. Can I assume DC on my DMM (good for up to 1kV) is accurate enough to shoot for while looking for 125mA or so of current draw?
My first tube with the strange plate blemish did 100mA with "960v" on the plate using my DMM...Seems to be a little under the weather? I am warming up tube number two...
EDIT: So I tested number two that came with the other tube that I just tested and its only doing about 60mA at "1kv"....It does look to be used a good bit, so maybe it has seen its days. I tested my last 833a which I got from a different seller. It looks to be a much older version of the RCA 833a tubes I just got in the mail. I tested it out and got 100mA at 1kv just like tube 1. Perhaps they are okay if both are drawing the same current for a given plate voltage? Matt
It could be that all the valves were thrown out because of low emission, but the average anode characteristics graph is so cramped at 1kV and below that I wouldn't draw any hard and fast conclusions from it. If all conditions of the test were identical, then I would assume that the 60mA one was definitely clapped out.
I expect that you will still be able to get the 250mA or so anode current from the two 'good' valves with 2kV on the anode in Class C Self-Rectifying Oscillator mode, which is better than a poke in the eye with a burnt stick.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Proud Mary wrote ...
EEYORE wrote ...
Hello, Thank you for all the wonderful tips! I am using a level shifter for my MOT supply. Can I assume DC on my DMM (good for up to 1kV) is accurate enough to shoot for while looking for 125mA or so of current draw?
My first tube with the strange plate blemish did 100mA with "960v" on the plate using my DMM...Seems to be a little under the weather? I am warming up tube number two...
EDIT: So I tested number two that came with the other tube that I just tested and its only doing about 60mA at "1kv"....It does look to be used a good bit, so maybe it has seen its days. I tested my last 833a which I got from a different seller. It looks to be a much older version of the RCA 833a tubes I just got in the mail. I tested it out and got 100mA at 1kv just like tube 1. Perhaps they are okay if both are drawing the same current for a given plate voltage? Matt
It could be that all the valves were thrown out because of low emission, but the average anode characteristics graph is so cramped at 1kV and below that I wouldn't draw any hard and fast conclusions from it. If all conditions of the test were identical, then I would assume that the 60mA one was definitely clapped out.
I expect that you will still be able to get the 250mA or so anode current from the two 'good' valves with 2kV on the anode in Class C Self-Rectifying Oscillator mode, which is better than a poke in the eye with a burnt stick.
Hello, So the other two ought to be fine then (at least for a vttc), and the other one (dud ) should be good for some abusing? My coil is nearing completion. Just need to put together the actual coils now. I will have to take some pictures and start a project thread soon.
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Vacuum tube without silvering still good?
