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Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Brtaman so very generously donated a nice 4cx250b tetrode to me. I received it yesterday, and the filament is good, it pulls 2.7 amps at 6 volts exactly, which is what the schematic specifies.
Well, no matter what i did, i could not get the tube to conduct From the plate to cathode. I am measuring the current with an analog moving coil meter and a shunt of 10 ohms, which shows any tiny amount of current. I have tested that much. With the control grid on a few tens of volts potential in respect to cathode, and a 350v supply on the screen, nothing would conduct, and with all grids at cathode potential, and some 1.5kv on the plate, no current was being pulled.
I am wondering if there is a bad connection or molten grid, or if the vacuum is bad. It is metal to ceramic seals, so it seems like the vacuum could have leaked. But, i would expect the filament to burn out if that was the case, within a few seconds.
Is there an easy way to test this without having to buy any measuring equipment?
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
look for a white spot on the top of the tube inside the glass. if found then the tube is 100% up to air :(
it should be silverish if the vacuum is ok.
also another test is to ping the glass with a fingernail. a good tube will "ring" for several seconds, whereas a bad one will go "Donk".
That said, if there is no obvious mechanical damage then the tube could be salvageable if you have access to a good two stage pump and small induction heater to reactivate the getter.
regards, -A "Bother" said Pooh, as he clicked "OK" to Install_Spyware.exe
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Conundrum wrote ...
look for a white spot on the top of the tube inside the glass. if found then the tube is 100% up to air :(
it should be silverish if the vacuum is ok.
also another test is to ping the glass with a fingernail. a good tube will "ring" for several seconds, whereas a bad one will go "Donk".
That said, if there is no obvious mechanical damage then the tube could be salvageable if you have access to a good two stage pump and small induction heater to reactivate the getter.
regards, -A "Bother" said Pooh, as he clicked "OK" to Install_Spyware.exe
That is the problem. This is a ceramic tube, so i cannot check the getter for a change in the gasses etc. Getters ARE reactivate-able? It'd be nice to be able to pump a tube back down, but even if i did that it probably would not work worth a crap, plus it'd be impossible to reseal the tube.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Try connecting the screen and grid and plate all together. Connect your DVM on the lowest voltage range between cathode and the junction of the 3 other electrodes. When connected, apply filament voltage and see if you can see any space charge current generated as it heats.
Those tubes aren't like lightbulbs that burn up when vacuum is lost. The filament temp is lower so the cold and hot resistance are not too different.
If you do hook it up for a emission test as its normal voltages and it is good the wiring may make it oscillate and it will RF burn you.
Bypass the pins to ground with caps
Those tubes require considerable drive to budge. Find a ham with a linear that uses it and try it.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Most things that go wrong with a vacuum tube will cause it to conduct too much, rather than too little.
Be careful, because the 4CX250B is a high-gain tube with delicate screen and control grids, aligned to a few thousandths of an inch. It doesn't need "considerable drive to budge", I've seen an audio amp application that made 600 watts from a push-pull pair of the things, with the grids driven off a single 12AX7.
If you connect the control grid to the cathode, and apply a voltage to the screen with nothing connected to the plate, you can draw heavy screen current and melt the screen. Likewise if you drive the control grid too far positive with respect to the cathode, you'll draw heavy grid current and burn out the grid. This renders the tube completely useless. It's too small to even be useful as a doorstop or paperweight!
I'd test this tube by connecting it as a triode. Try connecting G1 to the cathode and G2 to the plate, then apply some voltage between plate and cathode and watch the current draw. You should get several hundred mA of current flow at maybe 100-200V. I once tested some EL34s like this and they could draw enough current to get the plate red at less than 100V.
Here's how I tested a used 4-400:
I hooked it up to two transformers and let the plate get good and red. But note if I had got the two transformers out of phase, it would probably have burnt up the screen, so I checked the phase before I started.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
I was careful when i tried it. The control grid was at only a few volts, maybe 10-20, with respect to cathode. The screen was at 350v, which is 50v lower than the max. I used a fullwave rectifier for the screen, and the supply was also fullwave rectified (i heard you talk about how you cannot let the screen conduct to the cathode or whatever it was, so the supply and screen should have the same waveform, i paid attention). I hope i did not kill it, but i am fairly doubtful i did. At no point in my testing did i pull plate current.
I will do the test you suggested, Steve. I have always done that with my final amp sweep beam pentodes from televisions. I basically did the same thing already, though. Only difference was that i tried giving the control grid a few positive volts. Maybe something was wired wrong, though it is doubtful.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Just let the grids float, lad - leave them unconnected for a quick test in diode mode. Solder a resistor of 4K7 10W to the anode pin. Apply heater current for 60 seconds, and then very briefly switch on your 350V power supply with positive to the anode via the 4K7resistor, and negative to the cathode. . The valve should at once conduct significantly. If it does not, it means that the vacuum has either gone 'soft' or lost its vacuum completely. In either case, the valve is unserviceable.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
I tried what Proud Mary suggested. At first, absolutely no current was pulled. So, i started turning it on, then off a few times. About 2 out of 15 times, the meter shot to the positive side, and my 5kohm resistor was almost unnoticeably warm. But right after it conducted, it shot back to zero current.
I am confused! Do you think an arc formers due to a dead vacuum? At only 80v input to a microwave transformer, the sudden current draw happened about as often as with 120v into the transformer.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
If you look at Fig 1 in the RCA data sheet, with control grid 0 volts, you'll see that at the lowest voltage range where conduction is possible, the graph of Anode Volts/Anode current takes off like a rocket, so that a very small increase in anode volts causes a very steep increase in anode current. This is normal operation and most likely what is going on with your valve. If the vacuum was gone, there would be no conduction at all.
The most common mode of valve failure is heater failure, with the next most common being progressive loss of emission as the valve ages. A valve replaced because of reduced emission will still function to a limited degree, which is a good reason for only buying valves in unopened labelled cartons, since it was common practice for accounting purposes for a technician to put a failed valve into the same box from which the replacement had just been taken.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
I am fairly sure this tube is dead... Even with the filament off it looked like it pulled some current. It is possible that is not the case, because the resistor was cold after i was the meter move. I am just going to say the tube is dead. Every test i used failed. I would love to be proven wrong, but i am sure that simply will not happen. I have always tested other tubes this way, as well as some other ways, this one failed every one. I am not sure what is dead. I think it went soft, seeing that it is an old tube, with ceramic/metal seals.
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How to test the vacuum on a vacuum tube?
