In broadcasting, a fairly standard practice for squeezing an extra bit of life out of a very expensive tube whose emissions have dripped off is to up the fillament voltage say 10% and let it cook about 12 hrs with no HV applied. This seems to somehow free up a bit more cathode material. At multiple thousands of dollars each even an extra six months of time is worth a little effort.
My question is has anybody tried a similar practice with audio tubes and if so what were the results?
Doc
My question is has anybody tried a similar practice with audio tubes and if so what were the results?
Doc
Thanks for your replies. I know the cathodes are different in smaller glass envelope tubes. I knew that the main cathode material was thoriated tungsten. In broadcast tubes I've seen amazing results from the rejuvination process. Saw a Klystron that was basically dead come back to life and live another year and a half. At $36,000 ea. that was significant.
Doc
Doc
Got some direct heated transmitting tubes and a bunch of them have the following issue. With the heater on and 0V on the grid, when I bring up B+ there is very little current passing through. At around 300V on the anode a purple/bluish glow appears at the bottom of the tube and the current through the tube goes up abruptly. Looked around the net and found some advice on tube rejuvenation which involved baking the tube for 12 hours or so at about 100C, which I did, last night. But it didn't help even one tube. The tube needs 5V/7.5A for the heater, so I also tried to give it 10V for a few minutes. Tough tube, the filament didn't die; the plate was nice and red.
Not sure what else I can try. Any advice?
Not sure what else I can try. Any advice?
Run the filament up to 5.5 volts and let it cook a day. No HV on. Check for emmission then repeat. But it may not be emmission but a gas problem. Depending on the tube, you might check to see if they are rebuildable. Some are. Surprisingly you get better results out of rebuilt tubes because they run much longer without gassing up because all the components are pretty much through outgassing. Basically they just replace the filaments then resuck a vaccuum and reseal.
Doc
Doc
It would help to know the type/number of the tube in question. However, the symptom you describe is a large amount of gas that most likely makes the tube unredeemable. However, with certain smaller types, I have had fair to good results with this treatment. First, make a diode out of the tube by connecting the grids and plate together. Next, light the tube filaments to the normal level. Then, apply 120 volts AC across the tube (cathode to plate) in series with a 60 or 100 watt light bulb and let it run for several hours. If the bulb grows dimmer, the gas is recombining inside. You may also need to apply forced air cooling with an external fan and chimney setup depending on the tube type. Use an isolation transformer if you're squeamish about the AC.
edit: I just saw that it's a 4E27. Those are very old. If the bottom of the tube has green glass, these are always leaky and almost never work out well. And if they're anything other then Eimac, forgetaboutit.
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Nice trick, thanks. What do you mean about the light bulb? The bulb I'm using doesn't light at all.
Also, found this page with a few more ideas.
http://www.sm5bsz.com/recondit.htm
Think he was talking series current limiter on the 120VAC. Resistance of bulb element shoots up as it heats. Limiting current.
Doc
Black hole,I got that. But he's talking about the bulb growing dimmer, implying that it was lit. My bulb is as dark as a black hole.
man that's dark. My treatment was for a very gassy tube and a way to bring the gas down when normal operation wouldn't work. Assuming your tube filament lights at the normal intensity, you either have poor/no emission or the gas is not ionizing. Usually 120 volts is enough to get it going. If your tube lights up dull, then it has poor vacuum. In which case it usually smokes. But you said the plate got red, so it must be conducting. Your bluish glow at the bottom of the tube may be due to the uncontrolled conduction and excessive current. Or perhaps from a little gas that's not otherwise apparent. Difficult to tell without being there.There is a way to test emission on many directly heated small transmitting tubes up to 1000 watts plate dissapation. You need a filament transformer to light the tube, and a 100 volt DC power supply, preferably adjustable, that's capable of 1 amp with voltage & current meters. Before I built my big rack mounted class "A" tester, this is what I used to screen out the definitely bad from the maybe good. It doesn't test for gas.
As before, make a diode out of the tube by tying all grids together and to the plate. Light the tube up and connect it across the 100 volt DC supply. Minus to cathode & plus to plate. A good 4E27 will draw ½ amp as displayed on the amp meter. I know this because I've used this test for years in the past to screen tubes I bought. And I have a list for dozens of tubes and their current draw at 100v compiled from testing thousands of tubes. Mostly between ½ and 1 amp.
If the tube passes this first part, then give it the acid test. Reduce the filament voltage 10% and watch the current meter. A good tube will not move much. A weak tube will be spongy and fall back as the filament is lowered. And the greater it falls back, the weaker it is.
Also, and this is from Eimac (the horse's mouth), with a thoriated tungsten filament, filament current is very important and is used as an indicator of tube condition and life. These tubes have a definite range of filament current that is acceptable. As a tube ages, carbon is drawn to the surface of the filament which lowers it's resistance. this results in a greater filament current. When the current reaches 5% above mean (bogey), it is at end of life. Worn out filaments of this type become more fragile and break easier too.
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I should give more details. This particular tube (one out of five) has been sold to me as NOS. When I tested it first, 5V on the filament, -10V on the grid, and 350 on the plate, the purple glow was very concentrated at the base of the tube. After baking the tube overnight, and applying your procedure for about an 1 1/2 hours tonight, the purplish glow is not as concentrated as before. It's rather diffuse and goes up to about 1/3 of the tube. While on 115V AC, wired as diode as you said, the tube draws at first about 1.7mA, then over the one hour it went down to about 1.58mA.
The other thing I should say is that when the tube is wired as triode, with about -10V on the grid and 5V on the filament, when the voltage is increased on the plate, the current through the tube goes up once almost to normal levels of conduction, and then it decreases to about 2-3mA.
The one thing that gives me a bit of hope is that the purple glow isn't as concentrated as it first was. Besides time, I got nothing else to lose.
The other thing I should say is that when the tube is wired as triode, with about -10V on the grid and 5V on the filament, when the voltage is increased on the plate, the current through the tube goes up once almost to normal levels of conduction, and then it decreases to about 2-3mA.
The one thing that gives me a bit of hope is that the purple glow isn't as concentrated as it first was. Besides time, I got nothing else to lose.
You do know that this tube's a pentode, right? My class "A" test for that number is like this:
5V fil.
2500V plate
500V G2
-65V G1
Pin 5 & 7 tied together
PI should be 50mA
G1 current should be very low. <10µA (but it's always more)
NOS means little because the tube is 50 years old. And this number has a bad reputation for being gassy. There's often little one can do for this tube when it gets this old.
5V fil.
2500V plate
500V G2
-65V G1
Pin 5 & 7 tied together
PI should be 50mA
G1 current should be very low. <10µA (but it's always more)
NOS means little because the tube is 50 years old. And this number has a bad reputation for being gassy. There's often little one can do for this tube when it gets this old.
It would be hard to not notice it's a pentode.
It's a bit of a problem... don't have a 500mA HV supply, nor a 2500V supply yet. Yes, I did connect pin 5 (G3) to ground (pin 5).
Was hoping this was NOS and the only problem would be gas... but clearly I don't have enough experience with old tubes.
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