845 / 211 / VT4C
Test cards and Rejunivation cards
Description
Disclaimer: You are using there cards entirely on your own risk!
Since the Anode voltage of the L3 tester can go up to 300 Volts, this comes into a well defined range of the tube curves of the 211 and 845. The clearest print data sheets are by Amperex, and these are used to verify the bias points with these cards. After testing some Chinese 845, a Cetron 845, and some USA made VT4C, it seems these card works very good.
It is also clear however, these tubes show a lot of variation in bias point, but transconductance is more consistent. So matching of 211 and 845 is much recommended.
Another issue, and the reason why testing of used 845 / 211 is this:
These tubes are transmitter tubes, which are also officially recommended for audio applications. However when reading carefully the old data sheets, these are 100 Watt dissipation in Class C only. In Class A, they are 75 Watt. By itself not a problem, because these are very dangerous tubes, and we can assume only experienced designers would touch those and do a great job. Well this assumption is wrong. The problem is, buyers of those amplifiers, do not want to pay the high price for a real high voltage design, and also refuse to buy when the output power is not specified at 25 Watt. So here comes a real problem, because to get a realistic 18 Watt, you need to go to 75 Watt plate dissipation at 1300 Volt. So what are the "designers" doing? They use 1100 Volts or 1000 Volts, because it's cheaper, and simply load the tube with 100 Watt dissipation. Either in purpose, or because they don't read the data sheet. Whereas historic 211 (mostly called VT4C) will not get defective from 100 Watt dissipation, it is not the best you can do for the poor tube. The #Chinese tubes will not go bad immediately, but have lifetime of only 1500 hours this way. These are low cost, so financially it is not a problem, but it does mean may not no good tubes are playing in many amplifiers.
Vintage 211 / 845 will last 10.000 hours, and provided the heater survives it, they can be regenerated three times! So when you are lucky, and when you know how to regenerate them, you can get 40.000 hours from vintage 211. Which is in the end a lot cheaper than using #Chinese stuff.
Anyway... testing of those is very important.
The GETTERS of 211 / 845.
These tubes have normal Barium getters, and frankly, the larger they are, the better. Moreover, the edges should fade out gradually. So where the getter stops, it should not be suddenly. This is a sign of some gas which has come in during storage.
If the tube still works despite partially used up getters, then.... . what is nice about these tubes, they have a Zirconium coating on the anode, which coating becomes active when the tubes are cherry-red in the dark. So you might be able to get the gas out that way. This requires 100Watt dissipation, and the L3-3 can not do that of course. In case the tube is too weak to produce enough current, you can try to do the Rejunivation. of the heater first.
We have two Rejunivation. cards for the heater. These Rejunivation. cards work on 211 and 845 the same time. (But normal test cards are dedicated to 211 or 845!)
Wiring of 211 / 845 test set up.
It is known these are by nature transmitter tubes, but they are so linear, they can also be used for Audio.
While testing out these cards originally, I connected it with four long cables (each one meter) wired to Socket #17 directly. This was NOT working well! The 845 tube oscillated so wild, it became a very powerful RF transmitter. There was an analog multimeter voltmeter near by, with only one (!) probe lead plugged in. So the meter was just standing there, nothing attached to it, but one probe lead, which was also just hanging loose. This probe acted as antenna, and the signal made multi meter needle moving. And not just that, it was hacking violently into the corner, making a loud noise.
So do NOT take this card, intended for an adapter, and connect it with very long, loose wires to socket 17. Because that will go wrong.
The adapter should be a little box only, which plugs directly into socket 17, and it has an 845 socket on the back of it. It should have no wiring at the outside.
Rejunivation. Cards
Rejunivation. is tubes can sometimes be successful, and sometimes not. To a great extend, tubes have the ability to repair defects of shortcomings. This is provided, there are no such defects which can not be repaired at all, like a grid deformation by over heating, just to mention something. Yet several defects can be repaired, if you only knew how. In fact, a new tube coming right from production is terrible. Emission is too low, it has noise, and leakage all kind of bad problems. All it needs form there, is three things. These are, burn in, burn in, and burn in. To be more precise, there is the initial burn in to make the tube function in the first place, then comes the factory burn in, which ends at a condition where it can be stored, and is normal working even after many years of storage. Next is the customer burn in which is the longest, and only after this, final tube data is achieved, and not before.
Decades of storage is not doing good to the tubes, because there are all kind of contamination set free during this storage. During normal use, this contamination would also be set free, and at much higher rate of course, but then the tube is hot. Meaning the getter is able to maintain a much higher vacuum, and due to the heat, no contamination will attach to the cathode. During storage however, any contamination can attach to the cathode easily, as the cathode is now cold, and also the getter will not be able to maintain the vacuum very well, in cold condition.
With some tubes, the contamination disappears right away, after first use. WIth others, the contamination has formed some chemical substances, which will not be removed from the cathode easily. Out glowing the cathode at higher temperature, may be a solution, but is damaging any cathode by definition. So this is a balancing act. Some other methods are using rather high plate current, and an electrolysis reaction changes the contamination into pure metal and gas residue. The gas will be caught by the getter, and the metal residue will evaporate, and land on the glass. Note, at orange glowing temperature, many materials become conducting, even glass, and it can become subject to electrolysis. Which in the end may lead to metal contamination by the metals contained in glass, in oxide form. So all on all, this is extremely difficult, and most of the time, we can only say what works, what doesn't, and simply how to do it. Such instruction can be as simple as 60 seconds over heating. Yet that will only work for that particular tube only. Some tubes (like battery radio tube) may need several steps of over heating, and also tube types exist which should not be over heated at all.
Generally it is such, that Rejunivation. works best when the tube is not too low on emission. So lets's say, a tube is at 80% plate current, chances that you can improve it, are good. When the tube is at only 40%, it may be possible to improve it, but likely transconduction will stay too low, and plate impedance too high.
These Rejunivation. instructions used for 845 and 211, I found many years ago, inside an NOS Tube box, with a General Electric VT4C tube inside. VT4C is probably the best tube of it's kind ever made. Not topped by anything else. The Chinese 211 clones, are cheap junk, compared to USA made VT4C from 1943. This reflects to the lifetime of VT4C which can be 10.000 hours, where as Chinese 211, gets weaker already after 1500 hours.
I do not know how good the prospects are for refreshing Chinese tubes, but I do know, for original USA made tubes from the second world war period, chances are so good, that GE even recommended officially to try it.
Rejunivation Card #1
This is a non damaging card, but you do need to follow the procedure on the card, as this reflects the GE method. So either the tube improves, or at least it stays the same.
Do not just play around with it, but make good notes from the beginning about the test results. So you can see if it makes sense to repeat the process a few times. Say it improves the tube an 80% tubes only 5% each time, that means 15% after fives times, and perhaps the tube is at 95% then. Just try it, it costs only time.
Rejunivation Card #2
This card overheats the filament. This would remove some contamination, and also migrates some radio active Thorium from deeper layers, to the surface where it is needed. However this also may lead to heater breakage. Either now or at a later moment. So the voltage needed for this, and the time needed, we have to make sure to follow exactly the GE recommendation. Do not heat it shorter or longer, and not at another voltage as GE Says. Still, using this card is only when you see no other option. After this overheating the procedure of Card #1 must be used, to see if there was an improvement. If so, you may repair the Card #2 process ONCE. If there was no improvement, or even negative effect, it should not be repeated. Then the process failed, and one should be aware of this risk. So Card #2 is not recommended for expensive NOS tubes, which are basically "good" and you want to make then "perfect". This card is intended for tubes which test not very good any more, and you want to give it a try.
Burn in at 100 Watt
The tester can not supply that much, so it's just mentioned here for interest, but this is another one of those "possible" methods. 845 and 211 have a Barium getter, which function is only to remove some gas from storage and production. The real getter is Zirconium powder, which is soaked into the graphite anode. This getter works best at red glowing temperature. At 100 Watt, in the center of the anode, will develop a 2cm round spot which can be seen cherry red glowing in the dark. This will activate the Zirconium very much, and with some luck improve the vacuum to a very good level. This is of course very meaningful when the Barium getter seems partially used. This may be done for several hours, and any improvement afterwards can be detected by increase of transconductance at the same test conditions.
Bias 211 or 845 at 100 Watt? No sir!
It's a traditional misunderstanding that 211 and 845 can be used at 100 Watt dissipation. This is only so for Class C (Transmitters) but for Class A or AB, it's 70....75 Watt only.
Same (badly!) designed amplifiers use the tubes at 100 Watt already, and it should be clear, in such a case, this method of Card #1 or Card #2 is not going to do any good, because here the problem is rather the Thorium has disappeared, and not bad vacuum. Yet, Card #2 may give a small improvement still. Sometimes your luck or bad luck, comes unexpected with tube Rejunivation. Though tubes which have been fried too hot and loo long, are most of the time permanently lost.
Disclaimer: You are using there cards entirely on your own risk!
For the socket adapter, here is how I build it myself. Due to High Voltages, you have to build this in your own reponsibility, all I can offer is some of the materials, and the cards. The socket used here, is a standard Chinese 845 socket. I filed off removed the solder wings, which was easy, the matal is some weak alloy. Then I glued on the UX4 base, which works nice because the 845 socket ceramic is raw from the inside. Once the base is on, I soldered the wires from the inside. Connection scheme is same as 300B.
In case you like this little neon lamp.
I took it from an old switch. It is a normal small neon lamp without series resistor. The way I connected it, was like this: First a 10k resistor across the heater. This protects the tester in case the tube heater is open. Then the neon lamp with one end to the heater (doesn't matter which end). The other end is connected via a 2M2 resistor to the plate. The lamp current is only 0.1mA so this little error has to be accepted. The left switch (if set to "Paramers") switches on the anode voltage. So with the Neon Lamp in there, you have better awareness, to remember to switch on and off the HV before exchange a tube to test .