Portrait of a tube
C3g, C3g-S, C3m, C3o.
Author: Jac van de Walle
Perhaps the best small signal pentodes ever made
The "3" I believe is to show they are third generation post tubes. These were low noise audio tubes, made by Valvo, Siemens and Telefunken, exclusively for the German post. SEL (later ITT-SEL) provided their telephone electronics to the German post, and the tubes were private branded for ITT, but still made by either Siemens or TFK. (As they all are). The TFK have a nice logo stamped into the metal. Most have a banderols on it, with a series number, but not all of them have this. These were not for sale for other customers. They were used in repeater amplifiers for long distance telephony.
Admitted, in the steel can they are not glowing so nice. Believe me, once you feel these tubes get warm, you start to see them as tubes also.
Who can remember the 1970's with a long distance call from Europe to the USA? A call cost you 5$ per minute, and still the voice on the other end was not loud enough, there was a lot of noise on the line, and a one second delay. So you started to speak very loud into the phone, but that didn't help much, and I remember people in the offices would (try to) look important, by shouting into the phone with long distance calls.
Anyway this is not what good signal transmission is about, but it was the limits of what was possible with ocean cables. If there is one enemy of a long distance call, it's low signal and noise. The problem is, a long distance telephone cable is a noise generator, and an attenuator too. So after a certain length, the noise gets more and signal gets less. This limits the length of a telephone line, and simply amplifying the low signal that comes out, would not help since you can't get rid of the noise.
If this is unclear to you, imagine the following, on a loud market place in the open, somebody shouts a message to you from a distance. You can understand it, regardless the noise. You could repeat the message, and shout it to the next person. So you are a "repeater", and with enough repeaters, you can send a message over great distance, regardless the high noise level. Provided no repeater distorts the message of course. Now suppose you don't want to use repeaters, this is not going to work. The sending person can not shout any louder as maximum, and suppose you go all at the other end of the market place, it will not help to amplify what you receive with amplifying equipment. You would amplify all the random speaking and noises too, and what you try to hear gets lost in the other babble and shouting on that market place. Indeed the only solution is, to repeat the message before it gets lost in the noise. You see, with a long distance transmission cable, it works the same way. So the only solution is, to re-amplify the signal before it gets too noisy. It increases the maximum length of a cable if by putting in repeaters, by amplifying the signal before the signal-to-noise-ratio (SNR) gets really bad. Doing so on our planet, using good cables and finest tube equipment, they could overcome distances from one continent to the other. Though practical situations are extremely difficult, and such projects are higher arts of project management. Imagine the weight of such a cable on a ship, or the trouble with repeaters in salt water, with a 4000 meters column of water pressure on it, inaccessible for ever, somewhere between Honolulu and Paris, on the ocean bottom.
Going further back to the 1960's, this had to be done with tube equipment. Of course they tried to use as little repeaters as possible. The ideal repeater amplifier adds no noise, has no distortion, draws no current, is very small, and needs no service. A partial solution you get from adding an auto transformer (Pupin coils) at certain distances, but there comes a moment where you need active amplification. So the engineers desired the "ideal" tube. However these ideal requirements do not go together well: High Amplification, low noise, long life, no distortion, small size, low filament power. As most of you know, a tube with more heater temperature will have better electrical performance, and more lifetime, but uses more power too. So the solution is a bit more complicated than you might think. Tubes that meet all of the requirements are masterpieces of design, and non-commercial since they are EXPENSIVE. The price of C3g was 295DM for one tube. So that's about 200$ for one tube. In those days you could buy a new car for 5000$. So a standard 50-Pack of those had the price of two new cars. Obviously, cost was no issue with those, and that's logical since with those you could for instance work with a cable that had a bit higher loss, and add repeaters inside to compensate that. Imagine 1000km (1 Million meter) of cable can be made 20% lower cost, who cares if the tube that makes this possible costs 200$. This is the world of C3g, C3m, C3o.
I think most of the secrets of how to make C3-tubes are gone with the wind, but at least we can still buy those miracles of tube art from new old stock. If the days come where we only can buy used one, it's no problem too, since the end-of-life is clearly specified in the data sheets. So you can always tell if the tubes are still good. I found the Funke W19 tester very reliable in picking out used tubes with good lifetime in it.
These German post tubes were first designed in the heads of the amplifier engineers, that finally wanted to have something without compromise. The engineers just said, what must an ideal tube have, and then let the tube factories try to make it.
Let me bring to your attention, the C3g at it's introduction in 1952 was the fist Frame grid tube, and it was not available commercially. In short, a frame grid means the grid wire is not self supporting, but is wound around a hard metal frame. Like this, they could use wire which is so exceptionally thin, as it was never possible before. You can not see this wire with the bare eye. This is quite strange to observe, when you take a frame grid tube apart. The grid wire is there of course, and you can "see" there is something there, that you can look through, but you can not see what that is. As if there is a gold colored, transparent layer, that you can see through. Things looks a bit "unsharp" when you look though. Like the low resolution picture below. It is just like this when you look at a frame grid wit the bare eye. Then, if you click the picture it gets enlarged, and you can see the wires.
This technology is more expensive, and was used for very few tube types, perhaps 100 only. Given the 10's of thousands other tube types ever made, this is not much.
NOTE that in the 1980's when tubes were obsoleted, there was a lifetime-buy option by Siemens for the German post. It stretched over a few years. It is from this period that BIG lots of tubes were made, and just stored for later service of old hardware. Because of the high manufacturing numbers, these were very good quality. Popular tubes were C3g, C3m, and also Siemens ECC801S of remarkable good quality. Probably many other tubes as well, it's just these three Types I ran across myself. Something similar happened in the USA, and from that period many very good 6922, 5687, 12AT7 and 5751 are around. These are those tubes with a barcode label on them.
Then, the whole hardware developed so quickly after the digital multiplexing was used, and analog repeaters were taken out of service everywhere sooner than expected. And so, a six digit number of those tubes were stored and never used. Through the years these were sold, and it seems the German government stocks of C3g and C3m have dried up since 2005 or so. However, these tubes are always somewhere, and find the path to their end user.
Check for a 16 pages (!!) Data sheet at our website / under Techcorner
I guess they came up with some more nasty things, that the tube manufacturers all had to comply with. Just look at how nice the triode connected curves are. These curves are so linear, I think there are very few triodes excising with such nice curves!
What to do, if you don't like the metal cap?
You can take off the metal housing, and inside is a very nice glass tube!
C3g, C3m, C3o, what's the difference?
C3g was made after C3m, and C3g has a frame grid and gold pins. Frame grids generally give better quality tubes, though it must be said that the quality of C3m is just as superb, and I make the statement here for C3g and C3m, that there is no better tube available of the same kind. If you think I am wrong, email me with facts from a data sheet, and if you find a better tube, I will add it here.
Generally with C3m and C3g it can be said they have the gain of a pentode, and distortion same as only the finest triodes like E80CC. However C3m will give that low distortion at a gain of 78. (and E80CC only at a gain of 25).
A gain of 78 It means you can drive a 300B with just one C3m, and you have less than 1V input sensitivity. In a few words, this explains why these tubes are so great for HiFi purposes. For applications like pre-amp tubes, or driver tubes for 45 or 2A3 the C3g can be used as well.
They show their extreme low distortion only as pentodes. They can be used triode connected, but then gain comes down a lot, and the distortion increases to a level just below triodes like 6SN7.
Gain of the C3g vs. C3m
C3o is a C3m with 6.3V filament. However C3o is exceptionally rare as NOS tube.
C3g, C3g-S, C3m SPECIFICATIONS (LIFETIME LIMITS IN RED)
There is "S" Version of the C3g that is said to me to be selected for higher minimum transconductance. I purchased those tubes from a retired Telefunken employee, who traced down big lots of C3g and C3m to where they were originally sold to, and actually found some. So I can say I have them from the first owner. However I can find no information about the "S", other than what he told me, and other than what is on the tubes and tube boxes. FOLLOWING I noted myself. The Telefunken and Siemens have the "S" stamped on the boxes, but not on he tubes. This would logically mean, these are selected out of normal production. One exception are the VALVO. These have this designation" C3g/s" also in original white paint on the tubes itself too. At least this proves to me, this is something real, and not some special action done, for some customer. Also it must have been a big customer. I have such tubes from Siemens, TFK, and VALVO. Do not pay too much attention to this now, I will make accurate measurements later, when I have the AT1000 set up for C3g, and then I will see what is the selection is exactly about.
The "S" Version C3g-S
C3g-S is a hidden treasure. They are EXTREMELY rare. I mean the real ones, not the fakes. Oh yes.... the "S" is faked, but that can be seen so EASILY by tube testing. Please read more about "fake" testing here, because some sellers do so with the U-trace.
So far, I was not able to find the factory specifications for this, but one day, I hope "coincidence" is going to help us out here. I have seen the original test tools for C3g and C3g-S selection myself, in Ulm. This was a dedicated plug in unit, for a universal test bench. The bench was like 1 Meter wide, and the operator had to add plug-in units to it, for C3g or C3m, C3o. etc. The unit had lots of knobs and instruments on it. The tubes were tested one by one, by hand, and many settings had to be made, before all parameters could be tested.
About the S-Version, the following I know for sure, because I was told so first hand by an Ex Telefunken Employee, who was at that time involved in this. The S-Version is selected for a SEVERAL things at THE SAME TIME. One of those the Transconductance exceeds the data sheet value by a minimum percentage. From my personal measurements, transconductance is from 105%...to 110% at normal (so 13mA) plate current. However, make sure, you don't get fooled by auction website sellers, shipping you normal tubes with an "S" printed on them.
WARNING ABOUT FAKE TESTING, AS I HAVE SEEN IT ON AN AUCTION PLATFORM.
This fake testing trick as as simple as it it easy. Don't be fooled! They begin with showing you official, printed test information.
This is hard to explain in just a few lines. Saying you have tubes with 105% Gm, testing this way, means only you have tubes with 105% Gm tested THIS way, and NOT the Telefunken way! This is really not a good was to test. For this, you need to understand the difference between auto bias testing, and fixed bias testing. If you think now, "...is there a difference? " don't worry, many think so. Now, don't take me wrong, but not knowing doesn't mean it doesn't matter. Much to the contrary! For those tubes, where it DOES matter, you better look twice who sells you what, because if it DOES matter, that is not without a reason. There are some (very few) tubes which have an official data sheet with AUTO BIAS specifications only. Such tubes are always high quality tubes, like C3g, C3m, C3o, or their little sister D3a.
What is the reason for auto bias testing? As we all know, auto bias, biases the tubes "automatically" the same. So an auto bias circuit for 13mA will (and must) produce close to 13mA, for a tube of flawless quality. Any tube from 11...15mA in fixed bias, will perform close to 13mA in auto bias. This means, you must use very tight limits for an auto bias test, much more critical as with a fixed bias test. You will find those limits of course in the data sheet. A new tube, with too much plate current in auto bias, that should better not happen. Such a tube has a tolerance problem, it is simply not build well, or it was dropped, and the inner structure is deformed. A tube with not enough plate current, that is something else. It can have any problem at all. So, whatever it takes, to reduce the plate current, that can be virtual any defect. The idea of an auto bias test is NOT to find all tube parameters. Auto bias is intended for tubes that have good parameters by design anyway, but we want to know if the tube is at the end of life, or perhaps like new.
HOW TO TEST Gm? There is only one way: This has to be done at 100% plate current. There no other way, not for these tubes here, and not for any other tube. And yes, all new made testers (100% of them!) ignore this knowledge, and I would say ignorance that is the best word for this indeed.
The tube tester W20 (Card from above...) makes the same mistake, but at least they have corrected the "good begins at" level to such which (probably) can be expected with fixed bias. So, the 13mA tube is considered to be "good" above 8mA. Whereas the German post, who designed those tubes, never said so, and also never said this is the right way to test them. They are not doing it wrong actually, just not by Post specifications.
This brings us back to auto bias testing, because in auto bias, a good tube is close or at 100% plate current by definition. (and if not, it is bad). Any Gm test at a random plate current, such as you would get with fixed bias, is indeed just something random, and little more than that.
Now for the S-Version. Very simple: These must have >105% Gm at 100% plate current. They must be like this, and nothing else.
A tube which has 105% Gm, but needs 115% plate current for this, is just a normal tube. If these are marked "S-Version" it can not be true.
Gm = 105%, Ia =12.4mA (95%) NOS condition Regular Version
Gm = 95%, Ia =105%: NOS condition Regular Version
Gm = 105%, Ia =13mA (100%): NOS condition C3g-S
Gm = 110%, Ia =13mA (100%): NOS condition C3g-S
Gm = 105%, Ia =16mA (100%): NOS condition C3g-S
Gm = 95%, Ia =105%: NOS condition Regular Version
Gm = 105%, Ia =11mA. NOS condition Regular Version, or USED tube S- Version
Gm = 105%, Ia =10mA. END OF LIFETIME tube, because <10mA is a bad tube.
1st. advise. If the above is not fully clear that is because this is indeed not easy to explain. But, I have tried, and best is, just read it a few l times, until a light begins to shine. Also do the next experiment if you have the equipment:
Experiment: Take a C3g, which is reasonably good, but you sacrifice it for this experiment. Suppose Gm=95% and Ia =12.5mA. So slightly used. Now knock the tube on the table. Such that you deform the inner structure, but not damage the tube totally. Now re-test it, and you will see, Gm=105% and Ia= 11mA. If there was no change, you didn't hit the tube hard enough. You can also try this with an old EL34, because these react to this much more sensitive than C3g. It is crazy, and you wouldn't want to know how many EL34 or C3g have been "improved" this way to sell it on an auction site. This will easily fool a fixed bias test, but it can never fool an auto bias test.
2nd. advise. Any "S" versions are rare and expensive. Makes sure, Gm is tested ONLY 100% plate current. Let nobody fool you.
Build a pentode pre-amplifier stage with C3m
Take a normal Cathode resistor stage, and use this table.
Good applications for these tubes are:
From the fact they both exist, you can see they both have their justification. So don't ask which one is "better". Same as with ECC81, 82, 83, there is c3g and c3m.
Some small note about low distortion.
In pentode mode, C3m has distortion figures compared with E80CC, but at much higher gain. (at 5Vrms out, E80CC has, with bypassed cathode, a gain of 25 and 0,16% THD. C3m has a gain of 78 and 0,12% THD).
Data sheets. can be found in the tech corner of this website (From the menu)