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. The seller is a retired production 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 I am told, 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. 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. 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 few things, one of then is the Transconductance exceeds the datasheet value by a minimum percentage. From my measurements, transconductance is from 105%...to 110%. Not many know perhaps, with NEW tubes, the transconductance of new tubes is a tolerance issue, not an emission issue. (Only with used tubes it becomes an emission issue). In production, if the Anode distance would be higher than normal, this increases the transconductance, and plate current becomes lower than normal. It is extremely important to understand, plate current in this case doesn't get lower because of lower emission, but because of a change in the tubes curves. So, simply due to normal factory tolerance, all tubes have this to a certain extend. In means a good tube, with lower emission than normal, should have higher transconductance than normal. And vice versa of course. Very important is to know how transconductance is measured. This may seem silly, but many do not know. You can not measure transconductance by applying the "normal" grid voltage to the tube. That is because it is not precisely known what will be the plate current in that case, however transconductance depends a lot on the plate current. So Transconductance must always be measured at that one specified plate current, which belongs to this test point the manufacturer tells you. This is 13mA for the C3g. So you need to change Ug1 accordingly to achieve this current of 13mA, and then you can measure transconductance.
Now it becomes difficult.. Pentodes with higher than normal transconductance, tend to draw less current, and to get the normal current, such tubes need a less negative grid voltage. This is so for all new pentodes, regardless Type or brand. If you multiply transconductance with the control Grid voltage (Ug1) needed to get the normalized plate current, you find....more or less a constant factor, indicating good emission of the tube. You can try this with EL34 or any other pentode. You take a new lot, and always the ones with higher transconductance draw less current, and vice versa. Also you will see, that transconductance and plate current show normal variation, yet the product of grid voltage and transconductance is remarkably constant, PROVIDED.... the tubes are new, and good quality.
Conclusion: To understand the above, best is really do some experiments yourself, and you will have the aha! effect. Now comes the C3g S-Version. What is interesting with those, I measure higher transconductance indeed, but I do NOT need less negative Ug1 voltage. By the above explanation, this indicates such tubes have their higher transconductance not by production tolerance, but by higher emission indeed. This is a very important conclusion, and for me the reason to regard such tubes very valuable. Also because not many are found.
Experiment: If you have good tube, you can just bias it at 10% lower heater voltage and see what happens. I would say this possible for a some time, but not for hours. You will see what is logical: Of course plate current goes down somewhat, and also transconductance goes down somewhat also. Now, THIS is lower emission. The measurement confirms it. However if a random, good tube, at normal heater voltage has LOWER plate current that doesn't have to mean emission is lower. If that tube has HIGHER transconductance at the same time, this is not lower emission, but just Anode distance tolerance.
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)