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Last Update: 02-Jul-2017

Portrait of a tube

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C3g, C3g-S, C3m, C3o.

Author: Jac van de Walle
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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.

I have an original 1992 price list (not a copy). Here you can see C3g has cost 257DM for one. That is 130 Euro. The factory price for the standard 50pcs box was 4898 Euro.

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.

  • Highest amplification
  • Lowest low noise
  • Lowest microphonics effects
  • Higher lifetime than commercial tubes
  • Stand-by possibility (very few tubes have that)
  • Metal shielding
  • If a pentode is triode connected, it must have curves like a real very good triode
  • Fully complete data sheet, leaving NO DATA out.
  • Optional: Individual series number on each tube

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.

Read here about frame grid tubes.

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!

C3m inside

C3g with cap


C3g Inside. 

Look at the two round plates above the pins.   The lowest is a metal shield (outside connected) for lower hum.   The other is the mica.   The anodes are open from the sides.   This open construction is the best for finest linearity.   It allowed plate distance adjustments after the tube was assembled.  This noncommercial construction was used already in DHT post tubes from the 1930's. 

Zirconium + Barium Getter

We are lucky to have a lot of the C3g version with the additional zirconium getter. All C3m we sell gave this double getter! So that is additional to the Barium getter ring (or plate).

A Zirconium getter is expensive and it works like this: A Barium getter has most of its function during the short moment (a few seconds) that it is flashed during production, so when it is in the tube in the form of a cloud, while being transferred out of the getter halo, and condensing on the tube glass. This cloud, at the moment it exists inside the tube, absorbs (at that short moment) almost anything whatsoever. Then, after it will be condensed on the glass, the Barium getter is only conditional active. Like during great heat such as with KT88 tubes. However little tubes like C3g can not really use the getter any more after activation. Of course there are some remaining functions left, amongst which is dust catching (yes!) but maintaining extremely high vacuum is not done. Here is where the Zirconium getter comes in. These need no flashing. They start to absorb gasses, whenever they have a sufficient temperature, and all you need to do is, mount it at a warm place. So really top class tubes have both getters. These tubes used to cost 295 DM when new, I have seen an original price list myself.

Zirconium getters can have various appearance. Some can be small square plates as you see here. With other tubes like EL503 they are hidden inside the plates. Tubes like 845 have Zirconium absorbed inside the graphite. With the 845 the Barium flash you see, is only used for initial vacuum during production. Maintenance of the vacuum is done by the Zirconium getter.


Removed cap

You can glue the metal base on the tube.  Put two components glue on the sides. Do not fill glue in the center adjustment hole. That could break the glass pipe in the middle. Note that one  pin hole has a different shape, and is used for positioning. 

Yamamoto Socket

NOS Socket

C3g Siemens
Order Nr: 114-146-50

C3g-S Siemens. From 1975
Order Nr: 114-147-63

C3g Valvo
Order Nr: 114-150-29

C3g-S Valvo
Order Nr: 114-151-93

C3g-S Telefunken
Order Nr: 114-149-97

C3g Lorenz. Some boxes
have Date code 1978.
Order Nr: 114-144-48
C3g-S Lorenz. Some boxes
have Date code 1978.
Order Nr: 114-144-48


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.

  C3g C3o
This is a 6,3V Version of C3m


6,3V 6,3V 20V
Pins Gold Normal Normal
Grid Frame Normal Normal


Gain of the C3g vs. C3m

C3o is a C3m with 6.3V filament. However C3o is exceptionally rare as NOS tube.



C3g-S New

C3g New

C3g after
10.000 hours

C3m New

C3m after
10.000 hours


Heater Volt





Heater mA






Rk-h (ohms)


R Pin-to-Pin (ohms)


Rk (ohms)

Ig1,max (uA)



Gm Pentode (mA/V)
Gm Triode (mA/V

Ri (Ohms)
Triode connected


Ri (kOhms)
Pentode connected


Gain Pentode connected *2

4200 (theoretical maximum)
1625 (theoretical maximum)

Gain Triode connected *1


*1 From TFK Datasheet
*2 Tested with AT1000                              
*3 My personal observations



The "S" Version C3g-S

C3g-S is a hidden treasure. They are EXTREMYLY 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. More about this later).

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 SEVERAL things. One of them is the Transconductance exceeds the datasheet value by a minimum percentage. From my personal measurements, transconductance is from 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.

To understand how transconductance selecting works, and what it means, you do need to understand the simple tolerance mechanisms causing Gm differences with NEW tubes, and on the other hand, how to select those tubes that are REALLY the better ones, and not just those with tolerance issues, having by coincidence 105% Gm, and not higher emission at all. To get the awareness of this topic, just imagine you are a manufacturer of electron tubes, and you observe buyers prefer the so called "better" 105% Gm versions, instead of "good only" 100% versions, or "bad" 95% versions. If this is so, wouldn't it be a good idea to use 5% more anode distance and have 105% Gm from this? (Tolerance works really that way indeed). So buyers would be more happy! Or why only 5%? Let's just adjust the internal geometry for 110% Gm! So buyers get fully exited. If they plug the tubes in a Hickok, they indicate 110% strength. And even when they lost 20% from the initial value, they still are at 90% on a Hickok. So guess why manufacturers did not do so. I mean really try to give the answer before just reading it here. Do you know it? Well basically the answer is like: Because magic doesn't work this way. I keep you waiting for the details a little more.

Here is another observation first: When a tube gets older, they loose Gm slowly, we all know that. Now take such a tired tube, and increase the heater voltage 0.3 Volt. It's not much. Now look what happens: Gm improves 10%! Mmmm... suppose you are a manufacturer of C3g. What a big deal, only 0.3Volt. Let's just build them with 6 Volt heater and let all the people use 6.3V heater voltage, and any old tubes will look 10% better again. I hear you saying... STOP! Magic doesn't work that way. And we all know why. The new tubes will wear out faster at too high heater voltage. As they are 6V only, and they are over heated at 6.3V. That is the right conclusion. The only thing you can do, is build the tubes at exactly 100% of parameters by default, and from such a lot you can pick out the better ones. You will agree that gives a better feeling about quality, as manipulating tolerances. Sorry for this brute explanation, but take good note, it was done that way by some inferior brands. But now let's look into this in more detail.

Not many know perhaps, with NEW tubes, the transconductance is a more a tolerance issue, not so much 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, but at the same time, 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.

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.


So at 13mA.

Any tubes with just Gm >105% and the plate current is accordingly lower, these are normally good tubes, but no S-Version. So when Gm is 5% higher and anode current is 5% lower, that a normally good standard version tube.

As a rule of thumb, the average of Gm performance and Ia performance must be 100% for an amuse tube. So Gm = 105% and Ia =95%. Average is 100%, this is a new tube. Or, when Gm=105% and Ia=80%, this is an average of 93%. This is not a BAD tube, but under the assumption this is NOS, this is definitely not an S-Version. Or if we are SURE it is an S-Version, like picked from original equipment, then it must be a used tube.

For NOS tubes, when Gm is at 105% and plate current is 20% lower like 10 or 11mA, this can NEVER be an S-Version tube. When it's printed on the tube boxes and tubes itself, do not be mislead by this, because S-Versions can easily be veried by corrent measurements.

Also read (and try...) the below experiment, it will open your eyes.


Some examples:

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.

Experiment: Take a C3g, which is reasonably good, but you sacrifice it for this experiment. Suppose Gm=95% amd Ia =12.5mA. So slightly used. Now knock the tube on the table. Such that you deformed the inner structure, but not damaged 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 theser react to this very sensitive. It is crazy, but they greatly "improve" test results on most tube testers this way. So allways look at plate current too, and nobody can fool you.


Build a pentode pre-amplifier stage with C3m

Take a normal Cathode resistor stage, and use this table.
More details in the official Telefunken datasheet.

Good applications for these tubes are:

  • Inside active microphones.
  • Very Stabile instruments, with DC coupling, like stabilized power supply.



  • Low microphonics applications. C3g mainly (frame grid)
  • triode connected, or pentode connected
  • driver or preamplifier stages. C3m for high gain.
  • phono amplifiers.  Searching for the holy grail? The C3g or C3m! Finally..  LOW NOISE!

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.

Yamamoto A09 amplifier with C3m (Metal cap removed by Yamamoto)


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)

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