In 1976 a Mig 25 fighter jet landed in Hokkaido in Japan, it was flown by a soviet military pilot that wanted to defect to the west. This was a real field day for western miltary people as not so much was known about the MIG 25 and the plane was completely dismantled.

Two things startled the military analysts in the west, the first was that the plane was built using a lot of high tension steel, it was earlier thought that the MIG 25 was built in Titanium to withstand the heat that is generated by flying at mach 3 that the plane was capable of, that it was built in steel made it heavy so the performance was actually less than expected.

The second thing that surprised people in the west was that electronic tubes was used in some parts of the radio equipment. After a while it was understood that the tubes where used so the plane should be able to withstand EMP. An EMP, (Electro Magnetic Pulse) is generated by the detonation of a large nuclear bomb in the atmosphere, the electric field so generated destroys most solid state electronics and EMP is thought to be used as a possible electronic warfare measure.

In the power supply of the main radio unit of the MIG 25 a previously unknown triode tube was used, the Soviet military designation was 6C33C and it was a voltage regulator triode. The data of this tube was much better than anything made before in the west and SanEi of Japan imported some 6C33C tubes for audio application in 1997, renaming them EC33C for the Japanese consumer. Mr. Dakesue in Japan was the first person to design a 6C33C-B OTL power amplifier circuits in 1977.

The main characteristics of the 6C33C is that it has very high transconductance, high current capability and low internal resistance and also as the tubes where intended for use in miltary aircraft they are mechanically very robust. The nearest western equivalent is 6336 but data of 6C33C is better.

Tube 6336A 6C33C
Gm 27 mA/V 40 mA/V
Rp 100 ohm 80 ohm
Max continuous anode current 200 mA 600 mA
Heater current 5A 6.4A
Data for the 6336A is for both sections in parallel

6C33C in OTL amplifiers

A tube used in OTL amplifiers should ideally have high GM, low resistance and be able to withstand high continuous anode current. I think the best tube today is the 6C33C but also the penthode EL519 that was manufactured for use in TV sets as vertical output stage can be used if connected as a triode, (I believe the 6C33C is the more linear of the two). As I had some experience of the 6C33C in my SE amplifier I decided to use the same tubes in my OTL amp.

Some special characteristics of 6C33C

Some people have reported problems when using the 6C33C tube in OTL amplifiers. The problem has usually been varying characteristics between different tubes which make it difficult to find mathched pairs.

The reason that these tubes show varying characteristics is NOT because of bad quality manufacturing. The reason is that as the 6C33C has very high transconductance the internal spacing between the electrodes is very small and therefore even a tiny difference in the spacing will have a big effect on parameters, especially on gm.

gm for a plane electrode triode is equal to:

gm = 3.5 * SQRT(Eg + Ep/)/(dc^3/4 + ((dc + dp)^3/4)/)^3/4

Eg is the grid voltage
Ep is the plate voltage
is the amplification factor
dc is the cathode to grid distance
dp is the grid to plate distance

It can be seen from the above formula that if a tube is to have high transconductance the distance between grid and plate also have to be small. Such a tube will of course be more sensitive to any absolute difference in this distance and the transconductance will therefore vary between different tubes. In other words as the grid plate distance is only ~0.2mm in a 6C33C it is more sensitive for a variation of +- 0.05mm than a tube that have 1 mm grid to plate distance. Even so the tolerance in gm of 6C33C is rather small ~10% at higher currents, this shows that it is a real high quality tube.

Dynamic matching is more important than static!
Or static matching is almost un-necessary! The grid voltage needed for a specific anode current for the 6C33C varies quite a lot between tubes, this is because of the high transconductance and the above mentioned tolerances. I have found that it is not necessary to select tubes that needs exactly the same grid voltage in an OTL amplifier. The reason is that it is quite easy to adjust the static grid voltage in the amplifier so the current will be the same in the tubes. What is far more important is that the dynamic characteristics is the same, that is what increase in voltage is necessary for a given increase in anode current. it is not un normal to see a difference in grid voltage of 10% between two tubes for a given current if the anode current is fairly low, this difference is not important if the amplifier have individual grid bias adjustment.

6C33C tube, note the sturdy design, there are supporting rods going trough the whole tube and fastened at the top.

Data of 6C33C
Max data
Max anode voltage 400
Anode dissipation 60 W
continuous cathode current 600 mA
Overall length 133 mm max
Diameter max 64 mm

Typical data
Anode voltage 120
Anode current 550 mA
Transconductance, gm 40 mA/V
Anode resistance 80 ohm
Grid voltage -19 V
Heater voltage 6.3 V
Heater current 6.4 A

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Created on ... december 24, 2000