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Sunday, July 22, 2012

Tube of the Month : The 211 / VT4C


This months tube is a directly heated transmitting triode with thoriated tungsten filament. The famous 211, also known under it's military designation VT4C.

This is a very early tube development as the 4 in the VT designation tells. Only two VT numbers preceded this one, the VT1 and VT2. A VT3 was never assigned to my knowledge. It was widely used in the C revision. I never came across a VT4A or VT4B, not sure if these got manufactutered in significant numbers.

The 211 was developed for transmitters for the AF modulator stages where it mainly was used in Class B service. It was also used for Class C RF amplification. And of course it is also intended to be used in Class A amplifiers both single ended and push pull. A Class B Push Pull amp with a pair of these can deliver 260W. In single ended service up to 20W can be achieved from a tube in Class A1. With an appropriate driver whith low output impedance and the capability to sink the grid current which occurs in Class A2, up to 50W are possible. However this will be associated with significant distortion. A good compromise is an amp which mostly runs in Class A1 and which can drive the tubes grid slightly into the positive region. This way 25W can be easily obtained. An example for such a circuit can be found here. Photos of a finished amp based on this schematic are here. The tube has a 4 pin Jumbo base. The pinout is shown on the left. Being designed for AF amplification the tube is remarkably linear, which allows it to be used without any feedback. The linearity of the tube can be esily seen in the set of plate curves from the datasheet:

This is an impressive set of plate curves, but how well does this hold up to reality? Below scope screen shots of actual 211 curves taken with a curve tracer:

My curve tracer only goes up to about 400V on the plates, so this is just a small area of the curves. As can be seen, the datasheet doesn't exagerate. These are some very linear curves. The set on the left is taken with 5V per grid step, on the right 2V per step. The X-axis is 50V/division and the Y-axis 2.5mA/division.

The 211 is a serious tube, not only due to it's physical size. It is about 20cm high with a diameter of 6cm. It requires high plate voltages. To get full power it needs 1250V. It is also demanding in terms of filament power: 10V at 3,25A. The maximum plate dissipation is 75W. With the 32.5W filament power this means it will produce over 100W of heat. The filament is of the thoriated tungsten type. These operate at temperatures of about 2000 Kelvin, about 1000 degrees hotter than oxide coated filaments. This produces a bright white glow, rather than the dull orange known form more conventional tubes. This is also why such tubes are called bright emitter triodes. The 211 is not easy to use in an audio amp due to these properties. In order to sound best it needs very clean filament voltage. Preferably DC. While AC operation is possible, controlling the residual hum becomes an issue due to the high filament voltage. It needs 150V peak to peak drive voltage which should be low distortion and low impedance for the best possible sound quality. Grid current can kick in way before the grid crosses 0V. If the driver tube is not up to the task, the sound of the amp will be dominated by the driver and not by the output tube. The plate resistance is rather high at 3600 Ohms, which requires a high impedance output transformer. 10k are quite common as for example with the excellent Tango X10S. Higher load impedances are better. This can be obtained with the X10S for example by using the 4 Ohm tap with 8 Ohm speakers. Another alterantive is the Lundahl LL1691B a 20kOhm transformer which is used in the circuit linked above. If all the difficulties associated to this tube are properly adressed, it will sound wonderful. very detailed and musical with enough power to also drive less efficient speakers. I have tried the 211 in various configurations. So far it sounded best, when dirven by a second 211!

The 211 was produced by various manufacturers.While the 211 is also available from current manufacturers, mostly from China, I prefer old stock american made 211s. They are far more rugged and reliable and sound vastly better. Availability of old stock tubes is still quite good today. Most commonly from General Electric. Here a collection of GE 211s:

These tubes all have the same internal construction but have slightly different markings.

Here a pair of General Electrics in brown military packaging:

They also came in blue and orange boxes. Often the spec sheet was included. Note also the soft wrapping which ensured safe storage and transport:

The plate structure of the GE:

The GE 211 is easily recognizable by the cross shaped ceramic supports:

Here a pair of 211s from RCA:

RCA 211s are much more diffucult to find. They have a different internal construction. Here a close up showing the ceramic supports:

The plate structure:

A close up of the electrode assembly at the bottom:

The base:

A pair of Amperex 211s:

The Amperex 211s have brass bases:

Plate structure:

The bottom part of the assembly:

The 211 from United Electric:

The ceramic supports:

The base:

Thanks a lot to Jürgen, who provided the photos of his United electric 211!

As mentioned above the tube needs the 4-Pin Jumbo socket. While such sockets are available from current manufacture, again the old stock sockets are superior. Most commonly known is the 211-socket from Johnson:

A 211 in a socket:

Now let's have a closer look at the internals, by dissecting a dead tube!

A 211 with the glass dome removed:

The glass is quite thick and rugged:

With the glass removed it is easier to take shots of details of the construction. Here some close ups showing the grid:

Some more details of the construction:

The plate removed from the assembly:

The grid assembly, part of the filament is still there, it broke when the filament tension strings got removed:

Close ups of the grid, showing how it's welded to the support rods:

The filament tension springs:

A closeup of the hook whoch holds the filament which is arranged in an 'M' shape:

A segment of the filament:

A closeup:

A section of the filament which was held by the springs:

The filament lit up, this photo shows how the filament is held by the tension springs:

A 211 in all it's glory, shot in the dark:

I have another pair of 211 monoblocks in the making. Here a photo of the metal plates rigged up for assembly:

These will be based on the same circuit as linked above. Stay tuned for an upcoming article about these!

Best regards