Tube of the Month : The 849

Hi!

This month I am presenting something very different. A large air cooled transmitting triode. Meet the mighty 849.

Like many  tube amplifier designers at some point I was obsessed with large directly heated triodes and collected some of those beasts.

The 849 is one of them. It is a thoriated tungsten directly heated triode designed for audio amplifier and modulator use as well for RF amplification. It measures a massive 36cm in length and 10cm in diameter. Dimensions and connection terminals are shown in the drawing on the left. Connections are made to the tube from both ends. There is a large plate cap on one end and a terminal with 2 massive filament pins and a blade type connection for the grid between them. The 849 has a maximum plate dissipation of 300W. This comes with a relatively high amplification factor of 19 and a plate resistance of about 3 kOhms. The Amperex data sheet lists single ended Class A operating points up to 100W out of one tube! That should be enough to drive even some of the more power hungry speakers. What a beast! Of course such a power rating is not easily obtained. A DC supply voltage of 3kV would be needed and the filament consumes 11V @ 5A. No off the shelf output transformer is available for such a tube and would need to be custom wound. An amplifier with this tube would be a nice room heater. 300W plate dissipation plus 55W filament power equals to 355W of heat just from the tube. That's 700W for stereo. Add driver and rectifier tubes and losses in the power supply and the heat will be beyond 1kW. Maybe something for the winter time. But I bet it wold sound simply stunning. Have a look at the evenly spaced plate curves:

My curve tracer only operates up to 400V. But this limited region of the operating area of the tube looks great already:

Now let's see some tubes, here is a Amperex 849:

What a majestic tube!

Check out the internal construction details:

Unlike other tubes the 849 has a mesh like grid.

The plate structure is massive

Which it has to be to be able to dissipate 300W

The filament and grid terminals:

The tube is  normally mounted with this end up.

And the plate connection toward the bottom:

These are the original sockets for this tube:

Amperex logo and tube type designation:

Some more detail shots of the internals:

Close up to the grid:

Mounting on the filament/grid side:

Westinghouse 849:

Close Ups:

United Electronics 849:

Sylvania 849:

An interesting feature on this one, in addition to the common barium getter which leaves a silver mirror area on the glass, this tube also had a zirconium getter which leaves no visible mark after activation:

On the next photo we see the barium getter on the top and the zirconium getter was in the little mesh pouch on the bottom:

The construction of these tubes is simply stunning:

My collection of 849s:

Since these photos cannot really give a real impression of the size, here a 849 besides a 211:

Now a few photos of a 849 with the filament lit up:

These photos are a bit underexposed. In real life the filament is brighter.

The filaments create a hot fire inside the plate.

I hope you liked the presentation of this rather unusual triode.


Best regards

Thomas