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Thursday, September 29, 2022

Tube of the Month : The 845


This month I would like to present a tube which is long overdue to get a spot in the Tube of the Month series. Meet the mighty 845.

The 845 is a directly heated transmitting triode designed for use as audio frequency modulator or amplifier service.

It uses a thoriated tungsten filament which operates at 10V and needs 3.25A of current. Like all thoriated tungsten filaments it lights up in a bright almost white color when in use. The 845 shares the same filament, bulb size and base with the 211. Both use a 4 pin Jumbo bayonet style base. But that is where the similarity ends. The 845 has an amplification factor of 5.3 which comes with 1700 Ohms plate resistance while the mu of the 211 is 12 at 3900 Ohms rp. In Class A1 single ended use about 25W can be obtained from a 845. But to get this the tube needs 1250V at the plate and a driver which is capable to provide a voltage swing of almost 400V peak to peak. These requirements make it quite a challenge to use the 845 as output tube. The output transformers not only need to be able to handle the power delivery but need to be isolated to withstand the high supply voltage. This voltage insulation requirement doubles since the plate of the 845 will swing to almost twice the B+ voltage when driven to full power. That is why suitable output transformers are bulky, heavy and expensive. When you look at the data sheet you will find one of the reasons (besides it's power capability) why this tube is quite popular. It has amazingly linear plate curves:

And this quality holds up in real life, here the plate curves of an actual tube:

I remember the first time when I read about the 845 in the summer 1992 issue of Sound Practices. Back then this magazine occasionally had an article named 'Meet the tube' which was the role model for my Tube of the Month series.

Little had I known back then that more than 20 years later I would be a manufacturer of 845 tubes myself.

When I started to use these big transmitting tubes I was more drawn to the 211 and therefore never bought many old production 845 tubes and since I have a tube factory at hand I stopped buying old tubes altogether. So all I can show besides the ELROG variants of this tube are some United Electron 845W.

They are packed in beautiful oversized cartons.

The United Electron 845 in all it's glory.

The UE 845 has some distinct features not seen in other tubes.

A little glass stem protrudes from the tube into the tube which is used to mechanically hold the internal structure at the top.

Unlike most tubes which have the getter flashed to part of the glass bulb. In these it is contained in a small cylinder.

No getter mirror anywhere on the glass.

Here we see the connection from the plate to the base pin, not your typical audiophile wire.

Also note the use of ceramic instead of mica discs.

The base:

Next I'd like to show the ELROG 845 made by Deutsche Elektronenröhren Manufaktur GmbH.

They always come as matched pairs.

The specs match those of the old production 845 tubes.

Each tube has a serial number:

Aluminum / Teflon base:

Next a few photos from the tube factory to show some of the production steps.

Sealing the glass:

Two video clips showing the sealing process:

The freshly sealed tube on the vacuum pump:

The plate gets heated through RF induction for outgassing:

Like old production 845s made by RCA, General Electric, Westinghouse and the United Electric the ELROG ER845 has a graphite plate. Western Electric manufactured a tube with the same specs as a 845 but used molybdenum as the plate material. They called it 284.

In appreciation of the Western Electric 284 we named our molybdenum plate version ER284.

It has the same specs as the ER845 and is fully interchangeable with any 845.

And now some photos of the tubes in operation. Starting with the ER845 in one of my amplifiers:

Here the tube is paired with an ER887 which we specifically developed as driver tube for the ER845.

Some close ups:

The ER284 gives more of a light show due to the slotted molybdenum plate which lets more light from the filament get out:

And this is then topped after the plate voltage is applied and the tube is at operating temperature.

In normal operation the plates show an orange glow.

And lastly some shots of the United Electric 845 in operation.

That is all I have to show about 845 tubes. If you have some nice photos of 845s from other manufacturers and want to see them here, please email them to me and I will add them to this post.

Best regards