Tube of the Month : The 955

Hi!

Here is another Acorn tube, the 955.

Last month we had with the 957 the second acorn tube in the tube of the month series. So far only directly heated acorn tubes had been shown. The 955 is an indirectly heated variant.

The 955 is an indirectly heated triode with an amplification factor of 25. It has a plate resistance of about 13kOhms which results in a transconductance of 2000 micromohs. The heater is operated at 6.3V and consumes 150mA. The complete set of technical parameters can be looked up in the RCA data sheet. The 955 is another rather strange tube which is long forgotten, especially since the acorn type tubes were never widely accepted. They probably mostly found some use in military applications in RF equipment. The difficult to find sockets make these tubes quite uninteresting for current day amplifier builders. The 955 is the last of the 3 different acorn type tubes I have in stock. So let's have a look at it in detail.

As always we start with the plate curves:

And compare them to actual curves from a tube sample:

Typical curves as can be expected from an indirectly heated triode.

Now some photos of my small stash of different 955 triodes, starting with RCA.

RCA 955 in military packaging:

Another one:

955 made by Hytron:

Tung-Sol:

Arcturus 955:

The box is still completely sealed, so I am not opening it.

A Raytheon 955:

This one came in a neutral box without any manufacturer marking:

Sonotone:

And lastly some photos of a 955 in operation.

What a beautiful little gem!


Best regards

Thomas

The Molybdenum Plate ELROG ER300B is coming!

Hi!

Here a sneak preview of our new 300B variant, a molybdenum plate version of the ER300B.

We just got prototypes of the new molybdenum plate 300B freshly off the vacuum pump.

This new 300B will not replace the existing ER300B but will be a premium version as we have done with the ER242 a d ER284 which are molybdenum plate equivalents of the ER211 and ER845.

Technical specs remain the same the only difference is the use of molybdenum as plate material rather than nickel.

Since the molybdenum as plate material makes a remarkable difference in the ER242 and ER284 it was a logical step to introduce a molybdenum 300B as well.

We still need to do some qualification and productisation steps before the tube can be released. But after I listened to the prototypes I had the urge to share the news. The molybdenum enhances the qualities of our ER300B which is praised for its airy and high resolution sound with a solid bass and strong tone colours.

I am very excited to share the news about this new tube development. An announcement will follow when the tube is ready to order and ship

Best regards

Thomas

Tube of the Month : The 957

Hi!

This month I would like to show a tube which most of you probably never heard of : The 957.

The 957 is an acorn tube. This series of tubes is named like that due to their shape which resembles that of an acorn.

Acorn tubes are baseless. The connection pins come out of the glass on the side of the bulb. This and their small size minimised parasitic capacitances. The intention was to develop a series of tubes specifically suited for high frequency applications. The 957 is a directly heated small signal tube, the sister of the 958A which I already presented in 2014. The 957 has the same very low filament voltage of 1.25V as the 958A but needs even less current. The filament only consumes 50mA which made this tube especially suitable for portable, battery operated applications. Both tubes have very similar amplification factors. The 958A has a mu of 12 and the 957 has a mu of 13.5. But the low filament current of the latter comes at a cost which means about half the transconductance and twice the plate resistance compared to the 958A. For the complete technical specs refer to the RCA data sheet. The 957 would be usable in resistance coupled audio amplifier stages. Needless to say that DC operation of the filament would be mandatory for hum free operation. It anyways was only specified for DC filament voltage.

As can be expected from a directly heated triode, linearity is very good. Note that the data sheet shows plate curves also for positive grid volts. Here a set of plate curves taken from a tube with the tracer:

A 957 made by RCA fro the military:

Another military 957 made by a company called Sonotone Corp.

Let's see more of the construction by dissecting this tube:

The electrode system is tiny:

The filament consists of a single wire held on the top by the rod at the left side:

The delicate structure is easily damaged. Unfortunately I completely deformed the grid.

Here the rod which held the filament with the filament string still attached.

As expected this tiny filament shows only a very dull glow in operation, which can only be seen from the bottom.

Microelectronics from the 1940ies!

Best regards

Thomas