Tube of the Month : The 800

Hi!

This months tube is a directly heated transmitting triode, from the RCA 800 series. It is actually the very first tube in that series, the 800 itself.

I have never done anything with that tube and most likely never will. But it is an interesting triode nonetheless and certainly usable for audio applications.

Like most transmitting triodes, it uses a thoriated tungsten filament. In this case with 7.5V and 3.1A. So rather hefty requirements not far off from the 211 or 845. The base is UX4 but only 2 of the pins are used. The filament is supplied through the base. There are two caps on top of the tube which give it a very distinctive look. One is for the plate connection and the other for the grid. This arrangement keeps inter electrode capacitances low which helps for RF applications. The tubes was intended for both RF and audio frequencies. The data sheet lists operating conditions for use as Class AB power amplifier and modulator and for Class C RF power amplifier service. There is a sample circuit of a PP amp with two 800 as output tubes driven by a push pull pair of 45s. The tube was mainly designed for grid current operation but like most such tubes could be used in Class A1 as well.
However no parameters are given in the data sheet for Class A1 operation so these would need to be characterised. I haven't looked at that in detail but probably an output transformer with rather high primary impedance would be needed. Output power in SE Class A1 is probably around 10W. But then why not use this tube as intended and drive it in Class A2 with a suitable driver or even AB as suggested in the data sheet for a healthy 100W of thoriated tungsten directly heated output power. An amp with 2 or even 4 of these with their cool cap connections would certainly look great. May be some day I will find the time to look into this in more detail. Here a quick shot of the plate curves in Class A1 region which look as good as you can expect from a directly heated triode:

Some photos of the few 800 tubes I have:

A nice sample in big RCA box. The tube is padded in soft card board inside:

Some more photos:

The base:

Here the grid is visible:

Details of the box:

This tube also carries the military designation VT-64.

Another 800:

This is a RCA de Forest.

Close up of the base:

Grey coated plate on this one:

The top:

Here the connections from the caps to the grid (right) and plate (left) can be seen:

Closeup of the grid connection:

Let's have a look at the tube in operation:

The filament gives of a bright almost white glow. When turning down the filament voltage some details become visible:

Close Up:

The next photo gives a better view to the filament:

A close up shows that the filament is actually a spiral:

Another view:

Turning down the filament voltage further makes it easier to see more details:

A very interesting filament structure which allows a nice cylindrical arrangement of all electrodes.

A great tube! If you have some actual experience with this triode in an audio amp, let me know!

Best regards

Thomas