The Beam Tetrode
The beam tetrode was developed as manufacturers were looking for a way to make audio output valves that would not infringe the Mullard (Philips) patent for the pentode. In the UK two EMI engineers developed and patented in 1933 the Beam Tetrode. MOV owned half of EMI but considered the design too difficult to make. MOV also had an exchange agreement with RCA of the USA and the development passed to America.
A beam tetrode employs principles not found in other types of valves in that
the electron stream from the cathode is focused towards the anode. The
control grid and the screen grid are made with the same winding pitch and
they are assembled in the valve so that the turns in each grid are in optical
alignment.
The effect of the grid and screen turns being in line is
to reduce the screen current compared with a non-beam construction. For
example, in a pentode of ordinary construction the screen current is about 20
per cent of the anode current, whereas in a beam valve the figure is 5-10 per
cent. The other feature was two shaped metal plates that ran the length of the grids and was placed outside the grid support rods but within the anode cylinder. The effect of these plates was to confine the electron flow to the path through the grid wires by forming an electrostatic field when connected to a low voltage. This had the effect of repelling secondary electrons back to the anode and by not having the secondary electrons reach the screen grid, the kink in the Tetrode curve was eliminated. The Kinkless Tetrode was born.
RCA did have facilities to successfully translate the design into a working valve. The result was the 6L6. It was more efficient than an equivalent Pentode, due to lower screen dissipation, hapilly it also produced one third less third harmonic distortion. The ear is reasonably tollerant of second harmonic distortion up to about 5% but not third harmonic distortion which must be reduced to very low levels for qood quality sound. The Triode produces inherently lower third harmonic distortion than other valve types. The Pentode produces significant amounts.
Within a few years most manufacturers had introduced Beam Tetrodes for audio output service. Mullard remained loyal to the Pentode. The top honours went to MOV for their KT range, especially the KT66 and later the classic KT88.
Beam valves were originally developed for use as audio-frequency output
valves, but the principle was also applied to many types of radio-frequency
tetrodes both for receiving and transmitting. Their superiority over pentodes
for AF output is due to the fact that the distortion is caused mainly by the
second harmonic and only very slightly by the third harmonic, which is the
converse of the result obtained with a pentode. Two such valves used in
push-pull give a relatively large output with small harmonic distortion because
the second harmonic tends to cancel out with push-pull connection.
The characteristic curves of a beam valve and a pentode of equivalent
size. The line AB is a load line drawn on the curve. It will be seen that the line
extends farther to the left before it cuts the zero grid-volts curve than is the
case with the pentode; this indicates a greater power output because the
power output is proportional to the product of the change in anode voltage
between points A and B on the horizontal axis and the change of anode
current between points A and B on the vertical axis.
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