(This is what the OCR produced, in time I'll edit this and insert the figures. For now I recommend downloading the PDFs rather than trying to make sense of this text.)
'rinciples in Design
By J. H. OWEN HARRIES
IN this article the designer describes the effect of applying a new principle to the construction of a power valve which is that if the
anode of a multigrid valve is spaced from the outer grid at a certain
NEW principle is employed in the j design of multigrid power out‑
put valves which have just been introduced.
The electrodes of one of the new valves are illustrated in Fig. i, the anode being cut away to show the interior. The electrode assembly is very simple; it consists merely of a cathode, an anode, and two grids. The noveltylies in the dimensions
of the space between the anode and the
outer grid. It has been found that if the anode of a multigrid valve is spaced from the outer grid at a certain "critical dis‑
tance," special characteristics are ob‑
Spacing the anode at the critical distance avoids the necessity for a suppressor grid or any equivalent structure, such as
is used in pentode valves. The undesired
retrograde passage of secondary radiation
Fig. 2 (a) and (b) show the characteristics of two of the new valves, and, opposite these characteristics, in Fig. 3 (a) and (b) are those of precisely equivalent commercial pentode valves.
The "turnover" or "knee" of the newvalves is sharper and farther to the left
than with the pentodes. Therefore the power output is greater, and the working dynamic input / output characteristics (shown dotted) do not curve over at the
top, as with the pentodes, but remain
The most important difference is that, in consequence, the individual grid voltage lines on the anode characteristics of the new valves are straight and of constant slope from the knee upwards. Those of the pentodes are continuously curved, and the anode impedance and mutual conductance change substantially with the working anode voltage. In other words, the
mutual conductances of the pentodes fall
off at the left-hand end of the load line,
whereas the mutual conductance is con‑
stant in the case of the new valves. As. the amplified wave swings towards zero grid voltage it will be distorted in the case
Fig. 2.—The characteristics of the new two-volt battery operated
valve and of the new mains type valve.
Fig. 3. The characteristics of a two-volt pentode and a mains pentode
A New Power Output Valve
of the pentode, and any overtone present
in a speech or music wave will be even more seriously distorted than the main wave. Over this part of the swing, waves of different amplitude, and different parts of the same wave, will be amplified un‑
The impedances of the new valves are
lower than those of the equivalent pentodes. That of the pentode in Fig. 3 (b)
Fig. q.—The variation of mutual conductance
is about 50,000 ohms, whilst that of the Barnesi valve in-Fig. 2 (b) is about r6,000 ohms. In practice, the Ioud speakers at present available work best with a fairly low impedance. This is probably because a low impedance reduces the effects of
capacitative feed-back at high audio fre‑
quencies. The mutual conductances of the new valves are appreciably higher than those of the old. Therefore the lower im‑
pedance`does not reduce the sensitivity.
The new valves use the same cathode size
and cathode to control grid spacings as
Comparison of Power Outputs
The power output of the new two-volt battery valve (HY22o) Fig. 2 (a) is 58o mW., and that of the corresponding pentode of Fig. 3 (a) is 400 mW. Both were measured at the same HT voltage of 15o, and with the same anode current and grid bias. The increase is i.45 times. The power output of the new valve (ACHY) Fig. 2 (b) is 2,400 mW. The sensitivity, in milliwatts per volt squared input, is 48. The power output of the equivalent mains pentode of Fig. 3 (b) is 2,000 mW., and the sensitivity is 20 mW. The most important improvement is in quality.
It is accepted that a triode valve gives
an extremely satisfactory distortion level. Unfortunately, however; it is so insensitive that the extra stage of amplification it usually requires renders its use uneconomic in the great majority of receivers. In consequence, nearly all commercial
receivers use pentode output valves. For
instance, a typical mains triode needs 34 volts input to give two watts output. A
pentode needs only if volts to give two
watts or so ; but pentodes, whilst possessing this high sensitivity, produce a peculiar so-called "pentode distortion." This distortion is due to the reduction of mutual
conductance at both ends of the dynamic
The variation of mutual conductance over the full swing of grid volts is shown for a triode in Fig. 4 (a) ; that for a typical
pentode in Fig. 4 (b); and that for one of
the new valves in Fig. 4 (c). All are typical results after tests on many valves. The mutual conductance of the triode is almost constant, and the overtones of a complex speech wave are distorted but little. The pentode characteristics are so curved at all working loads that frequency-doubling modulation distortion of overtones is produced. The result is a '' rough-toned reproduction. The characteristics of the new valve are like those of a triode for normal loads and drives, and the distortion is of the same kind. and degree,. and is therefore as unobjectionable as that
The feature of the valve is the abnormal
spacing between the anode and outer grid.
of a triode. The sensitivity is of the same order as that of the pentode.
Because of the reduction of capacitative feed-back and the fact that high notes are not distorted, the conventional resistance and capacity filter circuit across theloud speaker may be omitted. This will
give a pleasant brilliance of tone and in‑
creased apparent loudness. Preferably the load is then lowered below the rated optimum value by an amount best found by trial with the loud speaker in use.
The load presented by a loud speaker to a valve is not constant, but, if not overdriven, the new valve will accommodate this variation within the -straight part of the characteristics. In operation, distortion due to toohigh a load, and/or too high a drive, may be detected by watching the movement of a milliammeter in the anode circuit of the valve. A fall during the reception of loud passages indicates overtone distortion with any valve. As a test, the fall of anode current was observed with a sine wave drive for various power output values for both a typical commercial pentode and for the new valve of Fig. 2 (b). The same load line and operating conditions were used. With a given percentage of full drive the pentode gave two watts on a dynamometer, and the anode current dropped by 15 per cent. The Barnesi valve needed less drive to give two watts, and the fall in anode current was 5 per cent. The average domestic user works well below the level of two steady watts of output.
The new principle therefore results in a high sensitivity combined with the type and degree of distortion of a correctly
adjusted triode, provided only that the valve is chosen to be sufficiently large to
give the desired loudness. The valves discussed are being manu‑
factured by the High Vacuum Valve
Company, of iii, Farringdon Road,