Harries August 1935

(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.)  
 
 
 

nbodying Interesting

'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
11 critical distance " special characteristics are obtained.

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‑

tained.

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

is prevented.

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

straight.

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
exactly equivalent to the new valves shown in Fig. 2.

 

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‑

evenly.

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
with input grid volts for a triode, a pentode
and a Harries valve. The curves correspond
to the load values marked.

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

before.

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

input/output characteristic.

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,

London, E.C.r.

 

Ċ
Michael Saunby,
28 Jul 2008, 11:25
Ċ
Michael Saunby,
28 Jul 2008, 11:25
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