Projection engineering (Jan-Dec 1931)
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DECEMBER, 193
Page 25
from the curve gives a plate current Ip of 16 ma.
From the family of curves it will be seen that the amplification factor (/*,„) of this tube is approximately seven. Since we are using a load impedance equal to the tube impedance, (10) can be written.
\i = Y* |x,„ = 3.5 using (9)
3 Ep -f \i Eg Es =
(9)
900 — 88
406v.
Through the points Ep =z 300v, Eg = — 25v, and Es = 406v, draw the load line cutting the Eg = Ov curve at Ep = 193v, and the Eg = — 50v (the limits of assumed swing), at Ep = 385v. The plate voltage swing is thus 192v. The impedance of the load can now be obtained from (5A).
Ip .016
: = ; ZL = 6600co
1
Es-Ep
106
(10)
The amplification factor is actually 3.8, which is in fairly close agreement with the estimated value 3.5.
The plate impedance at the point Eg = Ov, Ep = 193v is approximately 3,000 ohms and at the point Eg = — 50v, Ep = 385v is approximately 9,900 ohms, the calculated value 6,600 ohms is therefore a mean value.
It is desirable, however, for the purposes of lessening harmonic distortion, to make the load impedance double that of the tube since, while this lessens the output but slightly, it has a relatively great effect on the elimination of the harmonic content.
Correction can be easily made for this from the above by allowing a sufficient increase in Es, to account for the drop in the extra impedance.
In the present case let it be desired that the load impedance be double that of the tube. From (3) and (5),
Z,
Es — Ep Ip
(ID
and the potential drop (E-) over this is L
EL = ZJp = Es — Ep (12)
The total supply voltage (Ers) required is then the sum of (9) and (10).
3 Ep + n Eg
Ers = ( (Es — Ep)
2
(13) which may be written
Ers = 2 Ep + ^ Eg (13A)
and the load impedance (ZL2) in this case is
ZL,
Ers — Ep Ip
Fig 1.
Functional characteristics
of the 205-D tubes.
iv> H
400 -360
Assume the same values as in examples 1, i.e., Ep = 300v. Eg = — 25v.
The amplification constant (ft) is now from (10). ? [X = — pu = 4.6 (15)
3
and the supply voltage Ers,
Ers = 2 Ep 4 I1 E« = 600 — 115 = 485v.
(16)
The impedance,
208 Zr,. =
(14)
: ll,500w (17) 0.025
Draw the new load line through the points Ep = 300v, Eg = — 25v, and Es = 485v. The Ip-Ep curve, Eg = Ov is now cut at Ep = 176v. and the Eg = — 50v. curve at Ep = 41 2v.
The plate voltage swing is thus 236v, the increase being due, of course, to the increased amplification constant with increased load.
The amplification is thus 4.7.
The impedance at Eg z= Ov, Ep =r 176v is approximately 3,200 ohms and at Eg = — 50v, Ep = 412v is approximately 7,600 ohms. ' (18)
Double these values are 6,400 ohms and 15,200 ohms, respectively, so that the calculated value of impedance lies closely enough to a mean between these two values.
It is interesting to note the variation in output power and in harmonic content by the increase in load impedance.
In the case of the 6,600 ohm load we have for the output power (Pi).
Ip max — min. Ep max — min.
Pi =
8 29.25 X 192
= = 703 milliwatts
8
(19) and harmonic content,
5^2 (Ip max 4 Ip min) — Ipo IT = —
Ip max — Ip min. = .055 = 5.5% (20)
In the case of ZL2= 11,500 ohms we have
20.35 X 236
P., = = 600 milliwatts
8
(21)
= .0283 = 2.83%
(22)
Thus, while the power is decreased only 14.6% of the maximum, the harmonic content is decreased by 48.5%. This power loss corresponds to a change of less than 2 db, an amount hardly appreciable to the ear.
Ho =
..1/5
20.35
THE SKATING RINK MUSIC PROBLEM
(Concluded from page 12)
the manufacturer who would employ a band — it need not be a large one or a high-priced one — but one which would play and record all the new hits and the old favorites in a manner that intended for skating only. That is, all of the music to have a beat of 56 per minute and this tempo emphasized, so that a dozen or two of these records could be played automatically and all have the proper cadence. Rinks would begin installing sound equipment immediately and they and the many who already have equipment would be each an outlet for six to twelve or even more records a week.
If the manufacturers of calliopes and mechanical music boxes can go to the expense of manufacturing paper rolls for skating only, it is reasonable to assume that the market would be even better for records made for this purpose, for the days of the calliope and the music box are over. The era of electrical sound equipment is here to stav.