B.B.C. Research Department. REPORT No. L.003/3

Serial No. 1937/5.

18th February, 1937.

Work carried out by: J. Story.

Drawing Nos. L.003.1 to L.003.5

SUMMARY.

MODIFICATIONS TO OBA/7 AMPLIFIER.

In order to increase the gain from 70/72 db.v. to 85/87 db.v. an OBA/7 amplifier has. been converted to a four stage amplifier utilising the feed-back principle. The same principle has been employed to provide the equalisation at the upper frequencies for the ribbon microphone. This form of equalisation has the advantage that there is no reduction of gain at a frequency of 1000 c/s. when the equalisation is inserted.

This report describes the circuit and shows the result of tests upon the modified amplifier.

The modifications are not being carried out at present, pending further experience with the amplifier in service.

INTRODUCTION.

In Report No. L.003/2 the reasons for the necessity of

increasing the gain of the OBA/7 amplifier are fully set out. In the

same report, proposals for increasing the gain to the required 85 db.v.,

utilising a three stage circuit, were also dealt with.

11~11~llilijllill~~111111 300008848 V

-2-

Attempts to carry out those proposals failed, partly owing to

the "pongll of the micromesh pentode valve which it was intended to use in

the first stage, but mainly because the variation of the frequency

characteristic of the high ratio step-up input transformer with input

conditions proved to be more than could possibly be tolerated.

It was therefore decided to abandon attempts to increase the

gain of the amplifier wi thcmt an increase of the number of stage s, and

attention was turned to the development of a suitable circuit employing

four stages.

The advantages of a feed-back circuit appeared very attractive,

and experiments were commenced using this principle.

TEE CIRCUIT.

General. The Circuit finally adopted was that shown in Fig. 1. It

should be noted that the modifications to the original circuit have been

made only between the secondary of the input transformer and the grid of

the AC/EL valve in the third stage. It is o~ly the first two stages,

then, that warrant description.

Neglecting for the moment the feed-back and equalisation

circuits, the simplified circuit for the first two stages becomes that

shown in Fig. 2a. The first stage consists of an MH.40 valve, selected

for its comparative freedom from noise, resistance capacity coupled to

the second stage in the conventional manner. Bias for this valve is

obtained by the insertion of a total of 1,100 ohms in the cathode lead

-3-

in the usual way, but this resistance is split for feed-back and equal­

isation purposes, and is not shunted by an electrolytic condenser in the

normal manner.

The second stage also consists of an MH.40 valve coupled to

the third stage by a resistance-capacity filter circuit. The object of

this form of coupling is to prevent instability at a very low frequency

when the amplifier derives its H.T. supply from the rotary transformer

or from any form of choke-capacity smoothed source.

Bias for this valve is obtained by a resistance of 1,100 ohms

connected in series with the cathode and bye-passed by a 500 mfd.

electrolytic condenser.

In order to reduce nOise, it was found necessary to employ

Varley Bi-duplex resistances for the anode resistances of both the first

two stages. Grid stopper resistances are inserted in the first three

stages.to improve stability and to obviate short wave interference fram

the rotary transformer.

Anode decoupling of the first two stages is carried out by

means of two 8 mfd. electrolytic condensers, qontained in one carton,

and resistances of 10,000 ohms. The latter may be Erie or Morganite

resi stance s, as any noise generated by them due to the Dassage of direct

current has been found to be sufficiently bye-passed by the electrolytic

condensers.

-4-

Feed-back Circuits. In order to facilitate the description of the feed-

back and microphone equalisation circuits, a simplified schematic diagram

of these arrangements is shown in Fig. 2b.

The main feed-back arrangement consists of a resistance of

310,000 ohms (Rl + R2), one terminal of which is connected to the cathode

of the first valve and the other coupled to the anode of the second valve

by means of a 1 mfd. condenser. Currents will thereby flow from the

anode of the second valve, through the feed-back resistance (Rl + R2) and

cathode resistance (R3) of the first valve, back to the second valve via

its cathode circuit. The eam.f. set up by this current flowing through

the cathode resistance (R3) of the first valve will be in phase opposition

to the voltage applied to the amplifier across the secondary terminals of

the input transformer. Obviously the magnitude of the fed back voltage,

and therefore the overall gain of the two stages, depends on the ratio

Advantage is taken of this to change instantan-

eously the overall gain of the amplifier. The 310,000 ohm feed-back

resistance is therefore split into two resistances of 0.25 megohms and

60,000 ohms, and provision is made to short out the 0.25 megohm portion

by means of a key and thus increase the feed-back or, in other words,

decrease the gain of the amplifier. By depressing the gain key, then,

the overall gain of the amplifier may be changed from 87 db.v. to 75 db.v.

The advantage of. decreasing the gain in this manner lies in

the fact that a better fade out by the gain control may be obtained when

-5-

the full gain of the amplifier is not required. This operation is

sometimes performed at the O. B.point. The ant icipation that a gain of

75 db.v. will prove adequate for the majority of outside broadcasts

justifies the inclusion of some means of reducing the gain of the

amplifier to that figure.

Since there is no bye-pass condenser across the cathode

resistance of the first valve, the anode currents will develop another

feed-back voltage across the cathode resistance which is in practically

the same phase as the main feed-back voltage. It is important to

realise, therefore, that any change in the value of the anode or load

resistances of the first stage would materially affect the overall gain

of the amplifier.

Microphone Equalising Circuits. Provision is made for inserting

equalisation by the operation of a key in cases where ribbon microphones

are used.

The decrease in gain measured at the lower frequencies is

catered for by a 0.03 fifd. condenser connected in series with the gain

control potentiometer, the key bei.ng arranged to short this condenser

out when equalisation is not required.

The increase of gain at the higher frequencies is achieved by

decreasing the magnitude of the fed-back voltage at those frequencies.

A portion of the cathode resistance of the first valve is therefore

shunted by a condenser (Cl) when the equalisation key is depressed.

-6-

In addition, a small trimming condenser (C2) is shunted across

the 60,000 ohms feed-back resistance when the overall gain of the amplifier

at 1,000 c/s. is set at 75 db.v. The object of this is to reduce the gain

at frequencies above 10,000 c/s o , and thus tend to aid stability under

practical conditions. The inclusion of this condenser is not necessary

when the overall gain is set to 87 db.v. at 1,000 c/s. as, under these

conditions, the feed-back is insufficient to maintain a flat frequency

characteristic above the working range.

Test Results. The frequency characteristics of the modified amplifier

under various conditions of operation are shown in Figs. 3, 4 and 5.

Since no change has been made in the circuit of the last stage,

it is unnecessary to show here the variation of the frequency characteristic

with changes in the load impedance. The variation has already been shown

to be very small in Report No. L.003/l.

Reference to Figs. 3, 4 and 5 will show that the frequency

characteristics satisfy the requirements subsisting at the present time in

this respect.

As a result of the incorporation of the feed-back circuits~ the

harmonic distortion introduced by the modified amplifier is of negligibly

small proportions. This is shown in Table la

The amplifier works satisfactorily when deriving its H,T. suppl.y

from a rotary transformer, the level of machine noise being below that of

the combined valve and thermal agitation noise.

-7-

Lay-out and CO!PJ2onents. So far as possible, the original lay-out of the

OBA/? amplifier has been retained in order to simplify the modifications.

Apart from removing the intervalve transformer from its mu-metal '~ox and

changing its position, the lay-out of the output and trap valve stages

remains unaltered. No serious interference was observed during the tests

which could be attributed to the lack of electromagnetic shielding of the

intervalve transformer.

It is suggested that, if the modifications specified in this

report are adopted, the lay-out of the actual amplifier which has been

modified should be followed as closely as possible as the additional

introduction or variation of stray capacities in such a high gain

amplifier might conceivably upset the performance.

A list of components employed in the modified circuits is given

in the Appendix.

JS/CHF.

-8-

TABLE 1.

OBA/?/250

HARMONIC ANALYSIS.

ZT - 600/600 ohms.

t----':...--~"_t_-----" ........ --"i_------+-'--"J;_2n_d_H_a_rm_~. ! fo 3rd Harmon_1-"~ 0045 I 0~2· I

Frequency: I Gain Sett ing , Output Test cyclesls.iof Amplifier. Level.

I i I I

I

I

1,000 87 + 20

j' I 0.09 !

I I 1 0.18 I

1

1,000 81 + 14

t 1,000 75

I + 20

1,000 69 + 14

0.16,

0.48

0.245 0.08?

50 87 + 20 . 0.35 0.425

50 81 + 14 0.25 0.33

50 ?5 + 20 0.48 0.415 I 50 69 + 14 0.3 I 0034 I

========== ===========: =============--::= ===============::: ============="; !

50 87

~- 600/100 .i."1..

3 volts rom.s. (Equivalent to + 10 db.)

0026 I I J

~--------~----------~------------~------------~------------~

-9-

LIsr OF COMPONENTS.

1st Stage.

1. Grid stopper resistance

2. Anode resistance

3. Cathode resistances

4. Anode decoupling resistance

5. Anode decoupling condenser

6. Shunt bye-pass condenser

7. Anode coupling condenser

2nd St~e.

8. Grid resistance

9. Grid stopper resistance

10. Anode resistance

11. Cathode resistance

12. Anode decoupling resistance

13. Anode decoupling condenser

14. Shunt bye-pass condenser

15. Grid bias decoupling condenser

16. 1st anode coupling condenser

17. 2nd anode coupling condenser

18. Anode load resistance

10,000 ohms Erie, 1 watt.

100,000 ohms Varley Bi-duplex.

500 ohms) Erie or preferably 600 ohms) Morgani te • 1 watt.

10,000 ohms, Erie or preferably Morganite. 1 watt.

8 + 8 mfd. T. C. C. carton electrolytic type 702 H/4.

500 mfd. T.C.C. electrolytic type 501.

1 mfd. T.C.C. type 87.

0.25 megohm Erie. 1 watt.

10,000 ohms Erie. 1 watt.

100,000 ohms Varley Bi-duplex.

1,000 ohms + 100 ohms Erie, 1 watt.

10,000 ohms Erie or preferably Morganite. 1 watt.

&3e item 5.

250 mfd. T.C.C. Electrolytic type 50l.

500 mfd. T.C.C. Electrolytic type 50l.

0.1 mfd. T.C.C. type 87.

1.0 mfd. T.C.C. type 87.

0.25 megohm Erie. 1 watt.

-10-

Equalisation and Feed-back Circuits.

19. Gain key

20. Equalisation key

21. Feed-back resistances

22. Feed-back coupling condenser

23. High frequency equalisation condenser

24. High frequency equalisation trlimning condenser

25. Low frequency equalisation condenser

3rd stage.

26. Grid stopper resistance

27. Grid bias decoupling condenser

-------000-------

G.P.O. type 207 or 73.

G.P.O. type 207 or 73.

0.25 megohm Erie. 1 watt. 60,000 ohms Erie. 1 watt.

1 mfd. T.C.C. type 87 or similar,

O.lmfd.micaT.C.C. + 5%.

0.0001 T.C.C. type 340 •.

0.03 mfd. + 5% T.C.C. type 87 X.

15,000 ohms Erie. 1 watt.

500 mfd. T.C.C. Electrolytic type 501.

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