Handbook for Sound Engineers

294 Chapter 11

many interleaved sections to achieve bandwidth
extending well beyond 20 kHz.
If the quiescent plate currents and the number of
turns in each half of the primary winding are matched,
magnetic flux in the core will cancel at dc. Since any
current-balancing in vacuum-tubes is temporary at best,
these transformers nearly always use steel cores to
better tolerate this unbalanced dc in their windings. The
relatively high driving impedance of the tube plates
results in considerable transformer related distortion. To
reduce distortion, feedback around the transformer is
often employed. To achieve stability (freedom from
oscillation), very wide bandwidth (actually low phase
shift) is required of the transformer when a feedback
loop is closed around it. As a result, some of these
output transformer designs are very sophisticated. Some
legendary wisdom suggests as a rough guide that a
good-fidelity output transformer should have a core
weight and volume of at least 0.34 pounds and 1.4 cubic
inches respectively per watt of rated power.^4

A single-ended power amplifier is created by

removing the lower tube and the lower half of the trans-

former primary from the circuit of Fig. 11-35. Now

plate current will create a strong dc field in the core. As

discussed in section 11.1.2.1, the core will likely require

an air gap to avoid magnetic saturation. The air gap

reduces inductance, limiting low-frequency response,

and increases even-order distortion products. Such a

single-ended pentode power amplifier was ubiquitous in

classic AM 5-tube table radios of the fifties and sixties.

11.2.1.6 Microphone Output

There are two basic types of output transformers used in

microphones, step-up and step-down. In a ribbon micro-

phone, the ribbon element may have an impedance of

well under 1ȍ, requiring a step-up transformer with a

turns ratio of 1:12 or more to raise its output level and

make its nominal output impedance around 150ȍ. Typ-

Figure 11-34. Double cathode-follower line driver.

Figure 11-35. Push-pull vacuum-tube power amplifier.

+250 V

R 2

680 7

V 1

12BH7

6

In 7

+120 V bias

C 1

220 nF^1

2

R 1

1M (^73) R 3
330 7
V 1
12BH7
(^8) C 2
22 MF Grn
T 1
JT 10K61 1M
Org
Brn
Red
Blk
Blu
321 P^1 Line output
Yel
Negative feedback (if used)
Amplifier
In front-end
+250 V
Bias
+250 V
+350 V
+250 V
7
7
Com
Loudspeaker
out
16 7