362 Chapter 11
Although there are a number of factors that determine the phase shifts within the
transformer, the two most important are the inductance of the primary winding and the
leakage inductance between primary and secondary; a simple analysis of this problem,
based on an idealized, loss-free transformer, can be made by reference to Figure 11.15.
In this,R 1 is the effective input resistance seen by the transformer, made up of the anode
current resistance of the valve, in parallel with the effective load resistance, and L 1 is the
inductance of the transformer primary winding. When the signal frequency is lowered, a
frequency will be reached at which there will be an attenuation of 3 dB and a phase shift
of 60°. This will occur when R 1 jωL 1 , where ω is the frequency in radians per seconds.
R 2 is the secondary load resistance, which is the sum of the resistance refl ected through
the transformer and the anode resistance, andL 2 is the primary leakage inductance—a
term that denotes the lack of total inductive coupling between primary and secondary
windings—which behaves like an inductance between the output and the load and
introduces an attenuation, and associated phase shift, at the HF end of the passband. The
HF –3-dB gain point, at which the phase shift will be 60°, will occur at a frequency at
whichR 2 jωL 2.
To see what these fi gures mean, consider the case of a 15- Ω resistive load, driven
by a triode-connected KT66 that has an anode current resistance of 1000 ohms. Let
us assume that, in order to achieve a low anode current distortion fi gure, it has been
decided to provide an anode load of 5000 ohms. The turns ratio required will be
(5000/15) 18.25:1 and the effective input resistance ( R 1 ) due to the output load
refl ected through the transformer will be 833 ohms. If it is decided that the transformer
shall have an LF 3-dB point at 10 Hz, then the primary inductance would need to be
833/2π 10 833/62.8 13.26 H. If it is also decided that the HF –3-dB point is to be
50 kHz, then the leakage inductance must be 833/2 π 50,000 2.7 mH. The interesting
feature here is that if an output pentode is used, which has a much higher value of Ra than
a triode, not only will a higher primary inductance be required, but the leakage inductance
can also be higher for the same HF phase error.
Unfortunately, a number of other factors affect the performance of the transformer. The
fi rst of these is the dependence of the permeability of the core material on the magnetizing
fl ux density, as shown in Figure 11.16. Since the current through the windings in any
audio application is continually changing, so therefore is the permeability, and with it the
winding inductances and the phase errors introduced into the feedback loop. Williamson