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RADIO AGE for January, 1927
Further Notes
on the
Henry Ly f ord
By ELMORE B. LYFORD
The Magazine of the Hour 9
IN THE previous article on the Henry-Lyford which appeared in this magazine, nearly all of the space was given to a description of the receiver and to constructional details, for those who wished to build one. Of necessity, much of interest to builders of this receiver had to be left out, and it is the purpose of this second article to provide additional information about this popular set.
It is necessary to use UX201A type tubes throughout in the Henry-Lyford with the exception of the second audio stage — the last tube. In this position a UX112 type power tube is necessary. With this arrangement of tubes, 135 volts of "B" battery will be required. The first two 45 volt blocks should be of the heavy duty type, for they supply current to all five tubes of the receiver. The third 45 volt battery may be one of the regular type, for it supplies current to the last tube only, and the drain on it is very light.
All of the "C" battery voltages, as was stated in the previous article, may be secured from one Burgess No. 5540 battery, which is a 714 volt battery with a tap at every IV2 volt.
Of course, any good "B" eliminator may be used instead of batteries, if desired. Because of the use of sufficient by-pass condensers, the receiver is particularly adaptable to "B" eliminators, and any good one will give very satisfactory results.
A power tube of the UX171
type, may be used in the second audio stage without necessitating any changes in the wiring of the receiver. The only changes that are necessary to use this type of tube are those of the battery supplies. The "B" max. lead in the battery cable runs to B plus 180 volts instead of to B plus 135 volts, and the C minus max. lead runs to C minus 45 volts instead of to C minus 71 •> volts.
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Isolate DC Component
WITH any type of power tube which requires a plate supply of more than 135 volts in this second audio stage, precautions should be taken to safeguard the loud speaker windings from too heavy a current through them. Under these conditions, the direct current supply to the plate of this last tube should not be allowed to pass directly through the speaker windings. One way to accomplish this is to use an output transformer between the plate circuit of the tube and the speaker, as shown in Fig. 1. Another equally good method is to use a choke coil and condenser combination, as pictured in Fig. 2. Here the choke coil should have an inductance of about 50 henries, and the condenser should be 2 or 4 mfd. filter type. One terminal of the loud speaker is connected to the blocking condenser, and the other side may be connected to either B minus or B plus, preferably B minus.
The use of a milliammeter in the plate supply lead of the last tube is a practical and convenient