Rob's web

Coupling 500 ohm phones to the receiver

Home - Techniek - Electronica - Radiotechniek - Radio amateur bladen - QST - Coupling 500 ohm phones to the receiver

Fig 1
Fig. 1. Proper operation of the output stage of the receiver when 500 ohm phones are used is provided by the connection of a d.p.d.t. switch, as shown. All other parts shown in the diagram are those usually found in the receiver.

With tube-to-500-ohm-line transformers still scarce, many amateurs are faced with the problem of using 500 ohm phones with their receivers without burning out the output tube, its plate by-pass condenser, the output transformer, or all three, because of high peak voltages developed as a result of operating the tube with improper load on the voice-coil winding.

Operating the output tube as a cathode follower, as shown in Fig. 1, solves all of these problems with the additional advantage of producing excellent frequency response and very low distortion. By the addition of a double-pole double-throw toggle switch as shown, and a little rewiring of the components that are already in the receiver, proper operating conditions under both phone and speaker loads can be achieved.

Partlist fig. 1
C10.01 µF 600 V paper.
C20.0047 µF 600 V paper.
C310 µF 25 V electrolytic.
R1470 kΩ, ½ W.
R2470 Ω, 2 watts.
S1D.p.d.t. toggle switch.
T1Tube-to-voice-coil transformer.

Robert C. Potter, VE5TO.