Valve Radio and Audio Repair Handbook

Chapter 20

Finding faults on output stages

Chas E. Miller

Since there is little difference in AC and AC/DC techniques other than in the power supplies, in this chapter and those following the notes will apply equally to either type. The only point to remember is that AC sets can be expected to have higher HT voltages, perhaps even twice as much, than AC/DC models and replacement components should be rated accordingly.

Figure 20.1 shows a typical output stage having the triode section of a double-diode-triode driving a pentode, with conventional matching transformer to a permanent-magnet loudspeaker. Condenser C1 across the transformer primary is included to reduce the high frequency response. If it should happen to become a dead short it would effectively ...