Radford Revival
Trade: Radford Revival
Can someone explain the diff amp? How do you get a balanced signal when one half is a pentode and the other is a triode? I kept looking to see if it was secretly a floating paraphase and I had somehow missed the feed to the triode grid, but it's grounded (at audio frequency) so it must be a diff amp.
With the given anode resistors (33k / 39k) and the operating point of the circuit, it just so ends up that the balance ends up fairly good - better than you'd expect looking at the circuit anyway! There's no inherent way the circuit maintains perfect balance (like it having a cathode CCS and matched anode resistors for example), the design just centres things for that to be roughly the case most of the time.