I have a tube radio that has really low output (I mean the sound is barely audible at full volume). Probably some capacitor in the LF amp failed. However, after I downloaded the schematic diagram, I was intrigues and confused by the design of the output amp. While I can (most likely) repair the amplifier by testing the capacitors, I want to learn about electronics too. In this case, why it was designed as it was.
Here it is:
The left pentode is a 6ZH1P small signal pentode and the right pentode is the Soviet equivalent of EL84.
Capacitor value explanation - if the number is integer (2200) then the units are picofarads, if the number has a comma (25,0), then the units are microfarads. Resistor values are in ohms, unless there is a "k" or "M" written or the number has a comma (which means that the units are megaohms).
I understand how a basic grounded cathode amplifier works, but this one has some weird (to me) parts, for example:
- As I understand, C4-9 is for negative feedback, does it also behave as a cathode bypass cap? I mean, if I disconnected it, would the output increase or decrease?
- Why are the screen grids of both tubes and the plate of 6ZH1P connected to a tap of the output transformer? Why is that tap used to provide power to the detector and IF transformer (and their tubes). It is connected trough a 20uF cap to ground, so audio feedback probably would not be there. Is the output transformer used as a choke?
EDIT: The actual fault was a leaky C4-7.