Electronic – Can someone explain this tube amplifier circuit for me


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:
enter image description here

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:

  1. 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?
  2. 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.

Best Answer

First, I think it's more likely these ancient tubes are no good anymore than the capacitors having failed. Except perhaps for the power, which is easy enough to check, these capacitors are probably wrapped foil or something else that is dry and should last a long time. Start by checking the supply voltages. Those appear to be well marked. If the power input diode has gone bad, then nothing else has much chance or working.

As for the circuit, I am somewhat confused too. I'll take a rough stab at it anyway. To really understand it would take more time working thru it than I want to spend on it.

The left tube seems to be a pretty straight forward amplifier. The amplified signal appears on the plate, which is then coupled into the power stage thru C4-4. Most of the mess between C4-4 and the control grid of the right tube looks to be a tone control. That's just from the general form. I haven't actually analyzed it. I think R5 is likely some sort of tone control. I'm less sure about R4, but R4 and R5 together may be something like bass and treble controls.

The strange part is how the two output transformers are hooked up. I'm guessing that the top two speakers are meant to be tweeters, the lower two the rest of the sound range, and the strange connections between the double transformer is like a crossover network. This also leads some credance to R4 being a treble control since its signal is driven from the feedback from the top transformer output.

C4-9 and R4-7 feed back a bit of the signal at TP3 onto the cathode of the power tube. This looks like classic negative feedback to provide predictable gain and a flatter frequency response.

The section of the circuit you show here can be easily enough tested in isolation. First, make sure the two power supply voltages are as marked, then feed a signal into the line you labeled as audio input. That should be clearly audible on the speakers.