Electronic – Amplify Raspberry Pi Zero W Audio with 60s Tube amplifier circuit

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.

There are a number of things wrong here

1. There are no component designators in the schematic. This makes it hard to talk about, so I'll be more brief than otherwise.

2. You should use junction dots at all points where nets connect. The four lines crossing left of the transistor is ambiguous.

3. A quick calculation of the bias point not taking base current into account results in 1.3 mA thru the transistor, so your claim of 1.26 mA is plausible. However, this should tell you there is no way you're going to get anywhere near 1 W of output power. Even looking at the transistor as a perfect switch, the collector swing will be from 6.6 to 24 volts, for a swing of 17.4 volts. However all this is at a impedance of 8 kΩ. 17.4 Vpp sine wave is 6.15 V RMS. At the best case power transfer, the load is 8 kΩ so the voltage is half that, or 3.1 V RMS into 8 kΩ, which comes out to 1.2 mW. That's less power (although at a different voltaqe) than the MP3 player is likely putting out.

4. The frequency response is a mess. The gain of the amp only 2.7 at low frequencies. This starts going up 3 dB/octave at about 50 Hz, but the high end is unpredictable because it relies on properties of the transistor you don't know. Then the cap in series with the speaker forms another high pass filter, but this also allows the speaker to load the output heavily probably having a reverse effect for part of the range.

5. I can't even begin to guess what the 12 kΩ resistor accross the speaker is supposed to do. It makes no sense whatsoever.