In 2014, I bought a set of Geloso transformers to build a guitar combo. After some research, I found out these were from a G215AN amplifier, although I had been told they were intended for 6V6 valves. I drew my conclusions from a circuit diagram of the G215AN I found on the Elektrotanya vintage manual site and the NVHR website.
At home, I lifted the hood and took a look. All the valves were in their sockets and the transformers and other iron stuff were looking reasonably well, although the chassis and case were a bit rusty and dusty. After I removed the bottom, I could see that all the original capacitors were still in place. There were some changes though. The selenium rectifier for the heater voltage for the pre-amplifier valves had been replaced by a BY164 silicon bridge. The input sockets had been replaced by DIN sockets and a microphone transformer had been added to the microphone input.
First I started to reform the electrolytics before putting on power. These were doing quite well, they took less than 0.3 mA at 300V in total after I finished. I also checked the capacitors, that were in excellent condition. These are sealed in some kind of compound and might be paper capacitors, but the fact that they do not leak nor crack, may mean they are polyesther capacitors.
Time for power. I changed the output impedance setting to 8 Ω and connected a load resistor. I replaced the missing pilot light. Then I switched on. Nothing. No voltage after the mains fuse. I opened the fuse holder, finding the fuse was falling apart, looking corroded. I inserted a fresh 1 A fuse and retried.
Now the valves lit. The plate voltage rose to 340V and dropped as the amplifier heated up. The output valves were getting -12 V bias so the amplifier would function in class AB. I connected my AF generator and happily observed the sine at the output. I got about 26 Vtt. And a bit too much distortion. Now I had already noticed that both pre-amplifier valves had been replaced by ECC82's (12AU7), while they should be ECC83's (12AX7). Nothing had been changed to the circuits, so they were biased incorrectly and were giving less amplification than this amplifier needed. After replacing the valve of the paraphase inverter, I saw the output voltage had slightly increased to 28Vtt. The paraphase inverter was biased as it should be. The pre-amplifier valve only had 30V anode voltage. After replacing it, this went up to 70V and I got more amplification and less distortion.
This amplifier was working well and delivering about 13 W without clipping. Decently close to the 15 W Geloso had promised.
I checked the DC heater voltage to the pre-amplifier valves: almost 15V, which should be 12.6V. This was to be expected because the original selenium rectifier had been replaced by a silicon bridge having a lower voltage drop. A 27 Ω resistor in series with the rectifier corrected this. This also lowered the bias voltage of the output stage, that is derived from the DC heater voltage.
Time to modify the microphone input to become a guitar input.
This amplifier did work with a guitar but it distorted in an unpleasant manner. The first amplifier stage had zero bias and a high (3.3M) grid leak resistor. Fine for a mike but I suspected it couldn't handle the input from an electric guitar. I decided to change the input stage to the input stage of the VOX AC15. After this change, the amp sounded slightly better.