On this page:

On this site:

Geloso G215AN amplifier

The Geloso G215AN is a 15 W hi-fi amplifier from about 1960. I bought it because this amplifier type is the origin of the transformers I used in my first guitar combo project.
Geloso was a manufacturer of radios and amplifiers from Milano, Italy. Their vintage hi-fi amplifiers are still popular among valve amplifier enthousiasts. The G215AN dates from around 1960. It has a push-pull output stage with two EL84's with fixed bias, producing a nominal output of 15 W.

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.

    Familiar mains voltage selector and output <br>
    impedance selector on the back.

Familiar mains voltage selector and output
impedance selector on the back.

In late 2014 I stumbled across a small, rusty old Geloso amplifier on a radio swapmeet. I had to look twice before I realised this was a G215AN. The price was right so I didn't hesitate. I was curious to see how these transformers were doing in their original circuits. And I would have a spare set of transformers in case my son's guitar amp would break down.

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 The selenium rectifier for the heater voltage for the pre-amplifier valves had been replaced by a BY164 silicon bridge. The input sockets had been replaced by DIN sockets and a microphone transformer had been added to the microphone input.

    Hood taken off, inside view.

Hood taken off, inside view.

    Original view from the bottom.

Original view from the bottom.

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.


Making a guitar input

    Input connector replaced by jack input.

Input connector replaced by jack input.

I removed the two DIN input sockets and replaced the aluminium plate they were mounted on. I removed the microphone transformer and brought the input circuit back to original. I fitted an input jack and tested.

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.

Copyright © 2014 by Onno's E-page         published 2014-12-20, last updated 2015-04-19