Hi,

I'm Robert from Germany. I'm relatively new to tube ampflifiers. Out of curiosity I bought a diy kit on eBay a few months ago. A simple class AB push-pull amp with two 6F2 tubes and four 6P1 tubes. It sounded pretty good. I've replaced the chinese tubes with Svetlana 6p1p-ev tubes and Polam ECF82 tubes which made a noticeable improvement to the sound.

Since I wanted to learn more about tube amps and how they work I searched on YouTube and found this channel. After watching the videos about the A12 and A50 modifications I found a used A9 amp for sale not far away from me. The schematic I found looked similar to the A12 so I bought it. The seller told me it was barely used and it looked like new but cost a lot less than a new one.

At first I listened to the A9 in its original condition and wasn't impressed by the sound at all. The best I could describe the sound is "strange". I measured the output power with a 1kHz signal and a PC oscilloscope and calculated the output power at about 3,5W/8 ohms.

So I closely compared the schematics of the A9 and A12. The biggest difference is that the A9 uses 6N9P tubes and the A12 useses 12AX7 tubes. This is the schematic for the A9:

Mine is exactly the same except for the rectifier tube. In mine there wasn't a 5Z3P but a 5U4G. All voltages listed were a little higher on mine but not much.

So I made the modifications but didn't change any of the tubes at first. That already improved the sound a lot but I thought it was a little lacking in dynamics. The output power was raised a little to 4W. So I replaced the rectifier tube with a 5AR4 as recommended for the A12. That made another improvement but I still think something is missing. Output power is now almost 5W.

This is how the schematic of my A9 looks now. I used the schematic of the A12 mods as a basis for it. I hope that's ok:

The voltages in red is what I measured with 8 ohm loads and the volume at minimum.

The voltages are all a little lower than on the modified A12 but honestly I can't really tell if this is a bad thing. I would also like to know if anything might be problematic. Is there something I could change to improve it further?

What about the difference in the power supply. The choke in the A9 seems to be 10H (I couldn't find a label on it) while the one in the A12 has 3H. Also the capacitors are 33µF/220µF/33µF compared to 22µF/150µF/22µF on the A12. Would there be any noticeable effect if I change those to the A12 values? Any suggestions would be greatly appreciated.

Last but not least I want to thank Skunkie for creating this page and the YouTube content.