Valve amp

This project uses a Raspberry Pi running XBMC. There isn’t anything too exiting about that, and the construction is well documented on several other sites. I’ve connected it to a home network which allows it to access my MP3 files. So everything is fairly straight forward and silicon based up to the audio socket on the Raspberry Pi.

But from that point on, the silicon stops, the technology jumps back 60 years, and everything goes thermionic …


So here comes the update on the old technology…

I’ve based the circuit on a design classic from the 1950’s – the ultra-linear Williamson. Sadly the original transformers aren’t available anymore, so I’ve substituted ‘something similar’. Goodness knows what that has done to the overall performance, but to my untrained ear, they sound absolutely great. The first ECC82 (that’s the small valves) are double triodes. The first half amplifies the input signal, then the second half phase splits it. The +ve and -ve signals are passed to the two halves of a second ECC82 for further amplification. The signal is then passed to the two KT66 output tetrodes (that’s the great big things)  connected for class AB operation. The output stage uses  an ultra linear configuration, this operates the valves somewhere between tetrode and triode mode, giving the best of both worlds. The circuit uses a 5R4GY full wave vacuum rectifier to provide the DC supply.


Very first prototype – I was a complete wuss when it came to switching on, so I ran it through a variac which allowed me to build the volts up slowly (and from a distance). The downside of this approach was the heater winding was also throttled back, so I had to fit a second 6.3 volt transformer to get the valves to warm up.

So here it is running at 100v and sounding good.



So after that, I upgraded the smoothing capacitors to take the required B+ voltage  which can vary between 480 and 550 volts DC. I also repaced the silicon rectifiers with a valve (thats the tall one in the middle). The rectifier is the only part not in current production, so I had to source it through eBay. I got really lucky and found a pair of New Old Stock army surplus dated 1945. It amuses me to think that these come from a time when the only computers were a national secret at Bletchley Park, and have been sat on a shelf ever since.


I’ve used point to point wiring on this project, as it is in keeping with the era that the circuit comes from. I still need to create front and back panels for these units to stop the connectors from floating about.