Audio Research D250 MKll Servo

[SolderSlinger]

Replaced all of the electrolytic caps in my D250 MKll Servo with modern components. Newer passive components are far superior to old designs. Arguably the only benefit of the green agenda.

A major shortcoming of the D250 is it was physically designed to use the Sylvania/Philips 6550. The bottle diameter is approximately the same size as the base. Larger diameter 6550 tubes don't fit in the 2 lower regulator positions. The tube has an interference fit with the top of the capacitor bank PC board. You will see on some amps listed for sale where the seller has used a non Sylvania tube and it's cocked.

Newer capacitors pack a lot more capacitance in less volume. I picked replacement caps that are shorter to allow using larger diameter tubes in regulator circuit and fit the taller KT120 in the output stage.

I did try the KT120s just for fun. Amps is currently running 6550.

[admin]

Wow, that looks great. Look at that last picture with all those gorgeous tubes packed in! It's a great design that we don't often see in tube amps. That must generate some serious heat.

[SolderSlinger]

Just shy of 1kW at idle. There are 32 tubes, 20 output and 12 small signal. You get about as much heat from a pair of ART mono's. The main difference is the D250 is localized heat, and is fan cooled. The warm air blowing of the tubes is noticeable.