World Audio Designs (WAD) 300b Stereo

This amplifier is a clone of the original WAD 300b stereo amplifier. It was ninth workshop of repairs, as valve failure had taken out two cathode resistors. This version had been made with two 6D22 damper diodes for slow start and rectification and the owner was nervous about the top caps, which carry 550v, a potentially lethal voltage.

So we took the opportunity to convert the amp to solid state rectification and I installed a slow start module, to mimic the functioning of the valve rectifiers. We also lowered dissipation as the owner was keen to use Mesh Plate 300b valves, which don’t like to dissipate much more than 24 watts.

So how does the solid state rectification change the sound? I would describe the change as a faster sound with more low level detail apparent, so all in all a small increase in perceived sound quality.


The slow start module can be seen in the middle bottom of the above photo.


WAD 300b Stereo, Pre2 and Phono 2 Upgrades

I’ve built four WAD 300b stereo amplifiers and serviced many more. One of my own amps is a WAD 300b stereo too, so you can tel I love the sound. img_4137An upgrade for the Wad 300b stereo I’ve now done on six 300b stereos is to add an additional choke to the power supply for the input section. This improves an already great amp, providing more bass slam, detail and dynamics. The owner brought along his Pre2 and Phono 2 for upgrades too.

The Pre 2 had been converted to a cathode follower already, which overcomes the original design’s hum issues. So this time I increased the size of the power supply choke and converted the power supply to a pseudo mono set up. For phono 2 I added cathode bypass caps to the first valve.

Overall results are a very nice improvement in SQ for the system.