This preamp was in the workshop for some upgrades and repairs. The main upgrades were to upgrade the coupling capacitors to Mundorf ZNs, and repair the volume motor pot. Finding a replacement motor pot that fits in the space available and is a 500k log pot proved impossible, so I spent a long time stripping the original pot down and rebuilding it. It now works perfectly and the pre sounds very dynamic, detailed and spacious and much improved over the original grainy sound. Ming Da use poor quality coupling capacitors then bypass them with equally poor quality caps. In my view that can introduce phase shift effects and sound is much improved using single quality film /foil capacitors. Those bottle-shaped tubes may look cool but they are horribly microphonic. The 6SN7s are worse offenders-because higher gain-than the 2A3s so those were replaced with some very high spec Russian military valves which again contribute to the overall improved sound.
These venerable mono blocks came along for some upgrades. I went down and tried and tested approach of upgrading key coupling capacitors, and making switchable feedback. These amps really come alive with these mods, providing some of the best valve sound around. With no feedback gain can become too much, so I also tried bypassing the first valve and making a few mods to the second valve stage. This worked really well but with feedback switched on, the gain was rather low at around 4v for max output. So to maintain flexibility I reduced global feedback slightly in the feedback on position to enable the amp to work with a wide range of speakers and left the feedback off option in play. With feedback off and 88db speakers the noise level is acceptable and the sound stunning. With the feedback switched on there is a small reduction in overall sound quality but the amp still sounds among the best!
As one of the 805 output valves was running slightly pink I checked the bias to find the output valves were running at 135ma dissipation, way higher than the recommended level of around 100ma. So the bias circuit was modified to bring dissipation down to 100ma. Now the amps were really singing and making a great combination along with their accompanying Ming Da MC 2A3 preamp which was also in for some upgrades.