This amp is still in pristine condition despite dating from the 90s. Some new valves, a new selector switch, replacing the electrolytic capacitors and upgrading the coupling capacitors and it will be ready for another 30 years service!
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.
This Graaf GM20 came along for a health check and adjustment of bias and DC offset. It’s an interesting design that use the massive Russian 6C33 valves and is output-transformerless, thus avoiding any issues of transformer limitations, weight and cost. It sounds great but care needs to be taken with the 6C33 valves. Reputedly used in cruise missiles, Mig fighters and trawlers, these valves run very hot and can cause problems with heir valve sockets. Graaf use excellent quality sockets and overall build quality is excellent, as you would expect from a top Italian brand.
The 6C33 valve needs very careful burn in to perform reliably, and otherwise bias can drift alarmingly leading to short valve life. Therefore, it’s best to buy valves from a source that does this careful burn in process for you or do it yourself.
Once adjusted and checked over this amplifier was sounding excellent again.
This amp came in for repair after the client stated his girlfriend had been connecting up the speakers and had managed to short the speaker cables. There’s at least a couple of morals in that tale, the most important being ‘switch off ‘ before you connect or disconnect speakers-and inputs in fact as the resulting transient can easily damage speakers too.
Despite sophisticated protection circuitry the amplifier had blown its output transistors. Once replaced the amplifier is singing away sweetly again.
I was commissioned to build a high quality step up amplifier of moving coil cartridges and produced this little silver box. Using top quality Toshiba JFETs this step up amplifier offers silent non coloured amplification for moving coil cartridges that compares with the very expensive step up transformers. This version uses battery power (2 x 9v batteries) but it can also be supplied with a matching mains power supply.
It’s so good I now offer it as a standard product at £250 Moving Coil Step Up amplifier
Another Yaqin 13s on the workbench for the usual upgrades: upgraded coupling capacitors, upgraded grid stopper resistors and fitting of cathode bypass capacitors. These mods worked their usual charms and gave a very decent performance, even with the stock tubes. This time tho, I tried the amp with negative feedback removed. The difference was dramatic, and so a small switch was installed on a plate to replace the mains voltage switch which was re-mounted inside the chassis. In my set up, I never switched the feedback back on and both myself and my regular ‘golden-eared’ mates considered it way better. Here’s what the owner had to say:
‘Just collected my MC13S following David’s mods and I’m stunned! In addition to the mods described above David also fitted a feedback bypass switch and with the negative feedback switched off the resolution of this amp is incredible. I keep wanting to try another track to test the improvements but find myself having to hear the whole track before switching, which is not something that usually happens when I buy new kit. All in all I’m very pleased with these mods….’