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.
These diminutive Dacs pack quite a punch for their price and size. One of their claims to fame is a battery power supply, which is slowly charged by a trickle charger. This Dac came in for a battery replacement.
This interesting amp was in the workshop for hum reduction. There is a lot of correspondence on the internet about the amp and ways to reduce hum, and in this case the hum was as much mechanical as electrical. We swapped out the input wiring for screened cable and fitted screening cans to the Edcor autoformers all with little effect on the hum levels. The next set is to try a DC blocker on the mains to see if that helps the mechanical noise from the transformer.
Having reviewed the costs of restoration and options with the owner we are going to:
- Turn two into streamers using Google Chromecast Streamers and Class D amplification. Two of the controls will be used for on/off and volume and the tuning dial will be backlit by LEDs.
- The third, the Bush VHF, will be fully restored and a Chromecast model built in and connected to the Aux input.
All three will use the original speakers, in the case of the Bush that’s two 6ins bass /mid units and an electrostatic tweeter.
Just completed building the new Tent Dac board and mainboard for a DDDac commission. I’m excited to compare it to the previous version once completed!