Another really interesting amp has arrived needing a lot of tender loving care. The Radford TT100 is a hybrid solid state /valve amp. This one has been butchered over the years. While some components have been upgraded new components are held in place with Blue Tack, and others have been installed with no insulation so any movement or shock could potentially cause a major short circuit. Not the way I’d do any upgrades with 600v floating around inside! Thankfully all the transformers and chokes are in good order and having made the amplifier safe, it even made music on power up. Now for some restoration work to make this an everyday amp with decent sound quality and a lot of slam and dynamics!
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 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.
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.
These Unison Research pre and power amps are currently in the workshop for servicing. The power amp is the hybrid valve /solid state model. Repairs included the input selector and a choke for correct earthing and hum.