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.
These two Canary CA303 mono blocks came in for re-biasing and an open-ended ‘could I do anything in terms of upgrades?’. Like a lot of high end American gear, the componentry is top notch, so there’s no easy wins in terms of upgrading capacitors, resistors, or rectifiers for example. But what I did do was to install a switch for the negative feedback. With feedback switched off, with my speakers, these amps really hit their stride, open up and become seriously dynamic performers.
Those with keen eyesight will notice an Audio Innovations 700, and an LFD integrated solid state amp, both of early 90s vintage I believe.
AD Audio is pleased to announce a licensing agreement with Oddwatt Audio to build their amplifiers and preamplifiers. The first two products are the Cannonball KT150 Monoblocks and the Satchmo preamplifier.
For full details visit the AD Audio Sales Site here