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….’
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.
I’m constantly amazed by the results of upgrading Yaqin amps of which I’ve now completed quite a few. I remember my nervousness upgrading the first one as the received wisdom was ‘Chinese transformers are no good’. Yaqin’s transformers are made in house and they claim, are made of Japanese steel.Whatever the ingredients the result is an amplifier that sounds very decent as standard but which can easily be improved with some simple upgrades.
Circuit wise the amp is a fairly standard ultra linear output stage with input and phase splitter. The upgrades this time were to add improved Mundorf coupling caps, to change the grid stopper resistors on the output valves and to add Elna Silmic bypass capacitors to the output valves.
Once installed the amplifier performs at a new level. The light graininess of the sound disappears, vocals become more breathy, and bass and dynamics improve. All these improvements were heard with the stock Chinese valves and of course these too can be swapped for improved Western designs or NOS types.
The Onix Melody is an Australian design made in China and runs 6L6 valves or equivalents. It had a pretty decent sound for the money when released in early 2000 and one of its main claims to fame was the use of a stepped attenuator.
Another feature is its use of Russian military paper in oil capacitors (in the green cans in the picture above). These are not bad in terms of sound quality, but can be bettered with a decent audio grade capacitor. So, after discussion with the owner, we agreed we would upgrade the cathode bypass capacitors with Elna Silmics and the coupling capacitors with Musicaps. I also installed an earth lift system to stop the amplifier interacting with other equipment.
Overall the result was a very decent uplift in performance.
I’m selling these on behalf of a client who’s upgraded to my KT150 amps and preamp. These have been converted to cathode follower design as an improvement over the original hummy WAD design. These have been converted to cathode follower design as an improvement over the original hummy WAD design. The build contains every possible upgrade including: Tocos carbon pot in shunt mode with Vishay naked foil resistors, Vishay naked foil grid resistors, Jupiter beeswax signal caps. The power supply has an upgraded 30 Henry choke, Schottky heater diodes, and Hexfred HT diodes.*The valve is an ECC88 gold pin Mullard. Excellent sounding line pre in daily use. Front panels in good condition, some scratches on sides of black chassis.