Mastersound 845 Reference

This mega size amp is in for repairs. The fuse has exploded and the bias has failed.


Valve Output Stage for DAC

It looks simple but these valve output stages really bring my DACs to life. This one is destined for the latest DAC to roll off the production line. It’s a two DAC board version with valve output stage and special Audyn caps.

Audio Innovations s500

Yet another AI500 in the workshop. This time for a recap and new signal valve sockets along with a recap. The volume pot and select switch were replaced and the switch and balance controls were bypassed.

These amps really do represent excellent sound per pound-particularly the early ones-and there are many tweaks for improving the sound quality.

Tube Technology Unisis

One of these interesting amps came in for repairs and renovations. A polyester cap had shorted in the phono stage, taking out an anode resistor. In addition the amp was recapped, and had a new selector switch and new output valves fitted.

I’m always surprised how good these amps sound. The four valves inside are the pre and phono stages, the latter of which seems to be based on the old Marantz circuit. The main large power supply caps seem to have incredibly long life and I’ve never found any in need of replacement.

Audio Innovations s500

I’d been wanting to do this for a long time. Upgrade an AI 500 by converting to a choke power supply along with rewiring, capacitor and resistor upgrades, and upgraded selector switch and volume pot.

Instead of mounting the components in a separate case a la Border Patrol, I mounted the choke on the rear of the case. The sound quality is the best yet. While I don’t have a Border Control power supply for direct comparison this version is right up there, or maybe even better!

Radford STA 15-The Tweak

This Radford came into the workshop for a few reasons. Firstly the original cathode resistors had literally dropped out from their solder, through the action of heat over the years and finally overload with some dodgy valves. These were replaced with the superb Mills wirewound resistors.

Secondly the owner wanted attenuators fitted instead of the variable output impedance switch. I fitted the high quality Tocos carbon track potentiometers here.

Thirdly, removing the impedance switches meant hardwiring the output transformers to the speaker binding posts.

What I wasn’t prepared for was the great increase in sound quality that removing this 50 year old switch gave. Bass was much firmer and better defined, and there was greater clarity across the frequency range.