Archive for June, 2020

Servicing the JADIS JA30 MonoBlock Amplifier

June 14, 2020

JADIS JA30

The JADIS amplifier is a high-end domestic high-fidelity hand-wired creation from France. They are very expensive and also quite rare here in Australia. Production of the JADIS amplifier, and matching preamplifiers, began in the early 1980’s. The power amplifiers are built as MonoBlocks, so obviously a pair are required for a stereo system. JADIS products are clearly designed for the most fastidious music listeners, for whom money is no object (within reason). The power amplifiers are mostly designed to employ the thoroughly awesome KT88 power output valves (in ultra-linear configuration), which makes complete sense.

JADIS JA30

Our customer delivered a pair of the JA30 MonoBlocks to our workshop for a revalve (re-tube), and most importantly for replacement of all the valve sockets – a pair of octal and a pair of miniature 9-pin sockets for each MonoBlock. One only MonoBlock had unknown circuitry faults to be diagnosed and corrected as well. Each MonoBlock is loaded with a 12AU7/ECC82 and a 12AX7/ECC83 valve (tube), plus a pair of KT88’s. Each MonoBlock delivers 30 watts rms, operating in pure ‘Class-A’ (‘ultra-linear’ output transformer) configuration, with enormous over-engineered transformers.

JADIS JA30

Each KT88 is cathode-biased (ie, self-biasing) and drawing 90ma with a HT voltage supply of +475V DC. Therefore, each KT88 is dissipating 40 watts at idle, plus 10 watts each heater filament. That’s pure Class-A for you.  But what a sound !! This pair of MonoBlocks appears to have been built in 1986, give or take. The extraordinary amount of heat being generated takes its toll on many of the components, over years of regular use. In particular, the valve sockets are no longer fit for purpose – they are barely functional.

replacing the octal sockets

Our first choice of octal sockets would not fit on the chassis, so rather than make a mess we sourced an alternative with suitable dimensions. The miniature 9-pin valve sockets did not present any particular problems, they suit the standard 19mm hole, but the wiring and components had to be removed and reinstated in ‘layers’ (a bit like working on ‘Matchless’ amps), requiring extra time and a focused approach.

JA30 hand wiring

One only MonoBlock revealed destroyed screen grid resistors and cathode-bias components for the KT88’s. The primary reason for these failures turned out to be the cathode bypass capacitors, which under the extreme conditions went S/C (‘short circuit’), thereby removing bias for the KT88’s, resulting in ‘melt-down’. These capacitors are 150uF/200V axial units – actually not a common component at all. After some searching, we managed to source high-quality made-in-Sweden replacements, rated for  105 deg/C, from RS Components in Sydney. These are physically large, so were bedded down in silicone for stability.

JA30 hand wiring

The two MonoBlocks required hours of bench time to restore to original performance, but our customer was very pleased with the end result. Both MonoBlocks tested identical in performance and specs, so the time and effort was well justified and this is also a tribute to the success and long term stability of the JADIS design. Our one very significant complaint about the JADIS company is that they refuse all requests for service information, schematics, etc. We understand that they are very protective of their designs, but this is ridiculous – it is just not practical to ship these monsters back to France every time they need a service.

In this case – because I have a background in the design and building of valve (tube) power amplifiers, I was able to reverse-engineer the aspects of the design that were important, but at a later point in time we had to service the matching preamp, which was truly a nightmare without a schematic. JADIS (and others) – please take note !! IR.