Jadis JA-80 MK II mono valve power amplifier

Tubed power amplifiers
Jadis Electronics JA-80 Mk II

The rest of the design remains almost identical to the original JA-80, because change for its own sake is anathema to Jadis. The other valves in the circuit are an ECC83 as line driver and a ECC82 as phase shifter, which are flanked by four blue coupling capacitors. All the valves are housed at the rear of the amplifier chassis and can be kept from prying hands thanks to a supplied cage that no-one will ever use because the amplifier looks so much nicer without. In fairness, it looks pretty, even with the cage, but if you can drive the amp without the cage, you will at every opportunity because  – unlike most of humanity – the JA-80 Mk II looks so much better with its clothes off!

The power transformer at the front of the chassis is elegantly chrome-capped, while the output transformer is shielded, and has a gold name plate on its top. This is a core part of the Jadis ‘secret sauce’ and is proprietary and made in-house. A pair of larger high-capacity capacitors fill the gap between the two transformers. Real-estate is limited on the JA-80 Mk II chassis, and this means the layout of the power amp is perhaps slightly back-to-front, with the power inlet socket on the top and to the front of the chassis, and the valve layout at the rear of the amplifier near the single central RCA line input, and the two sets of speaker terminals. These are bi-wire terminals, rather than different taps for loudspeaker impedance. The amplifier is wired as standard for loudspeakers in the four to eight ohm impedance range, but internally altered for anything from one to 16 ohms. 

Once set and installed, user input is limited. The valves are automatically biassing and there are LED indicators next to each valve to indicate potential failure of that ‘bottle’. These are almost the only concessions to modernity, though. The amplifier has two large toggle switches on the front fascia. The first powers up the heater circuit, turning the amplifier from ‘off’ to ‘standby’, and the central indicator LED glows red. Ten or so minutes later, you are advised to throw the second ‘Operate’ switch, which moves the amplifier into full play mode, and that LED now glows green. Turning the amplifier off is to run the process in reverse, although there is no need for the intervening ten minute delay. The delay between the two actions allows the valves to come to their correct thermal operating temperature before play time, and shortening the time between throwing the two switches ultimately also shortens the life of the tubes. Most modern power amplifiers incorporate some kind of microprocessor or relay circuit to move from heating to full operation. Jadis places the onus on the responsible listener. However, if you are responsible enough to know never to fire up a power amp without turning the preamplifier on first, you are responsible enough to drive the JA-80 Mk II.

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