The MkII version allows the use of the KT150 new pentode tube.
The JA30, a pure class A 45 watt per side amplifier, is a very effective monoblock. Each tube is protected by a fuse and is self-biasing even when tubes are replaced, no adjustments are necessary (but the tubes have to be paired). Output tubes can be either 6550s or KT88s and the amps are designed to deliver full power at 15Hz, making for extraordinary bass.
When when switching from KT150 to another type of power tube such as : EL34, 6CA7, KT88, KT90, KT120 & 6550 a small modification has to be made inside the unit.