next to the ST70 stereo amp, the MK3 mono amps (60wpc) were the most popular product and are still in great demand today.
The MK4 mono amp (35wpc) wasn't as popular and has become slightly rare but is a great product.
The ST35 stereo amp is a fantastic little amp that really is rare, and gives great performance with lower power (17wpc).
All of the parts for these amps are available (even new reproduction).
Tubes4HiFi also does offer an upgraded VTA driver circuit for the MK3 and MK4, or can offer a new original style replacement PCB for the ST35
Roy www.tubes4hifi.com