I brought seven Russian built Priboj amplifiers back to Sweden from Estonia. All except one was sold and the last one was modified according to plans made up by A. Likhnitsky (the constructor of the AML+ amplifier). From a 60W PP amp using dual beamtetrodes (6R3S-I) it was turned into a 12W trioded amp using the same OP tubes.
The original Priboi was a heavy - 17kg - amplifier with heavy transformers but cheap power tubes rated approx 2x50W. The original 6R3S-I are a commercial version of a more reliable tetrode aimed for radio transmitters. A change of tubes could be recommended and worth a trial. Suitable tubes could be Russian GU50, EL34/6CA7, KT88/6550 or even heavier KT-versions like KT90 or KT120.
What Likhnitsky did was to hook up the 6R3S as triodes and bias them to run in class A and lower the B+. The latter was done easily. There are two HV-windings on the tranny, each feeding a rectifier and filter and then the resulting voltages are added and wired to the amplifier. I principle the connections between the two rectifiers are broken up and each is feeding one channel, thus resulting in a sort of dual monoblock.
Also the 6ZJ32 + 6N6P tube combo is replaced by a single 6N1P.
Out of the blue, a crazy idea: buy a pair of PCBs for any of Dynacos EL34 or KT88 amps and thro them in!
Description in Swedish (PDF)