From the manual:
The Telstar is partially cathode-biased (self-bias) at the output stage and does not require a bias adjustment when changing tubes. Each output tube has its own self-bias network. The stock octal-tube is a 6L6 which we find the most versatile match to the EL84. Any brand 6L6 will work. You may also use 6V6, KT66 or EL34 types in this location without rebasing the amp. Each of these has its own vibe and each will provide slightly different matches to the output transformer.
The attenuator:
Attenuator: when the knob is full CW, the attenuator is removed from the signal path and the amp produces 17W to a load. When the knob is turned CCW, the attenuator becomes active and will linearly decrease from 17 down to trace mW.
The Attenuator is a power L-pad and has a slightly coarse feel and action, due to the contour of the wire-wound component mating to the wiper of the L-pad.
At the very bottom of the track there is a slight jump from minimum power to zero, as the wiper exits the wire-wound assembly to the stop of the L-pad.