For this new tube preamplifier tube project Thomas Mayer built a test set up, that can operate with variety of directly heated tubes via UX4 base and also tubes with other bases can be plugged through a socket adaptor. This working prototype has two pairs of different line output transformers, selectable via switch. One is for tubes with lower plate resistance and the other for those with higher rp.
Bias is switchable between conventional cathode bias and filament bias and different bias resistors are selectable via a set of switches. Filament and plate voltages can be monitored via jacks at the top plate.