Cathode Compartment QBL 5/3500 VHF Amplifier
tube socket details
This is an overview of the cathode compartment.
The large brass plate in the middle is part of the Ug2 decoupling capacitor. This capacitor consists of three plates in a sandwich arrangement with Teflon as insulation. The top one is grounded, the middle plate connects to the screen and the chassis is the third plate.
The top plate consists of two identical plates on top of each other. A sheet of brass foil is pressed between them and protrudes on both sides. The brass bars press the sheet to ground, providing excellent rf grounding. This is necessary, because the cathode connects to this plate as well.
Note that the filament connectors are decoupled by capacitors and a 1/4 wave coax stub. Go to tube socket details to see more details.
The filament transformer is made of a 12/24 V transformer with a secundairy rating of 160W. The 12V and 24V windings were removed. (make sure you find a transformer with the 230 V AC windings directly on the core!!).
Two insulated copper wires of 3,5 mm diameter are wound in parallel on the core. The wire loop in the top right corner is "spare" so in case of old tubes an extra winding (= additional voltage) is still possible. Given the current of approx. 35 A the length of the wires is of influence on the voltage which can be measured on the tube connector. For correct setting and small adjustments of the filament voltage a 25 W resistor of a couple of Ohms can be inserted in the 230 V AC primary circuit.
The little transformer in the middle provides the 12 V DC voltage for the start-up circuit and the blower just left to it. This blower provides an airflow over lower part of the tube.
The capacitor in the bottom right corner is used in the main blower circuit.
On the right hand side the start-up relay and 25W start-up resistor can be seen.