Conditioning GU74b / 4CX800A transmitting tubes before installation April 2012
Many common amplifiers, e.g the Alpha 91B and the Acom 1000, use GU74 / 4CX800a tubes . These tubes were manufactured in the tens of thousands in the mid to late 20th Century, so all those available in 2012 are N.O.S. (new old stock) but have lain on warehouse shelves for 25+ years.
Before using in an amplifier, they have to be 'conditioned' - basically just applying filament/heater voltage only, for about 12 hours. The heater in a GU74b is 12.6V @ 3.6A = 45W. It would probably be OK to just let the anode of the tube dissipate this small amount of power without air cooling, but I decided to make a simple set-up which would keep the tube temperature down during the conditioning process.
Make a thin cardboard chimney for the tube - about 70mm high and slightly loose when pushed onto the tube anode (inside diameter of chimney 74mm approx)
Connect croc-clip leads to pins 3 and 7 of the tube as in the photo.
Use a small 12V DC fan (80mm x 80mm) from an old computer.
Use the original 4 mounting screws to lift the fan about 5mm off the bench surface, to allow cold air to be drawn into the fan from below and then pushed up through the cardboard chimney and tube anode.
Using Blu-Tac or similar material, make a 5mm thick sticky pad in the (fixed) centre of the fan, into which the pins of the GU74 will stick, holding the tube vertical.
Push the chimney down as far as it will go.
Connect the fan (red and black leads in the photo) to the +12V PSU and the tube filament (yellow and black leads in the photo) to the +12V PSU. I used a modified computer PSU for this.
About 15 minutes after switching on, the tube will have reached a temperature of about +35oC and should remain at that temperature for the rest of the 12 hours.
Disconnect the heater wire first, then leave the fan running for 5 mins to cool the tube (this is a completely un-necessary step, which just shows my obsession with cooling transmitting tubes!)