Francois...... Eimac and the other makers of the 3-500Z? used this chemical, called? 'Zirconium'? ?on the anodes.? ?The Zirconium coated anode IS the getter.? It is heat activated when the tube gets red.?
IF you want to operate the L7 at only 500-600 watts pep output....... use the LOWER? 1900 no load? B+ (CW)? position.? IE: 500-600 ma.....and aprx 1770 vdc loaded.? Then you will get better efficiency.
With 7.2 vdc of bias, and the lower 1900 vdc position used, the tubes will idle at just 40ma (normal).? ?IMD is superb? with 600 watts pep out.? I have measured the IMD on my spectrum analyzer.
Yes, you can use a Zener diode...BUT? remember that the zener is in the cathode..... and cathode current is the sum of the DC plate current and DC grid current.? ?IE: 800 ma of dc plate current and 300 ma of DC grid current = 1100? ma of cathode current.? ?1.1 amps? x 7.2 vdc (zener vdc) =? 7.92 watts of zener dissipation.? ?Zener's require a heatsink.? A 10 watt zener will run warm to hot.? ? You could use? 2 x 10 watt zeners in series, like a 4 vdc zener and a 3 vdc zener.?
The reason we use 10 x 1N5408 diodes in series for bias is..... they are rated for 3 amps CCS? and have a 200 amp surge rating.? ? The 10 x diodes can be mounted either horizontally or vertically...on some small perf board.? Non critical, since the vdc is so low.? 6A10 / PM600 diodes are rated for 6 amps CCS...and have a 400 amp surge rating.? ?Diodes are cheap to buy,? and do not require a heatsink.?
And yes, I use 2 x 100k? @ 3 watt MOF resistors in parallel (=50k)? ?for the cut off bias.? That's called....'redundancy'.? IF one resistor went open, you still have the 2nd resistor in the circuit.?
Jim? VE7RF
