Francois,? ?with zero bias on the L4B, using Eimac 3-500Z's ( MU=130)? the? idle current is 220-230 ma = 575w to 600 watts of idle power.... = blast furnace.??
If using Chinese 3-500Z's ( MU=200).... idle current is a little less.?
I installed 10 x 1N5408's in series? ?for bias on my L4B's.? ? ?Idle current is now 100 ma with 2600 vdc....and just 40 ma with 1900 Vdc.??
Linearity with 1290 watts pep out on 75m is superb.? ?Linearity with 625 watts pep out (CW / 1900 vdc lower B+)? is also superb.?
I tested 1N5408's? with my lab supply and a resistor in series...on the bench...with 10 in series.? ?They run fairly warm with 1000 ma? continuous . On the Drake L4B / L7...on SSB it pulls 800 ma on peaks.....but only 400 ma average? DC plate current ( with processing).?
The bigger 6A10 diodes run warm? with 2000 ma? continuous.??
The bias issue is a dead simple fix.? Just wire 10 x diodes in series for the bias.? Forget the zener.? ?Zener's are not adjustable.? ?Zener's? won't handle surge currents like 1N5408 (200 amp surge) or 6A10 (400 amp surge) do.?
As you pull more current through the string of diodes, the V drop across each diode will increase slightly.? If you want to be a purist, install a? 500 uf to 1000 uf lytic across the entire string of diodes. Then the V drop is constant, and won't budge.?
IF using Chinese 3-500Z's ( MU=200),? 7-9 diodes will be required.? ?Install 10 x diodes.? ?Then it's easy to tap the diodes.? ?( shunt the last 1-2 diodes with a wire).?
Wire a .01uf @ 1 kv bypass cap? from each end of the diode string....to chassis.?
With the lower idle current, it will take a HUGE load OFF the power supply.? ?With the pair of 50K @ 50 watt wire wound bleeder resistors removed from the? B+ supply (and also the 5k @ 7 watt)? you will remove another? 63 watts from the power supply.?
Stock, keyed, but no drive applied, power consumed by the B+ supply was 663 watts.? ?After the bias diodes installed, and 50K bleeders removed,? ?power consumed by the B+ supply is just 260 watts. ( keyed, no drive applied).
663 watts - 260 watts =? ?403 watt? reduction in power from the HV supply.? ?That's a HUGE reduction in power.? ?The L4PS / L7PS top cabinet lid will then run cool to the touch.?
