Hi Jim.
I tried a large value electrolytic across the bias string and
every time I keyed up, the ZSAC would take a while to stabilise at
the set level. When I stuck something like a 22uF in its place and
it seemed to be better. Without trying to work out time constants
and such, I figured that the 22uF would still provide the good
dynamic regulation during speech and CW transmissions. Dunno if I
did something wrong that initial time but for better or worse,
I've always settled on the lower value caps since then.
Cheers and thanks, Alek VK6APK.
On 28/02/2025 8:23 pm, Jim VE7RF via
groups.io wrote:
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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.?
