Date: Sun, 4 Nov 2018 20:49:18 -0600
From: Jim <jimw7ry@gmail.com>
To: amps@contesting.com
Subject: Re: [Amps] power limiting in 8410 amps?
<The Drake L4B only need 2-4 diodes for bias. The less ZSAC the harder it
<is to drive.
<73
<Jim W7RY
## I have bucket loads of drive form the 200+ watt output xcvrs. I
installed a
miniature spdt CENTER OFF toggle switch on the rear apron of the L4B RF deck.
Then used the switch to tap the string of 10 x diodes. Center off
position means
all 10 x diodes in the circuit. Other 2 x positions tap the string at 4 x
diodes..and 7 diodes.
## with all 10 diodes in the circuit, idle is reduced to just 100 ma with B+
in ssb position ... 2650 vdc.
And just 40 Ma...when B+ is in CW position.... 1900 vdc. Zero
complaints of IMD since day 1.
Depending on MU of tubes used, the thought was bias would need to be tweaked a
bit. With the grids
directly bonded to chassis, tubes like 4-400 A /B/C can also be used..in
which case, they would be run zero bias.
## The extra drive power to overcome the added bias of say 10 x diodes..vs
2 x diodes is not very much, 10 watts.
100 watts = 80 volts into 65 ohms. 65 ohms is what the cathode Z is.
110 watts = 84 volts into 65 ohms. The extra 6 diodes is just 6 x
.725 V = 4.35 volts. Another 10 watts required
to ovecome the 4.35 volts of extra bias.
But, the Drive power is reduced a bunch, typ 20-25 watts... when all 6 x
grids are bonded to chassis.
## On a side note, if the choke is removed from the grid pin on each tube..
carefully remove it, as they
are good quality chokes. Forget the exact inductance, but the DC resistance
is only .9 ohm on them, which is superb.
The chokes used on the heath SB-220 measures 25 ohms.
Jim VE7RF
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