11 diodes (7 volts) is way too much bias voltage.
Better to have about 3 diodes (1N5408). That is what I have found while
designing my QSK circuit board for the Drake. The L4 and L4B was designed
with Amperex 8802 tubes. They did not require
bias because the idled at a much lower current than Eimac 3-500 tubes. And
the old L4 was designed around the 3-400 (8163) which did not require bias
either.
Sure, if you have 3800 volts of plate voltage then 7 volts of bias is fine.
The SB220 used a 5.1 volt zenner and the SB220 plate voltage was around 3200
volts.
73
Jim W7RY
-----Original Message-----
From: Paul Baldock
Sent: Wednesday, December 24, 2014 1:23 PM
To: mikflathead@aol.com ; amps@contesting.com
Subject: Re: [Amps] L4B biasing
Yes you should add bias. I added 11 diodes in the center tap of the
filament trany. So that would be about 7 volts.
- Paul
.At 12:52 PM 12/24/2014, you wrote:
I just finished me Drake power supply. I used a Drake transformer and a
doubler board with 8 caps. I have 2600 volts showing on the high voltage
meter. I have eliminated all the bias circuit in the power supply that
Drake used. At idle I show 200 mills of plate current. My question is
should I add a bias circuit? I purchased a W4ZT board but wonder if I need
it. Thanks and Happy Holidays to all.
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