[AMRadio] High Bias for AM linear.


John E. Coleman (ARS WA5BXO) wa5bxo2005 at pctechref.com
Fri Oct 7 09:54:31 EDT 2005


Hi Don (K4KYV):
	I discovered this by accident years ago when I used to run push pull
813s in a conventional grid driven final with a fixed bias with grid leak
and fixed screen supply.  The plate supply was from a BC610 plate XFMR and
the original modulator was 810s with a BC610 modulation XFMR.  I don't know
if any of this matched at the time because it was all I could come up with.
I sure the modulation XFMR was too light because I ran 2500 volts at 400 ma
on the final back then.  Anyway the modulation XFMR gave it's all and
shorted out indicated by a lot of modulation current kick and little
modulation.  I had been using a Knight T-60 with and homebrewed external
modulator (pair of 1625s) prior to getting the big final built.  So I put it
back on the air and decided to push it through the final.  I just shorted
out the modulation XFMR with a clip lead and shoved in the link for max
output (don't recall what the current was but it was high.  I then reduced
the drive from the T-60 until the modulation was good.  That's when I
remembered that the grid bias was beyond cutoff on the 813s and that the
modulation level to the T-60 was very low to hit 100% on the output of the
813s.  
	I though I discovered it.  HIHI  Didn't realize that it had been
done before I was born.

John, WA5BXO

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of John E. Coleman (ARS
WA5BXO)
Sent: Saturday, October 01, 2005 11:26 AM
To: 'Discussion of AM Radio'
Subject: RE: [AMRadio] GG 813 Linear in GE Ham News

I have an SB200 linear and on AM it is equivalent to a DX100.
I increased the bias on it so to the point just beyond cutoff for AM so that
it requires a little more to drive and it reaches negative 100% modulation
slightly before the driver does.  But this seemed to help the modulation
linearity and efficiency just a little.  I really use this position mostly
for CW drive to the push pull 250th rig. But I have occasionally put it on
the air and switched the driver to AM.

John,
WA5BXO



-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Jim candela
Sent: Friday, September 30, 2005 6:47 PM
To: Discussion of AM Radio
Subject: RE: [AMRadio] GG 813 Linear in GE Ham News

Don,

    So your saying that 4 X 811's at maximum ratings in AM Linear mode are
nearly equivalent to 3 6146's class C plate modulated, i.e. a Johnson
Valiant? :-)
Years ago I bought a book from an ad in the old AM Press Exchange about AM,
and there was a detailed explanation of class C plate modulated versus AM
linear operation with the same tubes, and it clearly showed the differences.
I need to go find that book again...

Jim


-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net]On Behalf Of Donald Chester
Sent: Friday, September 30, 2005 1:58 PM
To: amradio at mailman.qth.net
Subject: Re: [AMRadio] GG 813 Linear in GE Ham News




>.hey don,what are your thoughts of am transmission
>with 4 811s?..i'm looking to get more output on 160
>am..

In linear service, the maximum carrier output would be half the plate
dissipation.  The pd of the 811A is 65 watts.  So for a quad, the maxumum
safe carrier output would be 130 watts.

In grounded grid service, some of drive power will feed through, so you
might add 10% to that, bringing the carrier output power to 143 watts give
or take a few.

With the pair of 813's you would add some feedthrough power to the 125
watts.  I would say more like 12.5 watts, to bring the total to 137.5 watts.

Most g-g linears take about 10% of the output power to drive them.

Don K4KYV


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