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Re: [Amps] AL-80B questions

To: "" <>
Subject: Re: [Amps] AL-80B questions
From: R L Measures <>
Date: Mon, 26 Jun 2006 10:26:24 -0700
List-post: <>
On Jun 26, 2006, at 8:44 AM, "" <> wrote:

> Hello Gentlemen,

some are, some not.
> I have a few questions about an Ameritron AL-80B amplifier that I  
> converted for
> 50 MHz mono-band service.
> The first question goes to tuning the amplifier.  I am using a  
> Vacuum Variable
> cap, 3 – 30pF on the plate and the original air variable plate cap  
> has been
> moved to the load position.  With 60 watts of drive, I achieve full  
> amplifier
> output of approximately 900 watts with a grid current of 65ma and a  
> plate
> current of 575ma.  This happens with the plate cap at minimum and  
> the load cap
> at ½ its range or set in the ‘middle’.  I can achieve the same  
> output power
> results by ‘screwing in’ the plate cap and turning the load cap  
> counter
> clockwise.  This setting yields a grid current of 100ma and a plate  
> current of
> 450ma.  The latter produces currents more typical of how it  
> operated as an HF
> amplifier.
> I do not have an O scope to check linearity.

An oscilloscope won't.  Listening 4KHz above and 4KHz below the  
transmit window when using human voice modulation will give one a  
fair idea of how many db down the feculence is.

> That is on the list of items to
> purchase.  The first tuning method produces a more ‘linear’ gain  
> reading up to
> 800 watts output.  In other words, if I graph the input wattage  
> against the
> output wattage, the resulting gain stays very close up to the 800  
> watt mark.

A healthy 3-500Z noticeably looses linearity above 420mA, which  
should be no surprise since the rating for linear service is 400mA  
max.  At 3000v, this takes roughly 70w of drive.

> The second tuning setting results in this gain measurement  
> fluctuating up and
> down and drops of more at the 800 watt mark.  Any comments as to  
> how this
> amplifier should be tuned?

I would tune by adjusting the drive to produce 400 to 420 anode mA,  
then adjust the Tune and Load caps for max RF out.  Once this is  
accomplished, the drive level should not be changed.
> ...
> The third question, when using the ALC,

I would not use ALC.  A better fix is to add a RF-NFB resistor in  
series with the cathode's  input coupling cap.   To not overdrive  
with 100w, it takes 20 to 25 ohms.

cheers, Doug

R L MEASURES, AG6K. 805-386-3734

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