[Heath] SB-200 loading question.

[Kenneth G. Gordon]

On 10 Dec 2016 at 7:27, kevin_ward at juno.com wrote:

> You say you "run out of loading capacitor before achieving full load", and "the loading capacitor 
> is up against the full-capacitance stop."  I believe increased loading is achieved by reducing the 
> loading capacitance of the tank circuit, not increasing it.

You are absolutely correct. I was thinking "backwards". Gee....

Sigh....sometimes I think I am too old...I forget simple things....

>  This would require readjusting the plate 
> tuning (to dip the plate current).  The point to stop this process is when the output power no 
> longer increases.  You are watching the output power, aren't you?  You didn't say what the grid 
> current was.  

I keep the grid current to as low as I can keep it, consistent with proper output: about 600 
watts for a new set of RF Parts 572Bs. Normally, the grid current is about 35 mA per tube.

I believe that GRID current is the single most important parameter one must watch for GG 
amps....which is the reason I find the Yaesu FL-2100 so interesting: there is no way to 
measure grid current in that amp.

 > I believe Ken's (WA2LBI) suggestion about the dummy load is on the money, BTW.  

Agreed.

> Most amplifiers work most efficiently at their lowest operating frequency.  Full output is then 
> achieved with the lowest amount of plate current.  If you are getting a full 600 watts output, but 
> the plate current is less than expected, be happy.  Your amp will run cooler and the tubes will 
> last longer, as long as the grid current is not too high.  

As I said above... :-)

> One other thing.  What are you using to check SWR, and where is it connected?  Is it between 
> the amp and the tuner, or between the exciter and amp?

Ha ha! Amp and tuner, of course.

>  I am assuming it is after the amp 
> because most Heathkit amps have nothing like a 1:1 SWR at their input.

Correct. Unless they have been modified correctly. Mine has been. Input SWR is normally 
1.5:1 or less in those portions of the band in which I normally operate.

>  The point here is that if 
> the input tuned circuit favors the high end of the band, that can prevent the amp from achieving 
> full loading at the low end.  

Yes.

> Do keep the list informed.  We will all be interested to know how you make out.  

Well, Kevin, as you so astutely pointed out, loading increases with REDUCTION in loading 
capacitor capacitance.

I was thinking backwards.

What I do on 80 meters is adjust for maximum power output, yet not exceeding 600 watts 
and setting for lowest possible grid current while achieveing that output.

I then check plate current, and it is always below 500 mA on 80 meters.

I am happy now.

FYI, I also doubled the inductance of the filament chokes.

Thanks for the advice and reminders, folks.

Ken W7EKB

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