[Amps] Need Smarter Than I Am Help.

Sun Oct 10 12:24:11 EDT 2021

I did this once on a single-tube 3-500Z amp (an AL-80A) and raised the 
B+ about the same amount. A few back-of-the-envelope calculations showed 
that I needed more L in the tank circuit. It used a Pi-L configuration 
on the lower bands. All of my changes were fundamentally empirical.

Originally, the B+ was 3100 V no-load, 2700 V loaded. It is now 4000 V 
no-load, 3600 V loaded. At 2700 V, the plate impedance is about 3500 
ohms, but at 3600 V itï¿½s about 5000 ohms. To maintain the same loaded Q 
the output network needs more inductance. For all bands except 160 m, 
coil taps can be moved but for the 160 m band there are no taps. Thus, a 
small powdered iron toroid of approximately 10 uH has been added to the 
160 m inductor. I mounted this toroid onto an insulating pad underneath 
the 160 m coil. I moved coil taps around until the load control was 
about in the middle of its range on all bands. 15 m and 10 m didn't need 
much (if any) change, but I added inductance on all other bands. It was 
easier to drive and, by the book, had slightly better 5th order IMD at 
the higher B+. Make sure cooling is sufficient because you'll stand to 
make a fair bit more heat.

A drawback is that the RF voltages on the bandswitch are elevated quite 
a bit, so the amp will be very unforgiving of light loading. I never had 
a problem, but I always tuned the amp carefully after that making sure I 
was never in a lightly-loaded configuration. Overall, I realized a 
whopping 1.5 dB increase in power output from my mod, but I gained a LOT 
more than 1.5 dB in knowledge!

Kim N5OP

On 10/8/2021 10:10 AM, Joe wrote:
> As many of you here know, I have been making some MAJOR changes to 
> this Heathkit SB-220 amp.
>
> Many here, and many other places, have helped me both on these E-Mail 
> lists as well as directly.
> I am extremely grateful for all this help!
> These awesome people have given this guy that sometimes just does not 
> understand something a LOT of help.
>
> I won't waste space here, unless people here want the details.
>
> But I need help with the final last steps on bringing this AMP back to 
> life.
> The AMP works, I just want to make sure it is working correctly or in 
> it's best way.
>
> The help I need with is because of the last major change I have made 
> to this amp was the High voltage section.
> It is totally re-built, with a Major change. The High Voltage is now 
> roughly 3600 volts.
> This increase of voltage has thrown The Pi-Output network all out of 
> whack, because of the Plate load impedance has changed sooo much.
>
> So the first thing I want to confirm, is my calculated value of what 
> the plate load resistance
> will be for a pair of 3-500Z's running at that voltage. For when 
> running the reverse path output network testing.
> You know, lift the plate caps off the tubes, and at their common 
> connection where the two parasitic suppressors join,
> Run this value of resistance to ground. Then you can activate the T/R 
> relay, and using a VNA, or like an antenna analyzer,
> you can actually adjust the Pi-Output network to get a min SWR, so now 
> you know where the whole system matches and is happy.
>
> Obviously to make this test meaningful, I need to be positive, what 
> the value of the resistance to ground should be.
> So Looking for these second opinions of what that value should be. And 
> if you wish, explain how you got that value?
>
> Now, at least that above question, I am fairly confident on the value 
> I have calculated is right,
> or at least close enough it won't make that much difference.
>
> But this next one, or two, kind of co-related, I am 100% confused 
> about. A few have tried to help,
> but it is like they are speaking a space alien language.
>
> OK the amp has Standard old Eimac 3-500Z's a pair of course.
> Stock Caps on both Plate and Load.
>
> The Inductor Now is a 0 to 18uH Roller Inductor.
>
> The output side of the bandswitch, the ONLY thing it does now is to 
> add the C56 Cap to ground when on 80 Meters.
>
> Now here is the confusion.
>
> I have had Tube gear all my Amateur life since 1975, Drakes, 
> Heathkits, etc. And amps, Drakes, Heath's, Henry's etc.
>
> And when tuning them up it has always been that the Plate cap amount 
> of "C" always depended on the frequency of use.
>
> On 10 meters it was hardly any "C" at all. And as you went down in 
> Freq, you would have more and more "C" being used.
>
> And that's the issue! On 10 meters, With that Roller Inductor, I can 
> get a perfect SWR and I mean PERFECT, 1 to 1, Not even 1.01 to 1, but 
> 1.00 to 1.
>
> BUT..... and there always is a BUT isn't there?
>
> I can get this perfect match, with a Plate load Cap fully un meshed 
> (min capacitance) and easily as far as 50% meshed (like where it would 
> normally be for 40 meters) and anywhere in-between! just by using a 
> different amount of "L".
>
> What is the "BEST" setting? I mean it is a perfect match, but running 
> it with the cap like it would be for 40 meters doesn't seem right even 
> tho the match is perfect.
>
> So then I am told this is correct, but it all depends on the amount of 
> "Q"
>
> UG! "Q" This I have spent countless hours trying to understand and 
> still can not seem to grasp what "Q" is?
>
> What makes a "Q" of 10 a "Q" of 10.
>
> If you want a different value of "Q" What do you change to make the 
> "Q" change?
>
> How about even a simple question,,ï¿½ How do you measure to learn what 
> amount of "Q" is happening?
>
> And I guess a Important question,,, What level of "Q" does someone 
> want to have in an AMP like this?
>
> I hope someone is willing to try to help this old fart to try to 
> either understand this, or at least help me be an appliance op and 
> just get it set right so I can use it safely for the amp he he he.
>
> Anyone?
>
> Joe WB9SBD
>
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-- 
Kim Elmore, Ph

Kim Elmore, Ph.D. (Adj. Assoc. Prof., OU School of Meteorology, CCM, PP 
SEL/MEL/Glider, UAS, N5OP, 2nd Class Radiotelegraph, GROL)

/ï¿½I occasionally play works by contemporary composers and for two 
reasons. First to discourage the composer from writing any more and 
secondly to remind myself how much I appreciate Beethoven." ï¿½ Jascha 
Heifetz/