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 it�s 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,,� 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)
/�I 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." � Jascha
Heifetz/
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