Hi Gary, it sounds like you are close to success on this project.
On the issue of efficiency, it will be the better at higher drive levels
for a couple of reasons. One, as drive is lowered, the plate current swing
is reduced and that pushes the class of operation more in the direction of
class A and away from class B. And two, lower plate current swing means
higher plate load impedance when tuned for maximum power. Higher plate load
impedance in turn increases the loaded Q which is accompanied by higher tank
losses.
As the parasitic suppressors are in series with the rf current flow,
heating HAS to be a function of current squared times resistance. So an
increase
of resistance will cause an increase in dissipation. If the shunt
inductance is made smaller, more of the series rf current will flow through the
inductor and that of course forces the resistor current to decrease. Since the
dissipation is related to current squared, a 30% current reduction in the
resistors will yield a 50% dissipation reduction.
73,
Gerald K5GW
In a message dated 1/21/2011 10:19:48 P.M. Central Standard Time,
garymyers@powerc.net writes:
Hello....
I've just completed converting a Heathkit HL2200 amp, stripping all the
tank out as well as the input ckt. I left the existing plate tuning cap in
and am using it as the load cap. The plate tuning cap, now where the load cap
was, is a 100pf variable. This was all down according to an April '08
article on the same. The input is a T network and it by the way works
perfectly
with a 1:1 SWR.
The suppressors are as the article says - nichrome ribbon, 1/4" wide
formed into a U with 3 2W carbon resistors in parallel at its base. All
soldered
using silver solder for the nichrome. All seemed to go fine.
After installing the tank components I used a dip meter to try and see if
the tank coil was set right. I first confirmed the meter setting by
checking it on a radio set to 50.1Mhz... and then tried to insert its coil
into
the limited area of the tank coil... it measured 60Mhz! So I squeezed the
turns together - it dropped to 54Mhz... I decided to leave it as is and wait
until I could power it up.
When I first tried to run it I was only able to get about 60W out with
about 20W drive and the plate tune cap was at its minimum setting... I believe
this indicates that indeed the coil is too much inductance (does it not?)
and so I pulled the turns apart as best I could. This allowed me to get
about 350W out (with about 840W in) with maybe 35W drive (I didn't measure
this by itself - just going by where I had the power set on the 746Pro). But
very quickly, before I could increase the input, it started to smell! The
carbon resistors were getting VERY hot.
I quickly removed the cover (obviously made sure the HV was 0) and with a
thermal gun checked the temperature of the suppression resistors - one set
was at about 160 deg and the other set about 125 degrees. One resistor in
the one set appears to be higher than the others - but this might just be the
difficulty in checking with a thermal gun.
So my question is: WHY are these getting hot?! I would have thought, if
the article said use these resistors, that it would work fine. The U nichrome
wire is almost identical to the one they have pictured (mechanical picture
for scale) in the article. Note I couldn't find the 150 ohm resistors they
said - I used 3 180 ohm resistors - but I find it impossible to believe
that it would work with 50 ohms eqv and not 60 ohms eqv nor that the 60 ohm
eqv should dissipate MORE power, not less.
I'm also a bit troubled by the efficiency... but until I get this resolved
I'm not going to worry too much about this - however if someone things it
is related I'd love to hear it.
Thanks for your time.
Gary
K9RX
[the amp was bought from someone that said it had been converted to 6
meters replacing the 10 position and the other bands still there... it didn't
work on 6 which didn't make me happy so I decided to rip it all out and just
make it a 6 only amp... note however it would put out about 1150W on 20
meters before the change.]
ps: Also note that when I remove the cover (the HV short is disabled) I
get 550W out with about 800W in but now the load cap is at its maximum value!
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