|[AMRadio] Ranger... good news, bad news|
John Coleman ARS WA5BXO
wa5bxo2006 at pctechref.com
Sun Nov 5 20:00:26 EST 2006
I was going to comment earlier about the statement that this Ranger
would not hold it's on against the rice box with the SB200. As Jim pointed
out there should not be but a DB or so difference if the ranger is working
properly and properly tuned. I am curious as to what power his ranger will
put into dummy load and what the modulation looks like. I think I read in
this thread about someone reducing the drive on the Ranger's grid. That is
a sure fire way to ruin the output tube (6146). The single 6146 in class C
service as it is in the Ranger should have 2.5 to 3 milliamps of drive on it
at all times. Do not reduce the grid current drive to reduce power on the
Ranger. If memory serves me right the 6146 is to be used at about 65 watts
plate input on AM plate modulation. It should be putting out between 40 - 45
watts carrier and develop about 180 watts PEP. (Could be a little more with
a good modulation XFMR) That's only about 2-3 DBs below what the SB200 will
do as an AM linier when operated properly. Now 3 DBs is not to be taken
lightly when the going is rough but if your going to do a lot of work for a
DB or 2 then put the work in the antenna or a bigger rig that will get you
10 DB gain.
I have used the SB200 on AM and it does OK but I would not want to
do it for a long time. Those tubes get real hot. I use the SB200 as a
driver for my big rig. The input to the SB200 is about 5 - 10 watts and the
output of the SB200 is about 50 - 70 watts carrier this drives the grid of
my big class C plate modulated final. But even using the SB200 to put out
50 - 70 watts it get real hot and the tube show some color. I think the
best thing I can do to increase the efficiency of the SB200 at low power, is
to reduce the plate voltage to about 1500 instead of 2500. I added a switch
to the front panel of mine to increase the bias voltage making it harder to
drive. I thought that this would decrease the conduction angle enough to
reduce the heat but the trouble is that the plate is still not reaching
saturation at the low power level so I figure that if I reduce the plate
supply voltage and increase the drive then I can come closer to the non
linear class C service for continuous CW and the efficiency will surly
increase. The idea is to use the rice box rig at low power so it will last
a long time and have the SB200 to make up the slack. But I want it to last a
long time as well. BJ and I like the flexibility of this type of operation.
That is to be able to flip some switches and use the SB200 as linier on SSB
as it was intended and then flip the switches back so as to lower the output
but raise the efficiency for continuous carrier operation as required by the
big class C final. We have been operating it linearly with the 2500 volts
supply and reducing the drive from the rice box carrier source but it has
always bothered me that it is so inefficient and those tubes show color. As
I recall the PS in the SB200 is a voltage double type circuit perhaps a
little circuit change up with some HV switches might be in order here?
Any Ideas on this?
BTW the stock plate RF choke in the SB200 is NOT large enough on 75 mtr
and a lot of RF gets back into the PS causing weird AC modulation at low
carrier levels. BJ and I thought it was the Filter Caps so we got new ones.
No help. We found that the weird modulation on the output carrier would
come and go at different levels of drive at low power and only on 75 mtrs.
We measured the DC at the POWER supply B+ point as it went through the
chassis to the plate choke and found some RF there and guess what the RF was
getting back to the diodes in the PS and modulating with the AC. It was
real weird. Placed a RF choke under the chassis and an extra RF bypass
cured the whole thing.
73, John, WA5BXO
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