> Good testing Chuck. You confirmed what I had said earlier
> this week. The
> maximum plate load impedance for the 852 is around
> 1400-1500 Ohms and
> responds best down in the 1000 range.
Bob,
After just having dinner with my wife and here sister, I'm
probably a little too sensitive about drama. A helicopter
lands down the street, and the drama queens say it landed in
the front yard. A two inch Pine Vole comes up and eats an
apple, and I have to cut the whole tree down because it
attracts giant rats.
It's the same with tank circuits. People write as if there
is some mystical magical point where the tank quits working
on the hi-Q side of minimum Q. People are fixated on that
exaggeration.
Let's look at the real numbers in tank with a Q of 12 at
1500 ohms with typical vacuum caps and a B&W coil and see
how much is fact and how much is drama:
3.75MHz
Anode loadline 1500 j0
Anode RF Power 1500W
Anode required C 330.2 pF
L 6.45uH
Load C 1632pF
Loss 2.8%
Phase delay 147.5 degrees
Effective Q 12.0
Inductor current 11.6 amperes
3dB bandwidth (without retuning) .31 MHz
Now let's increase the anode loadline to 3000 ohms and see
if the tank "stops working"
3.75MHz
Anode loadline 3000 j0
Anode RF Power 1500W
Anode required C 320.2 pF
L 6.45uH
Load C 2400pF
Loss 5.3%
Phase delay 158.5 degrees
Effective Q 23.8
Inductor current 15.8 amperes
3dB bandwidth (without retuning) .16 MHz
It doesn't "quit working". The anode capacitor doesn't
change, the loading capacitor increases to about 1.4 times
the original value, the loss increases from ~3 to ~5
percent, the bandwidth (selectivity) doubles.
Now let's look at the same component quality and a 3000 ohm
tank at an effective Q of 12.
3.75MHz
Anode loadline 3000 j0
Anode RF Power 1500W
Anode required C 166.5 pF
L 12.1 uH
Load C 992pF
Loss 2.7%
Phase delay 134.3 degrees
Effective Q 12
Inductor current 8.2 amperes
3dB bandwidth (without retuning) .31 MHz
For all the popular drama that "without a Q of 12 the amp
will stop working", the overall efficiency of the amp might
change from 65 to 63 %.
The REAL major effects we would notice are only in the value
of the capacitors required. It doesn't stop working. It
isn't "only good for 1500 ohms".
If I had an amp as nice looking as the one Chuck built, I'd
just increase the size of the loading cap and ignore the 2 %
change in overall efficiency. The loading and tuning caps
should have at least 50% capacitance headroom anyway in case
the load isn't perfect.
73 Tom
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