Jim Henderson wrote:
>
>> Have a 3KD that won't load to over 400w into a known good load,
>>works FB (1500w) elsewhere. Seems to run out of tune/load range on 160m.
>>
>> Consulting the schematic, I find it calls for 675pF combined but
>>there is only 650 in-circuit. The caps on both 40, 80, and 160m switch
>>positions differ slightly from the schematic as well. Any 3KD users on this
>>group, or with experience on this one, I would like to e-mail with you.
>>
>> This unit was rebuilt by Henry recently, supposed to be stock and
>>working. The 3KD Guru at Henry has been hard to find, so users' comments are
>>appreciated. Perhaps it just needs another combined few pF, but the
>>discrepancy between schematic and actual bears investigation.
>>
>>73 de Jim, KF7E
>
And Rich Measures replied:
>The first thing I would check is the operating Q of the tank. If Q is
>too low. or too high, power output decreases. Tank operating Q should be
>kept between 10 and 15. (*Care and Feeding of Power Grid Tubes*). In
>order to determine the operating Q, I would:
>- Tune for max. output on 160m using full drive.
>- Shut the amplifier down, pull the mains' plug and wait until the
>filter C discharges.
>- Disconnect the tank L from the tune C.
>- Measure the total C at the tune capacitor.
>- Calculate the capacitive reactance (Xc) at the operating frequency.
>- Calculate the output load R of the tube or tubes (R-load).
>- Q= R-load/Xc. If the Q is outside Eimac's recommended range, I would
>correct the problem.
>
>Another possible source of low output might be lack of cathode drive due
>to high input SWR. How much anode current is indicated on 160m, Jim?
>Rich---
>
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And Bill Turner added:
Rich's advice is good, but I would caution you about the step that
says "measure the total C at the tune capacitor". This is trickier
than it seems. Ideally, you would use a capacitance meter that uses
1.8 MHz energy for the measurement. Most capacitance meters use a
much lower frequency and can easily lead you astray due to the
presence of the plate RF choke, which has a bypass capacitor at it's
lower end. If you use a typical hand held DMM, it will use a fairly
low frequency, thereby "seeing" the bypass capacitor and any other
capacitors connected there.
If you simply disconnect the RF choke, you will then get a true
reading of the capacitance only, but remember, the RF choke has an
effect too since it has relatively low reactance at 1.8 MHz. What you
want to know is the total capacitance including the effect caused by
the reactance of the choke at 1.8 MHz.
Probably the best way for an amateur to make the measurement is with a
grid dip meter. Connect a slug-tuned coil from ground to where the
tank circuit was previously connected and resonate it at 1.8 MHz by
adjusting only the coil - don't adjust the capacitor. Then remove the
coil and connect a variable capacitor to it and re-resonate it at 1.8
MHz using only the capacitor - don't adjust the coil. Now you can
disconnect the variable capacitor and measure it with your DMM.
Whatever it measures is the same as the net capacitance in your amp,
including the effect of the RF choke. Now you can figure Q using
Rich's formula.
Doing it this way is a bit of trouble, but you will get a correct
value for the net capacitance.
73, Bill W7TI
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