[AMPS] Henry 3KD on 160m

[Rich Measures]

>        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

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---


R. L. Measures, 805-386-3734, AG6K   


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