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[Amps] 813 Amps

Subject: [Amps] 813 Amps
From: Dennis W0JX <>
Date: Wed, 3 Jun 2009 15:01:10 -0700 (PDT)
List-post: <">>
First, congratulations on trying to make the 813's go. Great choice of tube for 
a medium power amp.
The fact that your loading cap is higher than expected means that your 
operating Q of the Pi output is probably higher than you planned for. You did 
not mention what the input cap and plate L are on 160. See the following on 
line calculator to arrive at plate load and the proper components of Cin and L:
Did you check the accuracy of your wattmeter? Mine was reading way low until I 
bought an LP-100A. This resulted in my amp efficiency coming into a reasonable 
Also, is your plate voltage measurement accurate? I use a calibrated HV probe 
on a digital meter.
What kind of plate choke are you using on 160? That can really affect the Cin 
BTW, your efficiency is even lower than you think. Part of that 100 watts of 
drive is included in the 950 output.
Good luck with your efforts. You should be able to get 62%+ in class AB2.
73, Dennis W0JX/8

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