[Amps] 813 Amps

Dennis W0JX w0jx at yahoo.com
Wed Jun 3 15:01:10 PDT 2009

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 range.
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 value.
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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