[Amps] Alpha 76PA Plate Current

Jim W7RY w7ry at arrl.net
Sat Aug 1 09:50:11 PDT 2009

Here is what I have found on the Alpha 76PA metering.

The 5.1 volt zener was bad so the tubes had no bias. After replacing the 
diode, I put my Fluke meter in series with the plate supply to the tubes. 
The idling current is 130 mA. The meter reads slightly high... about 190mA. 
At 900mA shown on the fluke, the meter on the front panel reads about 930mA.

Bottom line is the metering circuit is working correctly... Have not decided 
if I want to put in the resistor to double the meter reading or not.

Thanks for all the suggestions and input.
Jim W7RY

Thanks for all the suggestions.
----- Original Message ----- 
From: "Jim W7RY" <w7ry at arrl.net>
To: <amps at contesting.com>
Sent: Thursday, July 30, 2009 8:03 PM
Subject: [Amps] Alpha 76PA Plate Current

>I have acquired an Alpha 76PA with 3 8874 tubes. I get 1500 watts output 
>with 75 watts drive but the plate idling current in the SSB  position is 
>almost 400 mA. At 1500 watts out, the meter is against the pin! Grid 
>current is around 75 mA.
> Doing some searching on Google, I see the solution to the plate current 
> meter indicating high plate current is to put a plate current doubler on 
> the meter.
> Why not just fix the circuit?  Why does the meter read so high? Bad 
> design? Bad component?  I really dislike fixing the symptom.. I would 
> rather fix the problem.
> I have run a spot check on the metering resistors and they seem to measure 
> ok. Has anyone actually fixed the plate current metering circuit?
> Thanks
> 73
> Jim W7RY
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> Amps at contesting.com
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