FYI The zero signal plate current with 2000 volts on a 8874 is 22ma per
tube. The Alpha would have 66 ma of idle current.
Jim W7RY
----- Original Message -----
From: "Carol Richards" <n2mm@comcast.net>
To: "Jim W7RY" <w7ry@arrl.net>
Sent: Thursday, July 30, 2009 8:33 PM
Subject: Re: [Amps] Alpha 76PA Plate Current
> Jim...
>
>
> I have a 76PA. Even with the meter mod which stops the pinning, my plate
> idling current is about 200ma. The original Alphas, circa 1978, were
> designed to operate at 1kw input (that was the rule in those days. To
> accomplish this, the mannual specified using it in the cw mode. Now that
> the
> limit is 1500w output, we use it in the ssb mode for that power level and
> lower grid current (Higher Plate voltage) The meter mods were put in
> because
> 2000 volts at 1A gave 2000w input which was close to 1500w output. This
> way
> the meter wouldn't pin.
>
> As for the 400ma idling current, that is quite high. I would suggest you
> contact Dick, N4UQ. He Is great at repairing alphas. I sent him 2 of my
> alphas and he did a super job. If anyone would know why your idling
> current
> is so high, he would. You can reach him at : 404 317 0245 or
> service@n4uq.com.
>
>
> Carol
> ----- Original Message -----
> From: "Jim W7RY" <w7ry@arrl.net>
> To: <amps@contesting.com>
> Sent: Thursday, July 30, 2009 11:03 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@contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>
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