[Amps] Alpha 76PA Plate Current

[Carl]

The 76PA and 2 hole CA used the same meter and the "doubler" suggestion 
originally came from Alpha. Mentally doubling the readings shouldnt present 
a mental challenge as compared to setting some other arbitrary FS reading.

The 400 ma idle indicates some sort of problem in the bias or a tube.

Carl
KM1H



----- Original Message ----- 
From: "Jim W7RY" <w7ry at arrl.net>
To: "Carol Richards" <n2mm at comcast.net>
Cc: <amps at contesting.com>
Sent: Thursday, July 30, 2009 11:51 PM
Subject: Re: [Amps] Alpha 76PA Plate Current


> 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 at comcast.net>
> To: "Jim W7RY" <w7ry at 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 at n4uq.com.
>>
>>
>> Carol
>> ----- Original Message -----
>> From: "Jim W7RY" <w7ry at arrl.net>
>> To: <amps at 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
>>>
>>> _______________________________________________
>>> Amps mailing list
>>> Amps at contesting.com
>>> http://lists.contesting.com/mailman/listinfo/amps
>>
>
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