[Amps] Alpha 76PA Plate Current
km1h at jeremy.mv.com
Fri Jul 31 06:19:06 PDT 2009
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.
----- 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
>> 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
>> 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
>> 2000 volts at 1A gave 2000w input which was close to 1500w output. This
>> 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
>> is so high, he would. You can reach him at : 404 317 0245 or
>> service at n4uq.com.
>> ----- 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
>> 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
>>> 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
>> fix the problem.
>>> I have run a spot check on the metering resistors and they seem to
>> ok. Has anyone actually fixed the plate current metering circuit?
>>> Jim W7RY
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>>> Amps at contesting.com
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