To: <amps@contesting.com>
>Date: Tue, 20 Feb 2001 20:40:33 +0800
>To: W0YR@aol.com
>From: Alek Petkovic <vk6apk@eon.net.au>
>Subject: Re: [AMPS] Runaway amp
>
>Do you have a bad doorknob cap in the loading circuit perhaps? It could
>cause those sort of symptoms as it heats up and changes value under load.
>
>73 de Alek,
>VK6APK
>
>
>
>At 01:58 PM 16-02-01 -0500, you wrote:
>> AMP: Harris RF-110A (Pair of 4CX-1500Bs)
>> PROBLEM: Plate current and power output creep up after about 15 seconds
>>under
>> "keydown" condition.
>>
>> The amplifier will "run away" with itself until the overloads kick in.
>>
>> Scenario: Amp is excited to about a kw output with carrier. Key down
>> for 30
>> seconds. Power slowly increases to 1400 watts and would keep on
>> increasing
>> until the overloads kick it off. When I let up on the key, the idling
>> current is high and then it slowly returns to its normal state.
>>
>> Have measured the grid bias, no change under "heat up
>> conditions." Same for
>> the zener regulated screen voltage supply. (I had thought the zeners were
>> heating up and causing the screen voltage to rise, thus causing the
>> increase
>> in amplification. Apparently not so, no measurable increase in screen
>> voltage.)
>>
>> I do not have extensive experience with ceramic tubes. Is this runaway
>> output because of "soft" tubes? They REALLY put out. Or maybe it is
>>thermal
>> runaway, though the powerful air system on this military amp is working ok
>> (and certainly makes enough noise).
>>
>> Under SSB conditions everything appears normal -- obviously due to the
>> markedly lower duty cycle.
>>
>> Anybody got any ideas?
>>
>> Mike
>> W0YR
>>
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To: <amps@contesting.com>
>>Date: Fri, 16 Feb 2001 08:14:27 EST
>>Subject: Creepy Amplifier
>>To: amps@contesting.com
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>>
>>AMP: Harris RF-110A (Pair of 4CX-1500Bs)
>>PROBLEM: Plate current and power output creep up after about 15 seconds
>>under
>>"keydown" condition.
>>
>>The amplifier will "run away" with itself until the overloads kick in.
>>
>>Scenario: Amp is excited to about a kw output with carrier. Key down for 30
>>seconds. Power slowly increases to 1400 watts and would keep on increasing
>>until the overloads kick it off. When I let up on the key, the idling
>>current is high and then it slowly returns to its normal state.
>>
>>Have measured the grid bias, no change under "heat up conditions." Same for
>>the zener regulated screen voltage supply. (I had thought the zeners were
>>heating up and causing the screen voltage to rise, thus causing the increase
>>in amplification. Apparently not so, no measurable increase in screen
>>voltage.)
>>
>>I do not have extensive experience with ceramic tubes. Is this runaway
>>output because of "soft" tubes? They REALLY put out. Or maybe it is
>>thermal
>>runaway, though the powerful air system on this military amp is working ok
>>(and certainly makes enough noise).
>>
>>Under SSB conditions everything appears normal -- obviously due to the
>>markedly lower duty cycle.
>>
>>Anybody got any ideas?
>>
>>Mike
>>W0YR
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