[Amps] Drifting Power

[MU 4CX250B]

I’m not aware of any vacuum tube RF power amplifiers that are
sensitive to room temperature changes. However, if fixed load
capacitors heat up under full RF power, a frequent occurrence, that
can affect the tuning. Ditto for marginal switch and relay contacts.
In my experience, a falloff in power output, accompanied by a decrease
in grid current, is also a frequent occurrence. This can be caused by
external capacitor changes, or by weak tubes.
73,
Jim w8zr

Sent from my iPhone

> On Oct 16, 2021, at 9:10 AM, Bruce Campbell <W8HW at comcast.net> wrote:
>
> I have noticed that many amplifiers are designed to be most efficient _NOT_ at room temperatures, but a bit above room temp. A good compromise in design. I have noticed this is both tube and solid state amps. I do not have your amplifier in front of me, rather I am speaking in general.
>
> 73, Bruce, W8HW
>
> =========================================
>
>> On 10/16/2021 10:32 AM, Pete Smith N4ZR wrote:
>> FWIW, I see something very similar in my KPA-1500.  I had assumed it was software protecting the transistors, just in case.
>>
>> 73, Pete N4ZR
>> Check out the new Reverse Beacon Network
>> web server at<http://beta.reversebeacon.net>.
>> For spots, please use your favorite
>> "retail" DX cluster.
>>
>>> On 10/16/2021 10:05 AM, Steve London wrote:
>>>   I have a Commander HF-2500 Magnum, with a pair of 3CPX800's. Great amp.
>>>
>>> It has one quirk, and only on 15 meters. If the amp has been dormant for a
>>> while, the output power is initially low: about 1100 watts. Key down it
>>> gradually rises up to 1500 watts over about 10 seconds. After that, if I
>>> intermittently transmit, at least every 5 minutes, it will stay at 1500
>>> watts. Only after being idle for ~30 minutes will it go back to the low
>>> output state.
>>>
>>> My thought is that something in the input pi network is heating up.
>>> However, I have accurate SWR/power meters on both the amp input and amp
>>> output. Neither the input power nor SWR is changing as the output power
>>> rises. As the power rises, the grid current drops, but the plate current is
>>> unchanged. Again, this is only on 15 meters.
>>>
>>> Suggestions ?
>>> 73,
>>> Steve, N2IC
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