OK - That is interesting. While you have it set up, check that the power
does not come back when you tune the input circuit. If you can not tune
the input circuit, this might be your problem. If it does, it is your
problem - the heating of something is detuning the cathode circuit. Here
it really could be a coupling capacitor. I don't know the Henry but I
assume it is grounded grid cathode driven.
The filament voltage increasing (by how much?) implies that the
filament is getting hot from RF, rather than from the supply, increasing
it's resistance. I am assuming here that you are really measuring the
filament voltage and there is no RF contaminating the measurement. A
failing valve might do this - as might one with some problem in it's
heater choke. 8877s use indirectly heated oxide coated cathodes, the
filament is not supposed to be driven with RF. They will suffer back
heating from the cathode at higher frequencies, e.g. 432MHz and it is
recommended that the filament voltage is reduced. I don't think that
will be your problem at HF. You have checked for a heater-cathode short
haven't you?
You have not said what the grid current is doing - that is likely to
tell us the most.
400W output for a minute may be a hard job for an amplifier power supply
designed for SSB service - I assume the PSU is capable.
If it gives 600W for nearly a minute there can't be that much wrong with it.
Mike
> Message: 7
> Date: Sat, 6 Oct 2007 14:03:32 +0000
> From: jordan moser <henry8877amp@hotmail.com>
> Subject: Re: [Amps] HF amp output power dropping
> To: <amps@contesting.com>
> Message-ID: <BLU120-W37AD224D4DAFEE41C71734DBAA0@phx.gbl>
> Content-Type: text/plain; charset="iso-8859-1"
>
>
> I'm not sure how to respond to my first thread. Sorry if this is a duplicate.
> I have been trying to find the cause of my amplifier's output power falling
> off. Last night I checked it by transmitting a 400 watt CW signal into dummy
> load. After 1 minute output dropped by about 50 watts. At higher wattages the
> loss is more pronounced, at 600 watts it may drop to 400 over a minute time.
> Now when I apply drive the filament voltage INCREASES. It always used to
> decrease under load. That might be a clue, I don't know.
> Thanks for all the help on this.
>
> wj2w Jordan
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