[Amps] HF amp output power dropping

[jeremy-ca]

----- Original Message ----- 
From: "Tom Rauch" <w8ji at contesting.com>
To: "jordan moser" <henry8877amp at hotmail.com>; <amps at contesting.com>
Sent: Saturday, September 29, 2007 10:34 AM
Subject: Re: [Amps] HF amp output power dropping


>> I have an old Henry 3000d RF heater that was converted to
>> HAM use by someone else. It runs a 3CX3000A7 tube in
>> grounded grid.
>> Recently the output power has begun falling off rapidly
>> whenever I apply constant drive such as during tune up.
>> The tube is new and the plate voltage remains steady as
>> always.Is there anything else that could be causing this?
>> It reminds me of a tube that has gone soft although I am
>> sure it's not the tube.
>> Any help is appreciated
>>
>
> Assuming the drive is constant and the meters are good, it
> sounds like a bad tube.
>
> It could be a capacitor or inductor drifting off value and
> making something get mismatched, it could be the bias is
> drifting up,  but a problem like that when it suddenly shows
> in an otherwise properly working amp is generally (but not
> always) the tube.
>
> Watch all the meters especially plate current and bias, that
> should tell you something.
>
> 73 Tom


It could very well be the plate blocking cap, especially if it is undersized 
for that power or not even rated for RF use such as the TV doorknobs.

Also check the filament voltage, filament choke and input VSWR. Ive seen 
underated filament chokes act up under steady drive.

A soft 3CX3000 usually drops in power gradually but is still fairly steady 
under tuning. What happens to the grid current?

Since someone else did the conversion I'd investigate that first and 
consider the tube as the last resort....hopefully.

What drive, bands and other parameters is it running at?

Carl
KM1H