Yes. I agree.
Here in VK6, many years ago, many of us were lucky enough to come by
some TMC Intermediate Power Amplifier units from the GPT10K
transmitters, which were used at the US Navy communications base at
North West Cape, Western Australia.
These decks ran a 6CL6, into a 6146, into a 8295A final. This provided
around 400W output to feed the 10kW Final Amplifier in the transmitter.
They were good for all frequencise from 2 to 30MHz. Just 100mW in would
drive it to 1kW plus out. Most of us disconnected the 2 low level stages
and just fired into the final tube.
The odd thing was that they all exhibited the same behaviour on 40m,
where the power would build up slowly over 30 seconds or so. You would
tune up to full power ok and then not touch the controls. Say it was at
1kW PEP out. Then, when you dropped the drive and let it rest for a
minute or more and then keyed up, the output power would start out at
around 300 Watts and then slowly creep back up to 1kW over the 30 second
period. Let the button go and allow a short cool down and she'd be way
down again next time you keyed up.
We changed out the loading padder capacitors which were the 850 type
doorknobs, for different types and this largely did away with the problem.
I have one of the same amps under the workbench here at my place and I
note that it has a factory modified loading padder arrangement, with
multiple smaller value cpacitors in parallel. My guess is that this was
a known trait/problem with these TMC decks and later models addressed it
with this modification.
73, Alek, VK6APK
On 4/06/2022 6:43 pm, Rob Atkinson wrote:
Could be one or more things. Component in input network of amp
heating and changing value. Component in output (fixed value loading
padder cap) heating up and changing value. Something in the antenna
system heating up and changing value. This includes: Low pass
filter, watt meter, traps, baluns, ununs, anything with a capacitor
other than air or vacuum dielectric or inductor with an iron or
ferrite core. A tuned circuit is changing somewhere over
transmission time. Any cheap ham gear in the RF path is suspect. The
Matchbox is probably okay unless it has been modified with some part
not up to the job. Does this happen on one particular band or all
bands. Run the amp into a DL that is a decent DL like a Bird
capable of handling over 1 KW for a 30 minute xmission and see if it
still increases power. Just the exciter right into the amp and from
that to the DL. Depending on what happens that should eliminate some
suspects.
73
Rob
K5UJ
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