Chris Gare wrote:
> This summer I have successfully built a 2 x 8877 amplifier for 6m which
> works very well. I have 4000 volts on the anode and have only taken it up
> 4kW into a dummy load.
> To handle this power this power level I opted to use three Jennings RJ1A
> vacuum relays - one on the input and two in parallel on the output.
> The two output relays were placed inside the anode cavity of the amplifier
> and were unscreened.
> I found that after a JT65 TX session the ambient background noise on the RX
> was increased by several dBs with this increased background noise only
> returning to ambient after around a minute or so. If I increased power it
> took longer for the noise to drop away showing that it was related to the
> temperature of the valves (which are well blown). I only noticed this when
> my antennas were elevated so the local ambient noise was considerably
> reduced.
> The 8877s are definitely biased off on receive as I am using the 'SEK
> control card and I have never come across this problem on previous
> amplifiers when I used coaxial relays.
> When I put the relays in a screened die cast box the problem disappeared.
> My question is; what caused the noise? I can only think that it was heat
> radiation from the valves but I have never come across this before or heard
> it mentioned by any org amp builders.
> Has anyone any idea as to the cause?
One can only guess, it is possible some insulation is breaking down under
the hV.
If you can operate the amplifier with the valves on the shelf you might prove
or disprove it.
You can also Hi-Pot the high Voltage components.
--
Ron KA4INM - Did you know ...
... that no-one ever reads these things?
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