[Amps] Increased ambient noise in a 2 x 8877 amplifier

[Chris Gare]

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?

 

Chris G3WOS