Chris,
I think that I've heard about this before with 4CX250Bs on 144MHz...
isn't it something to do with "shot noise" and incomplete cut-off of the
tubes? ... you probably have a few tens of uA of anode current flowing
when on receive whoucl could result in 5/10/20/50mW of noise power.
If you consider the normal arrangement with a traditional RF relay
outside the box you also have the isolation (twice) from NO-COM-NC sides
of the contacts (assuming the antenna is connected to COM) so you have
probably another 80-100dB or so of isolation - this is lost in the case
where you put the vac relays 'naked' in the PA compartment...
... just a thought.
Mike G8TIC
On 14/11/2011 19:11, 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?
>
>
>
> Chris G3WOS
>
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