I'm currently building a second unit. As you can see, I've did a couple of mods.
I used an external PSU to get rid of the usual power transformer AC hum.
I rewired the tubes for 12.6V heaters, I use an SMPS here.
I'm actually measuring -80 dBu noise floor flat.I omitted the output attenuator section and replaced it with a simple 1k pot (Ian's Neve trick).
In this build I use a Lundahl 1540 which works fine. OP TX is a Sowter 1232. The meter is an old Weston 301 stamped May 1964 on the back.
Note: in an earlier post I wrote that the meter circuit needs to be modified. I was wrong. The meter circuit works well
for the Sifam meter in my first build as well as for the Weston meter when the meter is strapped with a resistor.
I used a 22R which works perfect for both meters.