4-65a EIMAC NOS DHT

Just got a couple of NOS EIMAC which I will be using in my SE design

So did some test on distortion, transconductance and driving them to +22.22dBu output to check the quality of these two ones.

I used similar test rig as before. At some point will be able to get a proper filament supply for this valve, but for the time being I will continue to use the hum pot and the big electrolytic cap across my old bench power supply which can gently provide the 3.5A for the hungry filaments!

I tested them at the limit of my CCS and bench HT supply which at the moment cannot provide more than 360V @ 100mA.

Transconductance is in the region of 3,800 – 4,000 μmhos.

So biasing the valve at -2.5V and over 90mA of anode current, the harmonic profile looks like this:

Both EIMAC measured about 0.12% THD.