I just started playing with these 5672 submini pentodes on the breadboard tonight, basically to check that they work. I bought four - they were manufactured in 1958! They are tiny, and are completely dwarfed by my 6111 submini dual triodes.
They require a 1.25 volt heater voltage (at only 50ma), I used a LM317 to provide that.
When I hooked up the heater voltage I expected to see a glowing tube..............but...........nothing.
So I rigged up a simple test circuit to check out if the tube was actually functioning - it was!
All of a sudden I was overdriving my VJ clone to saturation - basically what I usually use an LPB to do.
Here's the circuit, the clean boost is about 10db - and very tubey: