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.