by Steve Anderson » Fri Jun 16, 2017 7:30 pm
The 3BP1s are common general purpose scope tubes, their downside is they require around 2000V to operate, but anyone willing to have a go it's quite an easy tube otherwise.
The CRT with the black deposit on the inside could well be OK. This may be the 'getter' which is chemical sprayed into the tube after vacuum pumping and before sealing. It's function is to mop up any residual gas (air) in the tube. As long as it's not white it should be OK, often the getter has a silvery finish, especially true on radio-type valves/tubes. If it is white air has leaked in rendering it useless, usually through the glass/wire or pin seal. When a TV type CRT starts to lose resolution/focus this is what's generally happening, the trade term is the tube has gone 'soft', it's not a hard vacuum any more.
First thing to check is the heater is intact, two pins on the base will have a low resistance between them, though not zero. By low I mean a few ohms, anything from 1-20 ohms. Using a variable power supply gradually (very slowly) wind up the volts until you get that characteristic orange glow from the heater, measure the voltage and current. It will usually be between 300mA and 1A, there are exceptions though. Very few CRTs have heaters over 6.3V. The vast majority are either 4V or 6.3V.
Look for any burn marks on the face of the tube, if there's a straight light-brown line this CRT has already seen considerable service or the burn was the result of some other failure in the equipment it was installed in. (A grid-bias failure).
A very gentle shake should not provide any indication of loose parts rattling around inside, put it base up first, then invert, this will also show up anything not quite mechanically right.
All of these checks can be done as you buy them with the exception of actually applying heater volts. If it passes these then there's a 90%+ chance you have a good CRT.
In this case you want know what the heck they are though!
Steve A.