CRT Question - Old Calculator

It's been done, but it';s very uncommon. I even heard about a direclty-heated colour CRT (3 guns), It was a right pain, you had to have 3 indpeendant heate supplies whcih were flating nad low capctiance to ground. You see, the video mopdualtion was applied to the heaters. In any case, the sahape of a filimanet is not ideal for an electron source in a CRT Waht is moderately more common is to have the cathod intenrally connected to the heate. This solves the 'shape' problem. It doesn't solve the fact that the CRT heater supply has to bouce around with the video signal, bit in devices that don't modulate the brightness msuh (like 'scopes, ratehr than monitors) thsi is not so much of a problem. It saves a base pin which can be useful. IT also reduces the versatility of the CRT, which may be a problem... I've jsut worked on an istrument (nothing to do with classic computers) which has a small (3" or so diagonal) slectorstaically-deflected CRT in it. It has 9 pins, no HV connctor aor side pins on the CRT. Jsut 9 connections. Of coruse 4 are for the deflection plates. The got everythign else onto 5 pins by (a) not having a control grid and (b) linking cathode ot heater intenrally. That means the 5 connections were : heater/cathode ; heter ; first acclerator anode ; focus anode; fianl accelerator anode. Odd... -tony