I really love old calculators, especially ones that print and have a Vacuum Fluorescent Display (AKA VFD). I picked up a Sharp Compet CS-1606 (for $3.99) at a Savers on a Sunday (06/09/2019)

This is missing the paper holder and the ink is dry and the case turned an awful shade of yellow but I just had to have it.

I used to like those printing ones too. There is a way to lighten yellowed plastic with peroxide, in the sun.

Also I like the early 70s first electronic calculators with the red digital display too.

When I was a child, calculators with VFDs were common here in Israel, but they disappeared very fast in favor of the modern calculators with LCDs.
Printing calculators with VFD (Mainly 230V ones) are still available here: https://www.lighting-gallery.net/gallery/displayimage.php?pos=-79944

I used to. Have a Texas Instruments printing one and a 1976 Rockwell handheld (quite thick) one.

I used to like those printing ones too. There is a way to lighten yellowed plastic with peroxide, in the sun.

Also, I like the early 70s first electronic calculators with the red digital display too.

That process is called "retrobrite" I believe. I also liked those red displays too!

I have two more old calculators.

One is a TI-1250 with the red LED stick display

and the other is a RadioShack 65-602 EC-202 (Made by Sharp)

I really like the printing one I mentioned in the first post if only I could figure out how to use it properly as there is a lot of different functions that I have never seen before

I've seen a Casio DR-120LB, in the storage of Carmel hospital:


It connects directly to 230V, 50hz, and have VFD:


Why the filament cathodes aren't incandescence?
Also: What is the purpose of the grid?

The filament is not that much hot, it is barely orange and that does not pass the bluish green filter.

The grids are used to allow electrons to pass only on one digit. It is used as part of the multiplex drive of these displays:
The equivalent anodes (segments) of all digits are connected together and to one lead. So the character is formed by connecting positive voltage to the corresponding segment leads (the unilluminated are connected negative). But without any grid, the same character would glow on the whole display.
The grids are used to allow only one digit to pass electrons by connecting the selected grid to the positive voltage, all others are connected negative.
By this the corresponding symbol is displayed on the selected digit, one by one in a cyclic sequence. The vision inertia them makes the whole display appearing to glow at once.

The best calculator I ever had was the Hewlett-Packard HP21.

Sadly, it died many moons ago and I've never been able to find a replacement. They do occasionally pop up on eBay, but never so far at a price I can afford 

I really like the printing one I mentioned in the first post if only I could figure out how to use it properly as there is a lot of different functions that I have never seen before

The printing one is oftentimes called an adding machine. It is a calculator optimized for adding long lists of numbers, although it does lots of other functions. You basically use the += key as an enter key when adding, and the -= key as the enter key when subtracting. You use the * key to terminate your calculation and print the result. Several presses of the * key will clear the working memory and make it ready for the next go (and print a series of 0* on the paper). There is also the key with the diamond, which I believe will print the number of numbers in your calculation. Let's say you make the calculator do 1+2+3=6, you would press 1 += 2 += 3 += (the display will read 6, but it won't print 6 just yet). You then press the diamond key, and it will print <diamond>3 because you have three items in your calculation. Then you press * and it will print 6*, which is the result of your calculation.

Hope this makes sense. Practice first with known calculations (or ones you can verify easily) and you'll get the hang of it. If the ink roller is dry, getting a new one will help as you will be able to see what it prints.

I have a Litronix with red bubble LED display.

I use it when doing bookkeeping.

Love vfd.

Love vfd.

Sorry, I must be particularly dozy today, what's vfd 

Sorry, I must be particularly dozy today, what's vfd 

Vacuum Fluorescent Display

Vacuum Fluorescent Display

Ah, thanks for that, my brain cell takes a while to get going sometimes