Heater and cathode sharing

[timmy]

Ok so if you have a crt that has the cathode tied to the heater and try to use it in a set that has a separate cathode by tieing that cathode to one side of the heater in that tube why is it that there seems to be what looks like ac ripple in the pic. Is there a way to get rid of that, besides using a separate heater transformer. I did this once befor and it had that ac ripple in the pic.

[Electronic M]

Pretty much need the separate transformer, but you might get an improvement switching which heato leg you tie the cathode to.... if the heater is grounded or referenced to B+ your probably not going to get a good result....one alternative would be to use an isolation transformer for HK shorts...they look just like brighters, but don't change the heater voltage..

[old_coot88]

Just curious - what kind of CRT has the heater and cathode tied together by design?

[jr_tech]

Some early scope crts were built that way, 5BP1 for example.

https://frank.pocnet.net/sheets/049/5/5BP1A.pdf

jr

[mrjukebox160]

Or use DC for the heater.

[old_coot88]

Hah. (Smacks forehead) Dumdumb shoulda thought of scope tubes. Senior moment.

[ppppenguin]

With a scope tube the cathode is usually at a high -ve potential, often more than -1000V. Since the H-K insulation isn't adequate to withstand that sort of voltage it's usual to connect heater to cathode (either direct or via a resistor) and power it from a separate and highly insulated winding on the mains transformer.

In a TV, the cathode is normally much closer to 0V, though it can be up at 100V or more in some designs. I was working on a professional video monitor yesterday where the CRT heater was supplied by a separate winding for just this reason.

If you have a directly heated CRT, ie no separate cathode, just a filament then you have to have a separate supply unless the filament is grounded at one end. I've not seen a CRT like this but perhaps they exist. Such a filament would normally need a DC supply to avoid hum on the picture.

Direct heating was used in the early days of valves (tubes) with battery supply for the filaments. Also for later battery valves with 2V and 1.4V filaments for portable radios. A number of rectifiers had direct heating from AC; the hum didn't matter here. A separate heater winding was essential for these, and was often used with indrectly heated rectifiers to reduce stress on H-K insulation.

[timmy]

Originally Posted by old_coot88

Just curious - what kind of CRT has the heater and cathode tied together by design?

Well it was a 5hp4 but I had to run out yesterday and missed it on eBay , it went for 16.00 .

[wa2ise]

One thing they did in more modern solid state CRT TVs was to use a few turns of wire wrapped around the flyback transformer ferrite core to run the CRT heater. The pulses of the flyback transformer are of course synchronized with the video picture, so interference from it may not be objectionable.