Picture tube 'expander'?

[miniman82]

WTH is this thing?

no affil

http://cgi.ebay.com/PICTURE-TUBE-EXP...item53e6fd57f8

[ctc17]

Hmmmm, something to malipunate the ion magnet?

[old_coot88]

Just a SWAG, but it fits under the damper tube and connects a cap between the cathode and plate (pins 3 and 5 respectively, typical of 6AX4 etc.). The cap would be ceramic, 150pf or so, at around 4KV rated.

What this does is boost the sweep width slightly while reducing the HV, further allowing the pic to 'bloom' slightly. Plate current of the HO tube is also reduced slightly. Old techies would sometimes do this as a "last resort" when fighting a problem of insufficient width. They would try cap values like 82, 100 and 150pf to see which gives the best overall compromise.

Old color techies would sometimes do it as a last resort with problems of high HO tube current. But the caps would be 6KV rated. Bill(oc)

[ChrisW6ATV]

My first instinct when I saw the picture was, this must be an early equivalent of using a green marker on the edge of a CD to "make it sound better". Attach this to your picture tube and make the picture bigger! It is interesting to know that it has a true scientific function.

[bandersen]

I was going through a junk box tonight and came across these picture expanders.

[old_coot88]

Well I'll be *bleep*. Never saw or heard of the gizmo, but deduced its use from the visual cues. oc

[ctc17]

I have played with the cap across the damper to lower cathode current. Most of the sets I see have to much sweep and the only time I would see a need for this would be low line voltage areas or a quick fix rather than proPper repair.
I don't have many sets that us octal damper tubes so that one went right over my head.

[old_coot88]

Originally Posted by ctc17

I have played with the cap across the damper to lower cathode current. Most of the sets I see have to much sweep and the only time I would see a need for this would be low line voltage areas or a quick fix rather than proPper repair.

Yeah, one genre of sets that consistently defied a "proper repair" was those 25" Admirals with 6KD6 H.output and that goofy feedback HV regulation. In 'normal' operation the cathode current would sometimes soar to over 400ma. at high brightness. And they would eat flybacks like crazy. With every fly replacement, the only way to get the current within bounds was the cap-across-the-damper trick and lowering the screen voltage on the 6KD6 while maintaining width and regulation. Quite a juggling act before letting the job 'burn in' for a few days. oc