Questions about SRS

[Beta 5]

SRS gear isn't something I have a huge amount of experience with so I've got a few questions if anyone can assist.

Capacitors - how essential is it that they are on their rated value, say for a 5ft 65W needing an 8.4uF capacitor, would it matter is the cap was only measuring say 8uF.
What would happen if a capacitor that was drastically lower in value was used, or the 8.4uF one had failed, say something like 4 or 5uF? Would it just fail or struggle to strike?
What would happen if a much higher value capacitor than the one specified was fitted say if someone stuck a 12 or 15uF cap in place of the 8.4uF?

Tube EOL - what happens? Do they just rectify and eventually go out? Like any magnetic ballast I assume they wouldn't appreciate a rectifying lamp being left in place long term.

[Slyspark]

CAP:

So, the capacitor used is essentisal for the tube to be able to strike. If the capacitance drops as it ages, the tube becomes harder to strike. I've never tried using an 8mfd cap where an 8.4 is required (mostly cos I have enough 8.4's that I've never needed to). I guess an 8 would probably still work, but other factors such as the age of the tube, the conditions in which it's being operated would be variables that would also affect it's operation. If you wanted to test this, you'd be best ensuring that everything else remains a constant if you want any kind of meaningful result.

Presumably, if you go OVER the required value, it'd work also, but again, going too far might have unintended consequences. I guess they picked the very specific value of 8.4mfd for good reason.

I've never tried using other caps, but now I'm intrigues enough to try it and as I run SRS in the workshop anyway, it;s easy enough to do.

It should be noted that the 8.4 will work on those Parmar ballasts that list 5, 6 & 8' tubes, but if you're using a 4' SRS ballast, the cap drops to 5.5mfd. I have a 3' SRS batten somewhere and I *think* this is also 5.5mfd, but I'd need to check that (which means I'd need to find it first).


EOL:

At EOL, the tube generally rectifies, you get a fantastic black sock at one end and assuming therte's no fuse or other protective measure to cut the tube out, you can get to the point of a rather spectacular failure where the cathode melts, the tube continues to rectify, using the metal cathode stem until the end of the tube gets so hot it either cracks, or the glass stem melts, vacuum is lost and it finally burns out. This can cause damage to the lampholder if it gets hot enough, especially if its made from one of the later, soft plastics, it'll literally melt! The ballast wonlt rerally appreciate this for very long either and gets HOT. Thorn especially, used to include a fuse in a little holder (either a loose barrel type fuse carrier, or within the terminal strip itself) designed to blow if the tube began to rectify. A rectifying tube will draw more current, hence why the fuse was a cheap and simple solution. They could be a VERY specific value - I have some here branded 'Churchouse' that are 1.25A and came out of something or other Thorn from back in the day.

It's quite cool to watch, you get a nice orange light show at one end of the tube, right as it finally fails. Just need to be in the right place at the right time.

[FrontSideBus]

Would be interesting to measure ocv, lamp voltage and current with different value caps.

[Beta 5]

Thanks for the answers!
Yes, this is what I have been wondering, the specific value of 8.4uF must have been chosen for a reason. I may try putting some of my standard 8uF PFC caps on to see what the effect is, I guess if you could find an 8.5uF motor run capacitor that would do as the nearest equivalent, as I assume 8.4uF caps aren't made any more?

I do have a 1980s RIC 8.4uF which has dropped to only reading about 7.9...8uF, testing that it still seemed to strike ok (the tube is brand new though so may be somewhat harder to strike if well used), but the case of the capacitor did get noticeably warm, maybe 10+ degrees above ambient whereas the others were all stone cold.
I think that is probably more because that capacitor is failing rather than the fact its value is lower though.

Yes I have a couple of 5.5uF caps kicking around, I figured they were for lower wattage SRS, again it is a specific value rather than just using 5uF or 6uF so there must be a reason for it, perhaps it is just the optimal value for the circuit.

Another thing I found while experimenting, if you remove the capacitor from circuit after the tube has struck the brightness drops a little and the ballast quietens down a bit, the total circuit watts also reduced from 85W to 70W with the Transtar KL65, so the capacitor must still be active in the circuit after the tube has struck.


Ok that's pretty much what I was expecting for EOL, I have seen some very hammered EOL T12's so I guess those would have been victim to an SRS EOL! If left rectifying for long enough I suppose they would mercury migrate too?
I've seen those fuse holders in a few fittings actually, the Marlin 22w circular bulkheads also contain one, again at a funny value something like 3.15A if I recall. A 22w Circline will rectify once the cathode breaks so the starter can't interrupt it.