GE 42L series caps

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Scott Hanson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <huil888-at-surfside-dot-net>

Terry -

I've got a datasheet on the General Electric 42L-series film caps somewhere
in my HV capacitor folders; when I find it, I'll scan it and send you the
file. For some reason, GE does not seem to be actively marketing these caps,
and quantities of new parts have been showing up on the surplus market for
the past year or so.

According to GE's datasheet, all the 42L series caps are of straight
metallized polypropylene film construction; no foil used anywhere. I
requested this data from GE  technical sales several years ago when
Hossfeldt Electronics had a quantity of the .1uF, 3000V caps for sale as
surplus. I built a 12 X 4 string MMC using these (.033uF at 36 kV) and have
used it with a 6" X 32" static-gap coil powered by a pair of 15/60 NST's.
Performance was identical to a pair of series-connected .06uF, 45kV
Maxwells, and there has been no evidence of any heat problems at the end
connections (not a whole lot of extended run time though).

I have a fair number of 42L series caps of several different values, and one
characteristic that is common to all of them is that they have a large body
diameter relative to their length. This gives a lot of contact area between
the edges of the metallized film and the sprayed-on metal contacts at the
ends of the rolls This allows the contact-area-per-unit-capacitance ratio to
be higher than some other metallized-film caps I have seen.

So far, my experience has been that these caps will work fine in a
conservatively designed MMC running on an NST power supply.

Regards,
Scott Hanson


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: "Steven Ward" <srward16-at-hotmail-dot-com>
Cc: <tesla-at-pupman-dot-com>
Sent: Friday, September 13, 2002 12:51 PM
Subject: Re: defective caps?!?!


> Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
>
> Hi Steve,
>
> The caps you sent are not metal foil!!  They are metalized film where a
> very thin layer of aluminum is vacuum evaporated onto the poly film.
> Unlike solid foil caps, metal film caps tend to have weak connections at
> the end caps that will fail under very high currents.  That is what
> happened to your's that were burnt at the end plates.
>
> I thought the GE 42L series were a solid metal foil type.  Others in the
> 42L series seem to work ok, but I wonder what is going on.  It is possible
> GE mixed foil and film types in the same series.  I never have seen any
> data sheet on these caps.
>
> Your 42L2332  1200VDC 0.33 uF caps are not able to take as much "peak"
> current as solid foil caps.  They run ok for awhile until the connections
> start to fail and then they degrade very quickly until the ends get hot,
> burn away, and they bubble up.  This is what happened to Garry Freemyer's
> caps too.
>
> Even metal film caps may work for a TC.  It all depends on the peak
> currents and if that caps can take it.  Normally foil caps can take many
> times the peak current of a film cap so we get them.
>
> I would not find it hard to believe that either GE mixes construction
> inside the 42L series or the other caps are just able to take the peak
> currents despite being metal film.  Testing on other of the 42L series
> seems to indicate that they were foil and not film...  GE may have also
> switch to metal foil at some point as an ECO...
>
> So the problem with the caps is obvious.  However, the GE42L series seems
> to be a rather unknown type of cap now.  Does anybody have any solid data
> sheets on them??
>
> I seem to remember cutting up the bigger GE 42L caps and they were metal
> foil...  But all big yellow caps look the same and it may have been
> something else...  Too long to remember...
>
> Cheers,
>
> Terry
>
>
> At 08:37 PM 9/8/2002 -0500, you wrote:
> >I now have a very sick feeling in my stomach,
> >
> >I may have bought about $120 worth of bad caps!  While my GE 42l series
> >3000v caps have lasted for a looooong time, the 42l 1200v caps may not!
I
> >may send one to Terry so he can check it out.  Heres the story.
> >
> >I just finished up a medium coil powered by a 12/60 and a static gap.  My
> >MMC is 20 in series of 1200v .33uf caps for .0165uf -at- 24kv.  Well the
coils
> >performance was poor to say the least with only 2 foot arcs.  So i tried
and
> >tried with no luck.  So just out of curiosity, i felt the caps after i
> >turned it off.  They were hot!  well 2 of them were.  I let them cool
off,
> >then ran it again.  Checked for heating, it was still in those 2 caps.
One
> >was hotter than the other.  So i took them out.  I took apart the really
> >warm one and found some depressing realizations.  The ends of the foil
were
> >totally burnt to a crisp at one end!
> >
> >Here is where the sick feeling really sets in.  The 20 caps didnt cost
120
> >bucks, but the caps for my large coil, and small coil did!  Now it gets
even
> >worse!  This is just speculation, but i think the large caps on my large
> >coil are going bad!  Last time i ran the beast, i had some trouble
getting
> >it to tune.  I did get the normal 5 footers, but i had lots of trouble
> >getting there.  I did check the temp on these caps the first night i ran
it,
> >and there were no signs of heat, but i didnt feel each cap individually,
nor
> >did i check it after the first night.  So i am praying that these caps
are
> >better because my current bank cost about 85 dollars.  I was starting to
> >feel confident in these caps.  I just upgraded to 2 strings of caps on my
> >big coil, but never ran it yet.  Each string is 24 X .68uf -at- 1200v for a
> >,0283uf -at- 28.8kv.  By the way, is a .0566uf cap too big for a 15/120
system
> >with no RSG, but a blower gap?  I like the idea of that huge cap for some
> >huge current handling, unless these are "crap caps" and they will all
fail
> >soon anyway.
> >
> >Terry, is there a way for me to send you a cap so you can test them and
find
> >any problems they may have?  Im not positive that the whole series of 42l
> >caps are bad, so i want you to look at them.  Im really hoping that this
was
> >just a defective cap, and that the rest will be ok.  The surprising thing
is
> >that, i could charge the cap to at least 1500v with a small HVDC supply,
and
> >it made a fat spark.  But i did hear some clicking noises when it was
> >charging.  So lets hope that maybe it is just a defective cap, or at
least
> >its only this specific model thats bad.
> >
> >Steve Ward.
> >
> >
>