Trying to bring Keithly 600A Electrometer back to life

snip tale of battery hell

1.5v is only a 7% boost over 1.4v, and should be no problem for any thermionic emitter.1.55v ditto. I used to boost CRTs 30% and never had any problems.

If you were really determined to eek out every last second from dry cells, you could always put a R on the batts to giev the 1.4v, but it would achieve very little IRL.

But why not make a mains PSU to drive it all, instead of using stacks of batteries?

NT

Whatever he does, I recommend replacing the tube amplifier with a nice NSC cmos low-bias-current opamp, like the LMC662 or LMC6001. Free samples,

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 Thanks,
    - Win

I have a number of tube manuals, and those two are not listed. I am guessing that the filament rating is 1.4V, like the 1R5, 1U4... all-american 5 lineup. "1.4V" means a nominal and is OK for 1.55V fresh dry cells. In the manuals, i see a number of tubes with filament rateing of

1.25V and most are rectifiers. I do not see a 1.25V filament rating for an electrometer tube, since they were designed primarily for use with batteries; much more logical to have a 1.4V rating for a standard dry cell. Perhaps the mercury cells were used for longer useful life of the instrument, and the lower voltage did not adversely affect the emmission of the cathodes.

Mercury cells were used because they had a very low internal resistance, and because their cell voltage dropped practically not at all until they were fully depleted. They thus were used anywhere an inexpensive, accurate voltage reference was needed. This included power supplies where changes in supply voltage would influence circuit operation.

Filament voltage would have influenced circuit operation (filaments, are, after all inherently parameter corrupters :-), but I don't remember Hg cells used in any of the VTVM's I had to calibrate and repair in the '60's and '70's, when they were still standard items.

John Perry

I work on that series. As stated above the mecury battery holds the voltage longer at 1.4v You need those steady voltages for these meters. There are circuits for the 1.4v as they were used in cameras. Try google search such as mercury battery subsitute or other variables. If you do serches on mercury bateries you will find them in other countries but not US. Doug

? Were you addressing my suggestion that Don replace the tube amplifier with an NSC cmos opamp, like the LMC662 or LMC6001? Were you pointing out that low-voltage parts (NSC's cmos opamps run from as little as 4V) are well-suited to powering from a few battery cells? The LMC6001 draws 450uA at 6V, and their slower LMC6061 draws only 20uA (it's specs are slightly degraded). If that's too much, Don can consider the micropower LMC6442, which runs from lower voltages and takes even less current, only 2uA at 2.2V, and is very well suited for 3 to 6V batteries.

Free samples are available for all these fempto-amp opamps,

Recommended.

--
 Thanks,
    - Win

Or not. My sub-pA HP instruments don't have hum problems, and all my fempto-amp Keithley electrometers, including the remote-head 642, run from the ac power line. It's a matter of knowing how to make the transformer and power supply.

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 Thanks,
    - Win

I was refering to keep the unit as is. The circuit uses a higher voltage battery source and just regulated it.Keithly made battery operated units and ac operated type. The AC operated still used the

1.4v mercury battery I believe for the ohms scale. I'll have to search for notes at work. dough

One can introduce (erlatively speaking) a *LOT* of objectionable hum.

(Snip)

I don't have any suggestions on the 8.4 and 30 V batteries.

However, most rechargeable batteries have a nominal voltage of about 1.2V. A pair of D cell rechargeable batteries should be very close to the correct voltage.

In addition, the filaments probably draw a lot of current. Using rechargeable batteries will keep your long term costs down.

The 8.4 and 30 V batteries probably provide bias voltages. The current drain on them will be much less.

Aidan Grey

Actually I originally posted this. I was just hoping to find some acceptable batteries, not trying to make this an exact replica of what came from the factory 40+ years ago, but trying to keep most of the original intent in place, get a cute old instrument running again and still mostly meet the original specs without gutting it.

It was never my intent to debate the qualities of one model versus another or whether it was possible to design lab quality ac supplies. And it certainly wasn't saying that half a century of progress in solid state devices couldn't produce as good or better an instrument.

Someone recommended looking at the web for pages describing substitues for mercury cells in cameras that would have stable voltages. All the results I found in that direction seemed to be replacing far smaller batteries. The two 1.34 mercury cells in the 600A are about D cell sized, these are heating the filaments of four (small) vaccuum tubes for 500 hours. The ones I found on the web looked to be button sized.

Some previous user of the 600A already substituted 9 volt cells for the original 8 volt mercury cells and I presume, based on the discussion, that these are also not going to have the stability needed for repeatable measurements.

But I guess for the moment I'll try ordinary D and 9 volt cells to generate all the voltages and see how bad it is.

Now if someone had a pointer to a description of building an electrometer with current components I might try that, just to show as a comparison.

I was groping for something really cute to have the 600A measure. Now I think I've found it, the output of a little electric field mill. If I can't find a plausibly priced old one I may have to build one.

seems obvious enough that it would need to be designed to suit the app.

Youre the first to mention an 'undefined' psu being used. I think its fair to deduce that if someone is capable of restoring old meters, they can define the requirements of the psu they need.

NT

Yes; but the poster was going to use some undefinable PS to replace the batteries...

...

Fortunately there is enough room in the case to give some flexibility for battery substitutions, unlike any current product on the market.

I'll see how it works out.

Don Taylor wrote in news:Rvqdnctdjf-HLSreRVn- snipped-for-privacy@scnresearch.com:

Maybe you could use a CR123 lithium cell (3V)and make a small regulator for it,so it all fits in the space of the original mercury cell. For the 30 V supplies,maybe 3 9V alkalines in series,that's probably a plate supply anyways.Their physical size would be larger,though.

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Jim Yanik
jyanik
at
kua.net

There are lithium 9V batteries with the same form factor as the "standard" 9V battery. This would give an almost constant voltage during most of its life.

I find the whole idea kinda cool. also in my experience you can use any recent "9V" battery in place of any 8.4V mercury battery in almost any electronics. other batteries may pose different problems or advantages.

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JosephKK