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 Post subject: Meter Protection Diodes for Hickok 600A
PostPosted: Dec Tue 15, 2015 6:13 am 
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I've had the misfortune of pegging the meter on my Hickok more than once; what a terrible feeling! How? For example, on the Gas Test, the Operating Instructions don't specify an initial bias setting. If it's on a low setting, guess what happens? So I've learned to initially set it on 100 when doing the Gas Test. But then there's also been plain old stupidity... :( That's why I need help!

The tester supposedly has a 233 ohm, 500 micro-amp meter.

What diodes do you recommend? From other posts, I've seen 1N4007 and 1N5819s mentioned. And what about germanium diodes?

I also see that some increase the size of the meter capacitor. For example, if it was a 47 ufd cap, they go up to 470 ufd. They claim this helps. Does it?

I found this (among other) post but would still appreciate any thoughts or guidance you may have: http://antiqueradios.com/forums/viewtopic.php?f=8&t=216422

Thanks for your help.

Mike

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 Post subject: Re: Meter Protection Diodes for Hickok 600A
PostPosted: Dec Tue 15, 2015 9:51 am 
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Well.....

Its a good idea in theory. 500ua at 233 ohms works out to only 0.1165v for full scale. Silicon diodes conduct at about 0.7v. They might save it, but I can see why Germanium would seem attractive, as they conduct at about 0.3v. I think the Germaniums would just open rather than protecting the meter.

Schottky diodes are probably the best option. They conduct at about the same voltage as a Germanium, and you can get Schottkys that can take some current.

The problem is most diodes are never completely off. You would have to make some lashup with a pot and a battery and check the linearity of your meter with and without the diodes, and see how much difference it makes, and whether you can live with that difference.

The big capacitor will just slow tings down a bit, and maybe give you a chance to undo whatever you did before the meter pegs, The downside is that if the meter does reach overload, the cap will keep it overloaded longer.

John

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 Post subject: Re: Meter Protection Diodes for Hickok 600A
PostPosted: Dec Tue 15, 2015 2:43 pm 
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Schottky diodes. You can try them in and out of circuit, and see what affect, if any, they have on a known meter reading.

Pete

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 Post subject: Re: Meter Protection Diodes for Hickok 600A
PostPosted: Dec Thu 17, 2015 6:15 am 
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Schottky diodes. OK, thanks.

Since my original post, I've been doing research from previous ARF posts, and from other sources, too. I've also confirmed, through testing, that my meter is actually 500ua at 233 ohms, which does work out to 0.1165v for full scale deflection.

Now I need to learn more about what type of Schottky diode to use (1N5819, for example?). Here's a link to data on the 1N5817, -18, & -19s: http://www.onsemi.com/pub_link/Collateral/1N5817-D.PDF

Then I need to test for any inaccuracies the diodes might introduce. Hopefully none or negligible. And I think I'll use 2 diodes in a "back to back" configuration.

It looks like some folks use a non-polarized capacitor in addition to the diodes. More research ahead on this, too.

I'll keep posting what I find out. It might help someone else with a Hickok 600A.

Regards,
Mike

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 Post subject: Re: Meter Protection Diodes for Hickok 600A
PostPosted: Nov Wed 13, 2019 5:55 pm 
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Location: York PA.
Hi Mike

I've been thinking about meter protection for my 800A and found this thread during my web searching.

The 800A has the same meter as your does, and I had come to the point of going between a Germanium or Schottky diode.

I'm interested in what you selected and the impacts it had on your meter readings.

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 Post subject: Re: Meter Protection Diodes for Hickok 600A
PostPosted: Nov Fri 15, 2019 4:51 pm 
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Let me find my records on this and get back to you; I tried several different configurations and remember settling on a simpler design. I remember it worked well without affecting the accuracy.

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 Post subject: Re: Meter Protection Diodes for Hickok 600A
PostPosted: Nov Mon 18, 2019 2:28 pm 
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I have an 800 and have watched in horror the bright fuse light and meter pegging :shock: Having the wrong setting or using the top cap in the plate jack instead o the grid jack are 2 common causes. I never test a tube without doing the shorts test first. One easy way of protecting the meter is to set the shunt or English control to 100 first and then dial it down to the required number while holding P4.


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 Post subject: Re: Meter Protection Diodes for Hickok 600A
PostPosted: Nov Sun 24, 2019 3:53 pm 
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Lou deGonzague wrote:
I have an 800 and have watched in horror the bright fuse light and meter pegging :shock: Having the wrong setting or using the top cap in the plate jack instead o the grid jack are 2 common causes. I never test a tube without doing the shorts test first. One easy way of protecting the meter is to set the shunt or English control to 100 first and then dial it down to the required number while holding P4.

+1; excellent advice, Lou.

On page 3 of the Hickok manual, under Section 8, Gas Test, item a, I have a note written under "Set the English dial at 73," to set Bias to 100. This has prevented a lot of meter pegs and the resulting words my wife doesn't like me to use.

I've attached a copy of the drawing I made of the circuit I finally settled on. I tried a number of different circuits; this one didn't usually affect the accuracy (never more than 5% on the samples I tested). I enclosed the "circuit board" in heat shrink.

See what works best with your particular meter; they all have their own "PERSONALITIES" for sure!

Attachment:
IMG_20191124_0001.pdf [460.33 KiB]
Downloaded 27 times

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