[R-390] R-390 Digest, Vol 61, Issue 39

[rdavis7 at comcast.net]

Ok Barry here's the deal. You forced me to move a mountain of "good junk" and come up with a perfect probe for your project. Its a military grade probe with a nomenclature (test probe mx-404-9B USM-116. Has the diode still in it. Ruggedized probe body with a nice rubber sleve covering it. Complete with cord (3 insulated conductors, 1 bare). Send me your shipping address off list and It will find its way to you. Only have 1 of these guys, sorry! Rick K8PJQ CCA 11437 Dayton bum. rdavis7 at comcast.net 
----- Original Message ----- 
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Sent: Saturday, May 30, 2009 12:00:20 PM GMT -05:00 US/Canada Eastern 
Subject: R-390 Digest, Vol 61, Issue 39 

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Today's Topics: 

1. Re: Parts Request (2002tii) 
2. Re: Parts Request (2002tii) 


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Message: 1 
Date: Fri, 29 May 2009 20:53:31 -0400 
From: 2002tii <bmw2002tii at nerdshack.com> 
Subject: Re: [R-390] Parts Request 
To: R-390 at mailman.qth.net 
Message-ID: <20090530005332.8F38411B835 at karen.lavabit.com> 
Content-Type: text/plain; charset="us-ascii"; format=flowed 

Barry wrote: 

>Still no luck with a probe for an HP410C either, but a question 
>about that one. If I use a 
>silicon diode for a probe (to at least get low frequence AC 
>measurements going temporarily), then 
>do I need to account for the forward voltage drop across it? 

Yes. You can minimize this by using a germanium diode (preferably 
gold-bonded) or Schottky diode (in either case, use a small-signal 
diode). The forward drop will affect the low-voltage ranges more 
than the high-voltage ranges, but note that germanium and Schottky 
diodes have rather low reverse breakdown voltages. 

>Does the EA53 and/or 2-01C have a 
>similar voltage drop so it won't be an issue? For larger voltages 
>it wouldn't matter, but on the 
>low ranges, it might be an issue. 

I don't have curves for the EA53 and 2-01C handy, but 
indirectly-heated small-signal diodes operated at very low current 
generally start to conduct when the plate is slightly negative WRT 
the cathode. A 1N34 would be one of the likely candidates, and it 
has a reverse voltage rating of about 60 V, IIRC. 

Why not use half of a 6AL5? Those are good for 330 PIV. The 6H6 
(the once-common octal dual signal diode) has a PIV rating of over 400 V. 

Best regards, 

Don 






------------------------------ 

Message: 2 
Date: Fri, 29 May 2009 23:50:33 -0400 
From: 2002tii <bmw2002tii at nerdshack.com> 
Subject: Re: [R-390] Parts Request 
To: R-390 at mailman.qth.net 
Message-ID: <20090530035034.9D4FE11B84B at karen.lavabit.com> 
Content-Type: text/plain; charset="us-ascii"; format=flowed 


Barry wrote: 

> >Does the EA53 and/or 2-01C have a 
> >similar voltage drop so it won't be an issue? For larger voltages 
> >it wouldn't matter, but on the 
> >low ranges, it might be an issue. 

I replied: 

>I don't have curves for the EA53 and 2-01C handy, but 
>indirectly-heated small-signal diodes operated at very low current 
>generally start to conduct when the plate is slightly negative WRT 
>the cathode. 

A kind list member did have data handy for the EA53, and it appears 
that it draws 1 uA when the plate is 0.5 V negative (yes, it draws 
positive current even though the anode teminal is still negative -- 
thank the Edison effect due to the space charge, which causes "space 
current," and, to a lesser degree, the contact potential due to the 
dissimilar materials used in the cathode structure). When the plate 
voltage is 0 V, an EA53 is drawing 10 uA; at +0.7 V, about 100 uA; 
and at 1 mA it is up to +4 V and rising fast (not surprising, given 
the 0.5 pF plate-cathode capacitance -- it takes a tiny cathode to 
get the capacitance that low, which is also why the useful life of 
these tubes is fairly short). Rated cathode current is 300 uA (peak 
5 mA for no more than about 5 mS at a time). Extrapolating from the 
characteristic curve, the current is probably about 100 nA for an 
anode voltage of -1 V, and 10 nA for an anode voltage of -1.5 V. 

Best regards, 

Don 






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End of R-390 Digest, Vol 61, Issue 39 
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