[R-390] Parts Request

[2002tii]

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