[time-nuts] Replacement A9 boards for the HP 5065A

[Poul-Henning Kamp]

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In message <5A9C4644.5030601 at yandex.com>, Charles Steinmetz writes:

>Some times, "just because it's better" is a sufficient 
>reason to overdesign, particularly where the incremental cost is low and 
>especially where the projected number of units is low, both of which are 
>true WRT the improved A9 board.

I fully agree, and if it were me, I would absolutely use the best
capacitor I could find at a non-insane price.

But that was exacly my point:  The only thing you (might!) get by
spending an *insane* amount of money on that capacitor is PTFE and
its better dielectric absorption.

But there are three good reasons not to.

First, they are HUGE, typically a couple of inches in diameter and
four or five inches long[1]

Second, there are no reputable suppliers of ~5µF PTFE capacitors
that I have been able to find, there are only audiohomoeopaths.

*Nowhere* have I seen anybody buy an audiohomoepathy PTFE capacitor
and publish a traceable DA measurement for it, much less information
about tolerance, lot variations etc.

I am certainly not going to shell out $785.07 for what is claimed
to be a newly produced PTFE capacitor[2]:

	https://www.v-cap.com/cutf-capacitors.php

Neither am I going to shell out $34.70 for something which may or
may not be USSR army surplus and which may or may not be PTFE[3].

	https://www.ebay.co.uk/itm/Lot-of-1-piece-K72-11-Teflon-Capacitors-0-22uF-4-7uF-125V-1000V-NOS-Tested/132443841947?hash=item1ed644a59b:m:mPF1aOO4F7dJXTSF0qHKcSQ

And third, the difference in DA between PTFE and Polystyrene is
barely a factor five and it is from 0.05%(PS) to 0.01%(PTFE).

That is simply not going to make *any* difference in an HP5065A.

The trick here is to do the math on the S/N ratio of the optical
signal:  The A9-capacitor is primarily a low-pass filter, and pretty
much any sane capacitor can do that.

Poul-Henning

[1] Almost any other type of capacitor is wound from two layers of
insulator on which a thin layer of metal has been deposited by
evaporation or sputtering.  PTFE must be wound from two PTFE films
and two metal films, which means a lot more metal, because it must
have the mechanical strength for the winding operation.

[2] Notice the claimed "dielectric coefficient of 1.45" ?  Either
he means "Dielectric Absorption" in which case the number is in %
and *horrible*, or he means "Dielectric Constant" in which case the
number is physically impossible.

[3] Because USSR wasn't very good at PTFE to begin with.

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