[Novice-Rigs] Modern Crystals Article

[Brian Carling]

Did anyone read the in-depth article in the latest (May 2005) issue of 
Electric Radio
about the use of my crystals in Pierce Oscillators by G3UUR?
David Gordon-Smith is a materials science guru, who has done quite a lot of
research on a variety of chemicals and materials. 

Dave went into the Q, equivalent circuits, ESR etc. in a detailed analysis.
He took a variety of my crystals and put them to the test in a punishing 
series of 
experiments including some in which they survived an internal 
temperature rise to
over 200 degrees centigrade!

This 8 page article describes the qualities of the newer HC49/U crystals
in detail and goes on to describe methods for limiting crystal current.
The Pierce oscillator is of course one of  the hardest on crystals.
A Colpitts style circuit where the crystal is not directly coupled to 
the plate circuit,
pulls considerably less current through the crystal element and is likely to
produce better keying characteristics.  Most commercial rigs from the 
1960s and
the typical homebrew tube transmitter designs, used the Colpitts type 
oscillator,
yet a small number of the rigs used the Pierce design. Many people have 
reported
success using my crystals in Pierce circuits, but again I do not recommend
trying this with an 813 power oscillator running 1500V on the plate! Your
normal 6AG7, 6V6 and 6L6 circuits etc. would do fine.

Dave's article answers a lot of questions for those who were interested 
in knowing
how much crystal current is too much. Ideally you would want to limit 
crystal current
to 40 mA, BUT success can still be had up to 60 mA or even 70 mA in most 
cases.
The old series 60 mA lamp trick still works as well today as it did back 
in the old days
and it is a pretty decent indicator of approximate crystal current.

The report says that it took more than 100 mA for several minutes of key 
down to damage
one of the little crystals. I suspect that he same might also be true of 
an FT243 if you
abused it that much.

Comments? Questions?