[Hallicrafters] rubberizing crystals

[Nash4447 at cs.com]

Hi Gang  - got a technical question for all you whizbangs out there in 
hamland.

Lets say you have a crystal that for whatever reason is not on frequency - 
off a bit.

I have seen some sets where a fixed or trimmer cap is installed either in 
parallel with the xtal or in series with the xtal. What is the significance of 
each of these two possible arrangements?  Does one method move the freq up and 
the other down?
Also, I have seen some sets where a slug tuned coil is in series with the 
xtal
for making frequency adjustments.   Is this supposed to move the freq up or 
down?

I haven't found anything definitive on the web about tweaking xtal 
frequencies. 

So far, I have been only able to move the frequency DOWN a bit using any of 
these techniques.  Never up. 

Also, does anyone know anything about "ceramic resonators" ? I found some 
info on the web about these. Will they just plug in place of a crystal for any 
given frequency or are circuit changes needed?  Will they even work in tube gear 
given the currents involved?

Comments?                      Glenn   K6PZT
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