Re: Tube coils

[FutureT-at-xxxxxxx (by way of Terry Fritz <twftesla-at-xxxxxxxxxx>)]

In a message dated 11/20/00 9:54:26 PM Eastern Standard Time, 
tesla-at-pupman-dot-com writes:

> Original poster: "Stephen Carpenter" <sjc-at-carpanet-dot-net> 
>  
>  Ive been thinking about my next coil....
>  
>  Thought of getting NSTs and this and that...then thought, why not try
>  a tube coil. However...I am a bit perplexed. I have found some scematics
>  and things, but very little good information (for an added bonus I can't
>  check the list archives here - the website is down it seems)
>  
>  I noticed a couple of things...
>  
>  It looks like a fairly simple tube based occilator. I havn't had time to 
>  really to a close hard look at them (its been a few years since i really
>  read any scematics) but they are efinitly more complicated than the
>  simple tesla coild tank curcuit.

Steve,

Tube coils are just a simple power oscillator with a tuned coupled 
resonator as you say.  Yes, they are definitely more complicated
than spark gap TC's.  More critical and touchy to tune and adjust also.

>  
>  So I have a couple of questions....
>  
>  1) What is a "grid coil"? 
>  2) Are both the "grid coil" and "primary" coupled with the secondary?

The grid coil only needs to be coupled to the primary.  The grid coil
receives a pulse from the plate or tank coil at every cycle and feeds
it to the grid in the proper phase to keep the oscillations going.  It's the
method for providing feedback to the oscillator.

>  3) Does anyone know of a good source for vacuume tubes (the MIT flea 
>  marker doesn't open again ttill april or I would look there) either
>  online or local to boston? (Going to check the store down the street
>  that buys old sound equipment...they have tube amps in there often)

There are some surplus suppliers.  Yes, hamfests and flea markets
are good places.  Some of the tubes will be bad though, esp if the 
price is real low.  Some of the sellers are crooks unfortunately.  You
may want to get their address and phone number, etc if you buy a tube,
unless it's cheap and you take a chance.  Some of the audio folks 
might over-charge (I'm not sure), but the audio folks really see these
tubes as super-valuable.

Tube coils will give much shorter sparks for a given power input
than spark gap TC's.  Figure:

    spark length (inches) = .42*SQRTinput power in watts (wallplug)
    for tube coils.

   For spark gap TC's figure:

   Spark length (inches) = 1.7*SQRTinput power in watts (wallplug)

(those formulas are for efficient coils).

         http://hometown.aol-dot-com/futuret/page3.html

John Freau

>  
>  Any advice or comments would be appreciated. 
>  
>  -Steve