Re: Help with first VTTC

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Mike Veldman by way of Terry Fritz <twftesla-at-qwest-dot-net>" <vmike-at-email.ceat.okstate.edu>


>Here goes with the questions
>1.  Do i need to ballast(current limit) my MOT??
 
You shouldn't, the circuit will only draw as much current as needed.
Although one 811A is only good for 250ma.

>2.  Why does the filiment transformer tie into the HV part of the coil?? Im 
>using a seperate filiment transformer, so do i need to do this?
 
The transformer used was probably an old TV or surplus plate supply with
multiple windings.  You can use a seperate plate and fil transformer.

>3.  The explenation says that the MOT does not have a level shifted output, 
>then why does it have both a cap and diode on the output?

The rectifier and filter are just to make DC, no doubling, and actually the
schematic is incorrectly drawn.  The filter cap should return to the diode,
and one side of your filiment transformer or preferably the center tap if
one is available should be grounded.

>4.  Can the value of C2 be changed from .02uf to another value, i have these 
>selections:.01 , .015 , .03         Will any of these work, particularly the 
>.01 or .03.  What else will change due to this?

 The .03 will work okay, a very small change in frequency.

>5.  Would this coil work better with a different pri/sec set up.  I am 
>making the one he has on the page, but i also have available a 3.5"x12" #28 
>wound secondary, and some 6"PVC available for the primary coil.  If i did 
>this, what would be a good number of turns for each coil on the primary 
>side??

As long as you are copying this circuit I'd stick with his coil.

>6.  Do i need a variac?  The page doesnt mention one, if i do need one i 
>dont have one(hmmm).

I use a variac on all of my tube coil plate supplies.  It allows me to
bring the coil up gently and to demonstrate other effects.

>7.  Why on earth did he put a switch off of the output of the MOT?  Its at 
>2000v 500ma, sounds really risky, but i guess thats so the filiment can heat 
>up with out the plate supply on.  I van avoid this with the seperate 
>transformer.

The switch is to allow the fils to warm up before applying plate voltage.
You can do this with a seperate primary switch on the plate transformer.
It used to be done a lot in tube equipment, not too risky but usually put
on the ground side of the HV winding.

>8.  How could i modify this circuit later on, to run 2 811A tubes(i have 2 
>of them, matched pair).

You can directly parallel the 811A's if they are matched, or use chokes in
the plate and grid leads.  I have one running with 4 811A tubes, I directly
parallel the grids and made four chokes for the plates by winding 20 turns
of #24 enameled wire on and parallel to a 47 ohm 2 watt carbon resistor.

Hope this helps some.  Good luck,
mike