[Prev][Next][Index][Thread]

Re: 500W Tube Coil Takes Off!



---------- Forwarded message ----------
Date: Wed, 15 Oct 1997 15:46:43 -0400
From: Thomas McGahee <tom_mcgahee-at-sigmais-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Cc: jparisse-at-ddlabs-dot-com
Subject: Re: 500W Tube Coil Takes Off! 



----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: 500W Tube Coil Takes Off! 
> Date: Wednesday, October 15, 1997 1:16 PM
> 
> 
> 
> ---------- Forwarded message ----------
> Date: Wed, 15 Oct 1997 00:40:49 -0700
> From: "Jeff W. Parisse" <jparisse-at-ddlabs-dot-com>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: 500W Tube Coil Takes Off!
> 
> All,
> 
> Many thanks to Brent Turner, Mark Rzeszotarski and John Freau (and others
> not mentioned) for their help with my first tube coil. I ran it for the first
time
> tonight and it worked great (for a first try of course). I managed to get
> 8-10" sparks and a heck of allot of radiated RF.
> 
> The coil uses two 250TH tubes in parallel and a 0-4200 Vdc plate supply.
> The primary form is 7" dia. and the secondary is 5" dia. x 20". The primary
> caps add up to .0021uf to .0022uf (variable) and with a 22 turn primary coil,
> resonate near 356 kHz.
> 
> Question: The grid resistor, although seemingly overrated, got red hot during
> an extended run. Since I had already adjusted it until I got the correct
plate
> current (700ma), I'm guessing that it was seeing too much voltage from the
> grid coil. I started to remove some windings from the grid coil but stopped
> after removing two, I thought I should ask the group about this first.
> 
> Observation: Using a 10H filter choke to smooth the dc really killed spark
> output. From 8" to say, 1". Without the choke, the coil has a 60Hz (120Hz)
> hum that I really wanted to filter away. I placed the choke between the 
> center tap of the filament transformer and ground. Maybe I should move it
> to between the negative side of the full wave bridge rectifier and ground? 
> 
> 
> Jeff W. Parisse, Art Director
> Digital Design Laboratories
> www.ddlabs-dot-com
> 
> 

Jeff,
Here's what you need as the filter:

                ===========
>-----------*---CCCCCCCCCCC----*-------->
            |                  |
          -----              -----
          -----              -----
            |                  |
>-----------*------------------*-------->

That SECOND capacitor is *VERY* important.
The first capacitor represents your existing power supply capacitor. Hmmmm. I
am making the assumption that you are using a rectified power supply. Or are
you using the tubes in a self-rectification mode? If so, things work somewhat
differently from that shown. But I'll continue with the assumption that you are
using some form of rectification first, before the first cap.

The CCCCCC represents the filter choke. The second cap is very important, as it
becomes the localized source of electrons (current) for the remainder of the
circuit. Value of this cap can be equal to the first, and oftentimes smaller by
half. You will get much better results if the original AC is full wave
rectified instead of just half wave rectified. That will make the ripple
frequency double, and means that any given choke/cap combination will be twice
as effective.

**********
If using self rectification, the above method will also help, but not as much.
The reason is that in a self rectification mode the tube is acting like the
diode, and so the circuit now approximates to this:

                ===========
>-----------*---CCCCCCCCCCC----*--------|
 AC         |                  |        | TUBE
          -----              -----      V Anode
          -----              -----     ===
 GND        |                  |        | Cathode
>-----------*------------------*--------|

Note that this arrangement has almost 100% AC on the first cap! That is because
it is directly connected to the transformer, and the tube that acts as a
rectifier is AFTER the choke. Not a very satisfactory arrangement!!

Now compare that with this:

   Diode        ===========
>---->|-----*---CCCCCCCCCCC----*--------|
 AC      DC |                  |        | TUBE
          -----              -----      V Anode
          -----              -----     ===
 GND        |                  |        | Cathode
>-----------*------------------*--------|

In the last arrangement the first cap sees a fluctuating DC, and the second cap
sees more DC level and less AC ripple. The use of the diode removes the source
of one part of the problem, and then the second capacitor removes most of the
remaining problem (which is that the choke refuses to supply current in large
chunks. But the second cap DOES). 

Hope this helps.
Fr. Tom McGahee