Re: Help with tubecoil
Hello, I would have to differ from Dr. R's opinion. I have had good luck
with using coils using a winding ratio between 4:1 to 4.5:1 with #28
magnet wire. A 3" X 13" coil using #28 produced 24" sparks at less then
2KW power using two 833A's (both tubes only had one flament working). "I"
would start with 25-30 turns of #12 PVC insulated wire close wound as the
primary and tap down to find best impedance match, which, by the way, is
the real hassle. Grid feedback is indeed good to monitor current on as Dr.
R said. You can see my Audio modulated Tube coil schematic at (produced a
max of 10" sparks Amplitude modulated CW) :
And John Freau's coil schematic at:
Both of these designs can be modified for any tube, etc.
I have never used parasitic supresion chokes on my coils, only RF bypass
caps to protect AC mains, etc.
On Sun, 6 Feb 2000, Tesla List wrote:
> Original Poster: "Dr. Resonance" <Dr.Resonance-at-next-wave-dot-net>
> No on the 4.5:1 for vacuum tube coils. This is a resonance transformer mode
> so height ratio should be around 10:1 to 14:1 for best results. Also, use
> very small wire like 40 or 42 AWG polythermalize. In this mode voltage
> buildup is more standing waves and transmission line effects than
> transformer action.
> For best impedance match you need a lot of turns on primary. I would
> suggest starting with some 14 AWG and around 40 turns. Tap it down as
> necessary until you get the best resonance. Run a variac to control only
> the plate voltage on the tubes during testing (not the filament of course).
> Be sure to use a meter in the grid circuit as you are adjusting your drive
> on the tubes and also a plate voltage meter to see what potential you are
> applying. These are only necessary during initial testing. Don't exceed
> the recommended grid drive to the tubes.
> Coeff. of coupling must be very tight on tube coils for best performance.
> Use parasitic chokes on both plates of each tube and also parasitic chokes
> in the filament supply (See older version of ARRL Radio Amateur's Handbook
> for help on this subject -- around 1970).
> Dr. Resonance
> -----Original Message-----
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
> Date: Sunday, February 06, 2000 3:58 PM
> Subject: Help with tubecoil
> >Original Poster: "Tero Ranta" <dixie-at-netti.fi>
> >Dear List,
> >I have just acquired two QB3.5/750 transmitting tetrodes. American
> >equivalents are for example 4-250A and 6156.
> >I'm planning to use them probably in push-pull mode.
> >Some class-C operating info:
> >Ua: 4000V
> >Ia: 312mA
> >Pa: 248W
> >Pout: 1000W
> >(ug1g2: 5.1)
> >What kind of a primary/secondary coil would best suite these tubes? I have
> >bad experience with candlestick secondaries. Would the Dr.Resonance
> >suggested 4.5:1 ratio be the best?
> >I'd also like to hear suggestions about what kind of parasitic/RFI
> >suppression to use with such a coil.
> >Please send your replies also to tero.ranta-at-bigfoot-dot-com