The AM Forum

In the few amplifiers I have built , I have always used a filament choke.  Some commercial, some homebrew chokes, never had a problem.  I have been looking at the Bill Orr #14 and there are several amp builds shown, 4-250, 4-1000, 450 TH and none use a filament choke..  This is 1956....  Just curious..
 

Are those amps grounded-grid/filament-driven? Always figured the choke was required when you're applying RF drive to the filament. If grid driven, with the filament at RF ground through a cap, you wouldn't need a choke.

Grid driven doesn't require a choke.

Filament tubes that are cathode driven do.

I've seen guys use fils chokes on grid driven tubes before. Nive also seen guys riding wheelies through traffic at 515 at night.

--Shane
WP2ASS / ex KD6VXI

Hey Shane

How the heck are you.

47

   I don't have Orr #14, but I do have #15 - one of my favorites - and it does feature both grid and filament driven amps. For the most part, the grid-driven amps don't use filament chokes, while the filament driven ones do, as expected. There is one interesting exception: a pair of 803 tetrodes with all grids grounded, and RF drive applied to the filaments through a center tap on the secondary of the filament transformer.
   Orr's explanation: "When operating in the 3.5 - 7 Mc. region it is entirely possible to apply the RF driving voltage directly to the filament circuit without the necessity of adding filament RF chokes to isolate the driving voltage from the filament transformer. This makes for circuit simplicity and lower initial cost. If this circuit is used with high frequency tubes such as the 813 and 7094, the use of filament chokes for operation above 7 Mc. is recommended."
   Interesting to note that he uses no input tuning, so even with the RF drive free to interact with the secondary of the filament transformer the input impedance must be close to 50 ohms. What's a bit counterintuitive to me is that I'd think that the reactance of the filament transformer to RF would increase as the frequency increased; thus making the filament transformer's presence even less notable above 7 Mc. Unless maybe he's concerned that the filament transformer's secondary might become self-resonant at higher frequencies?

I'm good.  Living the good life here in the Carribean!

Next year I should have towers and stuff here.  For now, a Ke wood 850 is all I have.


No amp, a single 40 to 10 vertical.

I sometimes here the 75 meter am guys, but they are really low.

--Shane
WP2ASS / ex KD6VXI

I think the capacitance to ground is the concern. There's an awful lot of copper and varnished paper layers and the core (and maybe an electrostatic shield) is at ground - sounds like a capacitor to me  ;)

That makes sense, especially considering that this capacitance is a path to ground.

There are low capacitance filament chokes.....  At least there used to be.

I used them, from Dahl, to run larger tetrode in GG and triode as well.

160A filament chokes where not easy to build in practice.  However, by mounting the filament xformer on Glastic standoffs and then putting a choke on the primary side of the xformer you could get away with just a few amps of filament choke!

Worked great.

--Shane
WP2ASS / ex KD6VXI

   Intriguing idea. I'm going through the preliminaries of designing a 3-500Z three-holer. I've got plenty of the traditional bifilar-wound filament chokes, wound with 10 gauge enameled wire, but my experimentation shows that passing the total filament current - 45 amps - though a single choke results in one very hot choke. I could certainly use three chokes - one for each tube - but that takes up a lot of room in an already crowded box. Placing a couple of vastly smaller (and cheaper) chokes in the primary, with the core floated, sounds like an attractive option.
   My filament transformer reads 170uh from each side of the secondary to the center-tap, while the secondary to primary capacitance is 200pf. I'd want to experiment a bit to see if it's resonant anywhere.