3CX3000A7 filament choke question.

[W3RSW]

That GPT choke does not have 1/2 in dia wire.  It's coaxial with a smaller inner conductor of about 1/8 in. Dia and braid of maybe  3/8 in. Dia. But with annular conductivity limited by , say 1/32 in.  Thickness.
RF of course mainly conducts on the outside of a wire but that conductivity decreases expomentially as freq. decreases.
The DC component or very low, say 60 cycle AC component is, of course, dependent on total cross section of a wire.

Hence the reactance is not what it first seems, let alone filament voltage drop.

[Opcom]

Yes it's coaxial; I think there's a beauty to that. As far as using it, note the original GPT-10K transmitter circuit applied normal bias and screen voltages to the 4CX5000A as in class AB setup, all grounded for RF. So the workings may be a bit different. I didn't intend to supplant the original suggestions, but would suggest a look at the schematic of that amp (it's online) if its filament choke is to be used for the 3CX3000. I see no issues but there's a lot of things I don't see right off. Thanks for the readings with the ferrite rods added, that is worth a lot!

[WD5JKO]

 Yesterday at the Belton Hamfest I picked up a choke that appears to be an exact match to Patrick's choke pictured in post 20 of this thread. I got it for free.

  If anybody wants that choke, PM me. It is yours for the cost of shipping.

Jim
Wd5JKO

[fg5fc]

Hello all
I have built some years ago a linear amplifier with a 3cx3000f7 tube, i was using a homemade filament choke with 4 ferrites inside but it was working warm because not enough cooling and not enough coil to work on 80 meters, i plan to use an 52 amp RF Parts filament choke i bought some years ago , but i wonder if this choke should be OK ,52 amps is just the power consuming of the filament; may i have some  advice ?

73's

[Opcom]

B&W also made an FC-series choke, a -60 or -90, I don't recall but it is big and is enough for the 3CX3000. The "Jaws" amp has one. I don't have a pic. Probably kind of rare now, as is why I bought a 10K fil. choke. MGS4U.com sometimes has goodies that help with the more fun projects.

[KD6VXI]

Float the filament xformer on glastic.   Put the chokes on the primary.

Now you need a 3 amp choke.   Or 1.5 amp.

--Shane
KD6VXI

[fg5fc]

Thanks for the reply
@opcom, i have tried the Fc 50 B  W, i have burnt it i guess not enough cooling
@Shane , the problem i have is that the wire between the filament transformer and the choke is quite long ( 80 cm), the power of this transformer is 100 amp and it's very heavy i can not put it close the socket.

[KY4SP]

Thanks for the reply
@opcom, i have tried the Fc 50 B  W, i have burnt it i guess not enough cooling...

...was using a homemade filament choke with 4 ferrites inside but it was working warm because not enough cooling...i plan to use an 52 amp RF Parts filament choke i bought some years ago...may i have some  advice ?...

It looks as if you already realize you may benefit from more cooling. Getting more cooling air may be easier than getting another choke. A surplus of air is far better than not enough.

[K1JJ]

Thanks for the reply
@opcom, i have tried the Fc 50 B  W, i have burnt it i guess not enough cooling
@Shane , the problem i have is that the wire between the filament transformer and the choke is quite long ( 80 cm), the power of this transformer is 100 amp and it's very heavy i can not put it close the socket.

Your situation sounds like the perfect opportunity to load these two wires with large ferrite sleeves. Put a string of ferrite as close to the tube socket as possible with both filament leads going thru them.  See my post about this back at the beginning of this thread. (second post)   Put .01 uf capacitors to ground right after the ferrite stack ends (one cap on each wire) - and the long fil lead length remaining back to the transformer will not matter RF-wise.

T

[fg5fc]

Thanks Tom for the input:
About the .01µf if i understood well, i have to put one condenser between ground and each wire, that mean two condensers?

[K1JJ]

Thanks Tom for the input:
About the .01µf if i understood well, i have to put one condenser between ground and each wire, that mean two condensers?

Yes, that is correct. Also, add one .01 connected between the two wires at the filament socket pins, as usual, to balance the RF flow through the tube filament.

[fg5fc]

Ok fine!

[Opcom]

Sounds like a nice project.

I have several homebrew linear amps that all use the same type of filament choke. It is simple and least likely to have any resonances....
...
To test it before installation, connect a 500 ohm carbon resistor (non-inductive) in series

Tom, K1JJ

That is probably the best answer. Cheapest for certain. Wanted to say, be sure it is really a carbon resistor. Some new resistors that look like old carbon ones are really some other kind internally and may be inductive so check the resistor before using it in the test.