The AM Forum

Anybody know what this is? 

I was digging through the junk box a few days ago and I found this thing.
It was made by B&W.  Part number F0-30.  Four contacts on each side.  Contacts jumpered on both sides as shown in the picture.  Heavy little bugger.  Sealed.  Can’t open it.  Don’t want to destroy the little guy.

5” inches long, 2 ½” high, 2” wide.  Feels like it weighs about a pound.
Very low resistance measured from end to end.  My Radio Shack multimeter can’t measure it.  About 80 pf across the contacts on each end.  Same capacitance measured from any contact to the case.

If I had to guess I would say it is some kind of filament choke for a high power GG tube linear.

Any ideas?

Jerry

Jerry,

It looks almost like a FC-30 filament choke for a GG linear.

I don't know about the "FO-30" however.  Put a 1000 ohm  resistor in series with it and put 3.8 mhz across the whole circuit with a sig gen. With a scope, measure how much voltage drops across the choke. This will tell you its approximate impedance on 80M once the ratio of voltage drop is compared to the resistor.  You will want it to be at least 600 ohms or more on 80M to work as a fil choke. This assumes the wire size can handle the current. (30A)


Here's a thread about the FC-30:

 ...And many thanks to the SEVEN respondents who took the time to address my
query yesterday re. the whys & wherefores of my old Barker & Williamson type
"FC-30" filament choke...

For benefit of any interested parties who may one day chance across one of
these assemblies at a Hamfest, etc., here's what I learned on the PLUS side
(file it away on the back-burner --- it may prove to be useful at some time):

-The FC-30 is, in essence, TWO type FC-15 filament chokes in ONE enclosure;

-Each of these FC-15 chokes is rated at 15-amperes: to safely pass 30-amperes
through them, you simply parallel the two, and,

-The unit has the flexibility of feeding two separate tubes from two separate
filament transformers via its two separate chokes (maximum 15-amperes per
filament, of course), or, one tube / paralleled tubes requiring 30-amperes
maximum.

Now, on the MINUS side:

-The FC-30 is specified for operation from 3.5-MHz to 30-MHz, inclusive:
because each FC-15 rod is only 3-1/2" long, it does not have enough inductance
for 160-meters;

-Even if I elected to SERIES-connect the two interior FC-15's to secure twice
the inductance for possible use on Topband, I would be robbing it of its
ability to handle 30-amperes of filament current...and my hoped-for 4-1000A
requires 21-amperes to make it glow (sigh!).

That does say FC-30 on the tag, the ends of the c are close enough to make it look like an o.
Shelby

Tom,
I used an FC 30 in my old 4-1000A rig. When my friend wanted to add 160 we had no problems driving the tube.  I never measured the inductance but it worked fine.The inductors are in parallel to handle the 21 amps. I have one in th eshack somewhere I'll measure it if I spot it.

Yes, that's an FC30...

I used one way back when I was out in KH6 in a  811A x 4  final that my old HT-32 drove. Still have that final in the garage along with a few hunderd pounds of old gear.

Never got it tamed on 10 meters but did run it later on top band CW, when 160 became available, with no serious problems... 

Thanks for the info guys.  I figured it must be some kind of heavy duty GG filament choke. 

I've been a ham for 55 years but this is the first time I've seen one of these.  I figgured if anybody knew what it was, I would find that person here. 

Shelby you must have eyes like a hawk.  I finally saw the number was FC-30 after looking at it through a magnifier glass.

Jerry

Yes, the "C" sure looked like a zero. I thought it might be a C too, but it was just too convincing in the picture as a zero.... ;D


BTW, to replace that whole choke, just run the filaments leads through a stack of ferrite sleeves. It does the same thing if the core permability and total inductance is enough. I do that for all my linears now and have gotten rid of any chokes or wires wound on rod cores, etc.   If anything, there is less fil voltage drop waste across the stack wire length and no splices to go bad.

T

Tom said:
Tom, do you twist your filament wires and run that through the stack of torroids? Do you have a picture?
TNX.

In Orr's 1959 handbook, on page 628, he discusses a GG amplifier for 80 and 40 using two 803 tubes.

The design doesn't even use a filament choke.

Just drives the filament directly.

This might not work on higher frequencies due to stray capacitance in the filament transformer, but apparently it works on 7MHz and below.

Hmmm.... I've never seen that done, but maybe Orr floated the fil transformer and used the inductance from the fil transformer itself as a choke?  You wud think RF wud couple into the primary and get into the AC lines.  The idea is to keep RF out of the fil xfmr in the first place, but I'm always looking to learn something new... ;D


Mike - I have a fil core choke pic I'll post later  for you, OM.

T