Salvaging Crumbling Coils

[W7SOE]

I have a B&W turret coil assembly.  All the plastic that forms the coils and holds them in place has turned to dust.  With some acrylic bits and glue from Tap Plastics I have it mostly together.  A bit of brain surgery trying to get it all straight.

I plan on installing it in my 813 grounded grid linear for input tuning.

I have the attached instructions but they don't make any sense to me.  I suppose the idea is to drive each primary of the coild and have the secondary across the variable cap.  I don't understand the B+, jumper, link notation...


Rich

[Opcom]

B+ is the cold side, could be bias. Plate is the hot side, could be grid if used as an input coil set. These can be used either way. This set of coils is the high impedance set.

The link line is a pair of connections on the two wafers there, and the proper link coil is selected by those wafers to correspond to the chosen plate coil.  That is the llow impedance or Output of the coil set, or could be the input if used as a grid coil set.

The switch position selects the proper coil and link for each band.

IIRC there is supposed to be a 150 pF cap or a dual section 150pF cap across the hi-z end for tuning, of course you have to try different values. Possibly part of the data sheet is missing?

The ones with the link at the cold end are for single ended. The ones with the link in the middle are possibly having the coils center-tapped. You can examine this on yours.

Good for you repairing it. Mine was also ruined and one coil had been changed or 2726KC an old emergency frequency but I gave it to a friend who builds period stuff wit period parts and he will fix it up as he is good at coil work. I put in an MB-40 and am happy.

The picture rfdeck1.png is the schematic of the grid circuit that I made when the TX had the ruined turret installed. It might help you understand yours. It is "Of The Turret".  Other pics are the actual grid compartment and turret.

[W7SOE]

Thanks for the insight Patrick.

After examining mine it appears straightforward.  There are two connections for the primary and two for the secondary.  All unused inputs/outputs are shorted together.  There is also the center wafer switch that taps the center of the secondary, for the bias?

What is the purpose of bias on the input?  How is related to any bias I add the the center tap of my filament transformer?  (Grounded grid)

My handbook shows a simple variable cap across the output coil, in this case 100 pF.  In this example the turret coils are homebrew and lack the center tap.

Rich

[Opcom]

The coil was designed for high impedance stuff where bias or B+ goes on the high impedance coil and the Z around 1000-5000 Ohms or whatever. It was not designed for for hi-Z section to drive a 200-500 Ohms cathode in grounded grid service. The bias tap would not be very useful in that case.

My feeling about it is that if you want to use it for cathode tuning, then the low impedance coil which is closer to 200-500 Ohms would possibly be useful connected into the cathode circuit, and the tuning cap across the hi-Z section would serve to tune the low Z section by the transformer action of the coil set.

I feel that the impedance step-up from the link to the hi-Z coil would step up a low 50-ohm impedance to a lot more than the rather low driving impedance of the 813 cathode.

A possible work around would be to use a broadband toroid transformer to go from 50 Ohms to the driving Z of the 813 (what is that by the way? about 200 Ohms?)

In any case you have to decide how to use it. It looks like there is some experimenting to do. The schematic is just wild speculation.

[w7fox]

Rich,
The coils and turret you have are suited to driving the grid of a tetrode, not the cathode.  I have a pair of 813s in grounded grid and designed the input section using the formulas in several handbooks.  It is a pi section that matches 50 ohms to about 300 ohms, with a Q of 2.  Your coils are made for matching 50 ohms to 5000 or 10000 ohms.  Hope this helps, good luck.

73,
Chris

[W7SOE]

I have the wrong coils!  Thank you for pointing this out.  Well that was an exercise in futility! 

I will save them for a grid-driven project... 

I will seek other input tuning methods.  As was suggested, this is an option:

http://home.earthlink.net/~wd7s/TU-6B.htm


Thank you all for the help

Rich

[w7fox]

Rich,

The wd7s input relay board looks like an elegant solution!  Lots of good information in the table too.  When I built my amplifier, I think I got the design for the input network from the "west coast handbook" as it is called, and it works quite well.  One issue I ran up against though.  If you look at the table, you see that a single 813 requires only 25 watts drive, or 50 watts for two 813s.  I am driving my amplifier with a Heathkit SB-101 which has about 100 watts output.  I had trouble holding down the drive, and it was very easy to overdrive the amplifier and cause too much grid current.  I had no desire to splatter or damage my tubes, so I reduced the drive to the control grids.  I found the tip in a couple of places, and it amounts to connecting the control grid half way up the filament choke.  The screen grid and supressor grid are still grounded.  In my case, I used a second choke made from a ferrite rod about 3/8 inch in diameter, and about 4 inches long.  I actually made a test choke with several taps to see which worked best, and half way between ground and cathode worked best.  Now I can drive the amp with the full output of the exciter with no flat topping or excess grid current.  If you are interested, I can draw a schematic.
73,
Chris

[W7SOE]

Chris,
That is an interesting method of attenuating the input signal.  Unfortunately I am using a B&W choke which is sealed in a box.  I am driving it with a Drake TR-4 and am hoping to be able to lower the output there.  If I can't i can always build my own choke.

Thanks for the good advice

Rich

[w7fox]

Rich,

I am using the B&W filament choke too, and I didn't want to pry open the enclosure, that's why I used an additional choke on the control grid.  Choke is probably a misnomer, it is really an autotransformer.  I'm not sure exactly how the thing works, it must lower the gain of the amplifier, hence, take more drive.  As I understand it, the additional drive is not wasted, but "passes through" and appears in the output as useful power.

73,
Chris

[W7SOE]

Chris,
Would you mind providing a schematic?

Rich

[w7fox]

Rich,

Forgive me for not answering your last request, I promise to use the "Show new replies to your posts" button more often.

Here is a pic of the circuit I got the idea from.  It's from the 1962 Radio Handbook.