K1JJ Tuna Parts?

[KF1Z]

You should avoid using the doorknobs...
IF your variable cap needs just a little more capacitance, you could try to add a doorknob in parallel with the cap (or each segment of the cap if it's split/ganged)

But the power you run and antenna load etc will determine how much circulating current and voltage, .... doorknobs may/may not handle the task.

7 to 10kv is a safe voltage range... but must be able to handle the current too.

I've used a 5kv... ok most of the time, but when you hit the right antenna+feedline/frequency combination, sparks can fly!

[K1JJ]

DJ,

The guys got it correct on the doorknobs.  They can be used as coupling caps in plate circuits and bypass caps at the bottom of a plate choke. However, resonant circuits, like tank circuits, have tremendous circulating current depending on their Q.

These old TV door knobs were designed to handle high voltage(30kv) but handle only low RF current, less than a few amps in some cases. So they will literally explode when used in a 1KW amplifer in the plate tank circuit as C1 or C2. I've seen a friend do it when using them as plate tuning caps with a switch on a 4-1000A linear. I warned him, but what a mess he had to clean up...


Yes, 260pf will be OK in the tuner. As I mentioned, if you run out of capacitance, there is usually another combination of taps where you can use more coil and less capacitance to find a good 1:1 swr match. There are usually at least two sets of combos. The Q will be different, that's all.

Worst case is look around for a suitable padder air variable. OR, find a small fixed vac cap. Some of those 100 -200pf fixed caps are cheap.  

Another solution is to use a high quality doorknob designed for current. . There are ones that are like, 100-200pf. They are designed for high current RF and good for 5-15KV or so. Some are beige with silver plated tabs. They are at flea mkts, usually <$10.

T

[K5UJ]

If doorknobs are a problem Fair Radio has 1 MHz tx micas for sale.

[KC2YOI]

OK, I'm getting a better idea of the value ranges.
I'm trying to wrap my head around the theory, not just Lincoln log this together.

While putting together a grocery list for the meat and potatoes components I don't want to jump on marginal stuff if holding out yields a cleaner, more rugged, set up.  

As far as vac var. caps, are the Eastern Bloc variety OK from anyone's experience?
Some neat turn counters offered also but not much use w/o the housing and their not giving 'em away   . The shipping is also the devil in the details. I'll check out the domestics sources mentioned at the top of the thread also.
For reference, would 10-1200Pf / 4Kv be marginal?  

Chris, I don't mean to co-op the thread here.

                                               D.  

[N5RLR]

Just skimmed over the thread as I'm about to head out the door, but...

Has anyone logged any typical capacitance values for various frequencies using the K1JJ Tuna?  Yes, the antenna/feedline combination would be a huge variable and no two setups may show the same C value for a given frequency, but a ballpark figure would be nice.

I wonder if a homebrew cap made from two lengths of bare/plated copper tubing, one smaller in diameter than the other, telescoping together, separated by Teflon sheet/tubing, would work.  Thoughts? 

[KF1Z]

I believe 375 to 500 pf is needed to get down to 160 meters, with a 40uH coil.


A 1200pf 4kv cap would be marginal, mostly because I think they are usualll more expensive than a, say, 500pf at 5kv...

I suppose if you came across a real bargain the 1200 would work... probably..
Not sure if it is designed to handle the current.

[W7POW]

OK, I have been able to scrounge a few parts for the tuner like this 220pf Cardwell, a few 100pf doorknobs, ceramic stand-offs, large insulated knob and this tapped tank coil which I dont think will have any use in the build.  It is only about 12 microhenries (18 turns, 3 inch diamater, 4.5 inches long) and besides it will be fun to build the large 3/8 copper coil with about 40 turns.  The coil will be cut in half and attached with jumpers for series or parallel hook-up.  I think this weekend I will start the build and go ahead and do it minus the input cap C2 until I can locate one.  Thanks for the input and help guys.
Chris.

[w3jn]

Don't count that coil out.  It all depends on your feedline length etc - IE the total complex load the tuner sees at its terminals.

On 75 the output is tapped at +/- 3 turns on my coil; the capacitor is applied to about the +/- 15 uH points from the center of the coil.    Wind an input link and see what happens!

I find that the parasitic capacitance in my huge coil pretty much precludes its use above 20 meters, so don't go nutso with winding a huge one.

[flintstone mop]

Don't count that coil out.  It all depends on your feedline length etc - IE the total complex load the tuner sees at its terminals.

On 75 the output is tapped at +/- 3 turns on my coil; the capacitor is applied to about the +/- 15 uH points from the center of the coil.    Wind an input link and see what happens!

I find that the parasitic capacitance in my huge coil pretty much precludes its use above 20 meters, so don't go nutso with winding a huge one.

YUP..........JN.
That's what happened to mine.
I thought the total push for the K1JJ tuner was the lower bands to overcome the losses from the crappy expensive tuners.
Fred