Valiant "pinwheel" cap replacement

[stevef]

What an odd device on the output coupling  In the midst of my rebuild, I measured capacitance and two of the sections are open.  What should I replace the unt with?   I have some Vishay 330pF 6000v ceramic disc caps, or is mica preferrable?

Thanks.
Steve KK7UV   

[W5SUM]

Steve
this little cap arrangement has been a bad source of trouble in Valiants. I replaced mine with a big old 6KV  Mica Cap. Yes, you can find room for it in there! Certainly do NOT use a combination of the little 500V Mica's, t his is just inviting trouble.

While your at it, check the other output loading caps, that are parallel/series mica caps. I bet you find at least one with a carbon track.

change them too

if you need some of these mica caps, let me know, I may have them in my junque box

Ronnie - W5SUM

[stevef]

I have quite an assortment of the old micas (1" square caps made by Sangamo, Cornell, Aerovox, El-Menco, Solar).  They are either 600, 1200, 2500 DCW (=WVDC?) rated.  I could kludge something together with those I suppose.

I take it that mica is preferred over the 6000v ceramic disc caps I referenced?

[W2JBL]

lots of rigs (and amps) suffer this problem. the thing to look for in the ratings is current capacity, not voltage rating. the voltage involved here at 50 ohms is not very high. low Z=higher current lower voltage. the postage stap micas fail because they can't handle the current. those bigger ones that have the mounting holes and solder tabs on them work very well. be warned however that the ones in the aveage junkbox could be 50-70 years old and leaky or damaged by moisture. best to test before use. i don't used doorknobs in this application- most handle high voltage OK but not much current.

[Ed/KB1HYS]

I used several small doorknobs in parallel.  I tried some special Hi-current jobs, (expensive) and they failed.  The DKs were a little tough to fit, but they work ok for me, but I tune into a 50 dummy load and use a tunner most of the time so maybe I haven't stressed them enough yet.  YMMV...

[stevef]

Well I've had no luck so far finding what I need in terms of suitable mica caps for both the 'pinwheel' replacment cap and the output loading caps.

I looked into doorknobs but it would cost $260 for all of them.

So then,  CDE makes little surface-mount RF capacitors (MC, MCN, MCH, MCHN models) that would take up no room at all, but they don't list current ratings in they're caps.

Any thoughts?  Meanwhile I keep hunting for the old micas...

Thanks, Steve, KK7UV

[Ed/KB1HYS]

$260???  hhmm I don't think you are looking at the same Door Knobs I was.  I purchased mine through a couple different online vendors and only spent about $25 total.  These are the small 1 inch diameter jobs.
 You only really need Four ( I think I don't have my notes out as I type this)  they can be parralleled together to make the larger values using some alum or copper sheet. (use a stout gauge to keep it from waffling around) I used an existing chassis connection to ground, and a 1 inch copper strapp to tie the commons together, the other ends go to the switch terminals.

You put a 300pf on the first contact, then another 300pf on the second.  then I think I put in another 300 and a bigger value in the last spot. I set it up so the last settings would add them in parallel to get the bigger values I needed. Don't worry about getting the exact same PF values listed in the schematic, just get as close as you can, as there is also the variable cap in there for fine tuning.

[stevef]

I was looking at RFParts, SurplusSales, and, FairRadio for the various combinations of values that I need.  I'll see if I can work up cheaper combinations by not trying to hit the exact values.

[w0ng]

This was my solution to the problematic "pinwheel" loading capacitor in the Valiant. Fabricated a HB bracket from scrap aluminum to hole the doorknobs. Works really well.

[Steve - WB3HUZ]

Very nice! Those ceramic jobs will be much more FB.

[WA1GFZ]

It is best to use NPO temperature compensation type door knobs. Ther are the most stable. I don't think they come over 300 PF in value. stay away from high values like 1000pf in this application, they go off like an m80.

[Ed/KB1HYS]

Very nice work OM.  Ya done good.

[WQ9E]

At the Valiant power level, you should be able to use the heavier "snubber" type micas also to replace the "turnstyle" combined cap.  I have used this CDE series without problem and they are cheap and readily available through Mouser:  http://www.cde.com/catalogs/CD-CDV16.pdf  The MCM/MIN series are probably even better but I have never tried them:  http://www.cde.com/catalogs/MCM-MIN.pdf and also readily available through Mouser.

The Valiant coarse load is set up so that the fixed capacitors give you 300 pf increments and the fine load interpolates in between these coarse values.  For a conservative current rating, for the lower capacitance values use two at twice the capacitance in series to take advantage of the higher current ratings of the higher value caps. 

Rodger WQ9E