Pi-network Cap Advice

[W7SOE]

I am think about my 813 X 813 RF deck.  I am going to use my B&W 850A as an inductor.  According to the 850a's spec sheet the required capacitance to resonance is 50-150 mmfd for 10-80m.

This seems low as compared to other dual 813 rigs I have seen, I usually see ~500pF on C1.  (I assume they are talking about C1)

Would y'all recommend a vacuum cap here?  If so does anyone have any experience with the Russian jobs on Ebay?

Thanks guys

Rich

[K1JJ]

Rich -

I would definately use a vac cap for C1. You will not need more than about 300 pf for 160M depending on your desired Q.  The standard vac cap size is usually about 500pf at 10KV, that's why you see that value suggested in the schematics. Perfect for the job.

Add a turns counter and cheat sheet - then you can row around the bands rather quickly.

The Russian caps are fine. In fact, most any vac cap you find should be OK. If you can pull the opposite ends apart with some effort, then the vacuum inside is good. Then turn the plates all the way out and look at the inside mating plates for signs of arcing. If clean copper, then you are good to go.

T

[WA1GFZ]

Rich I agree get vacuum cap but a bread slicer will work as long as the minimum C is less then about 25 pf. The 850A is a fairly high Z network. Running it at higher power will work but as you reduce plate Z you also reduce tank Q. The way to get Q back is to short out inductor turns or remove them. You can also move taps on the higher bands. When you remove L to lower Z to increase Q you then need more C for both C1 and C2.
The effect is noticed as you tune up and decrease the load cap value to increase power out. You hit a spot where you load heavier but power out does not increase. That is the point where you short out a turn and retune.

[Opcom]

An old 2x813 amp I repaired had a 500pF bread slicer, and it appeared it had never arced over. I think it was a 3500V one. the amp used right at 2000V and was grounded grid. It did not have 10M, but 160-15.

I have a question about output capacitors. I sometimes find that when a random piece of gear is tuned up, the output cap ends up being almost fully meshed. This is the typical 1100PF gang of broadcast radio receiver type caps. What is really normal, and shouldn't a 1500pF cap be enough in any amplifier to cover the range down to 160M? What does it mean when the whole cap is being used? This has to be an indication that something isn't right, maybe with the load.

[W3SLK]

In the HN-500, (813's X 810's) I use a 500 pfd breadslicer. I'm running only 2KV but never suffered any arc-over.

[KC4VWU]

Vacuum variable vs. breadslicer... guess it mostly depends on how much room you have available; and probably to a lesser degree, how much money you wan't to spend. That is, providing the working parameters are well within specs. Those air variables seem to grow when you sit them on a chassis.

Phil 

[KM1H]

In a shunt fed pi network amp a breadslicer can be smaller gapped than in a series fed PP stage. Just be sure to use a RFC safety choke at the antenna output to prevent voltage build-up.

There are also a lot of small 365pf 3-5KV glass vacuums around fairly cheap if you dont want to opt for a ceramic version.

[W7SOE]

Ok, thanks to all for the help, very useful info.

Rich

[ke7trp]

I say use two vac variables if you can for both load and tune caps.  My transmitter has a big jennings for the tune but a bread slicer for the load due to the high capacitance needed.  Its arced already and I have had to clean it off and file it down.    The load cap does not have to have a high voltage rating if its tuned at 50 ohms. Its when you go off that or key with out an antenna or your tuner arcs over, then you need that higher rating. Build it big now.

c

[ke7trp]

SOE, I was digging around and found a brand new in the box never used "BUD" air cap. 27 min 340 max loading cap.  Its my brothers but if you need it, I will get it for you cheap.  I have no use for the thing.

C

[ke7trp]

pics

[Opcom]

classic beauty!

[W3SLK]

Clark said:

I say use two vac variables if you can for both load and tune caps.

Why? Your voltage at the LOAD side is relatively low. I use a standard broadcast tuning cap at ~1500 pfd. A helluva lot cheaper than a vacuum variable.

[KM1H]

Mistakes can be annoying with crystal set caps

[ke7trp]

If you can get a large 1500 to 2000 PF cap for the load then great.  I have a 1600pf cap in mine and its arced over plenty of times. Go to far, It will buzz, If you make a mistake, and key up and you are not resonant ect.. You arc the damn thing out.  Once its arced, then it tends to arc easy.  So for me, I would rather have it over built so this wont happen.  Maybe I am being foolish here.. But I wish I had a vac cap over on the load side also.

C

[flintstone mop]

What's with the high prices on ePay and vac variables??
hefty prices and most are coming from Russia with additional $80.00 shipping fees.

Fred