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Author Topic: Where to get a Vacuum Variable for new transmitter project?  (Read 9386 times)
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steve_qix
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« on: March 19, 2011, 09:19:11 AM »

I have MANY years building transmitters, but for everything I've built, none have ever had the need for a vacuum capacitor - until now.

The newest project requires a 1000pF 10kv or 15kv tuning capacitor.  This is just too big for air variables, so a vacuum really is the smart choice.

What is a good source of these, and what should I be looking at with respect to quality, etc.

Also, I guess a turns counter is in order so one can know where in the range the capacitor is set.

I've seen the Russian capacitors on ebay - are these any good?  Are there any domestic sources?

Any advise would be appreciated!

Regards,

Steve
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« Reply #1 on: March 19, 2011, 09:44:00 AM »

Steve,

I get mine from Max Gain Systems: http://www.mgs4u.com/index.html#microwave

Allen is a great person to deal with and I have purchased a lot of stuff from him in the past 15 years or so.
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Rodger WQ9E
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« Reply #2 on: March 19, 2011, 09:50:51 AM »

What besides 600M requires a 1000pf input cap?
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« Reply #3 on: March 19, 2011, 12:31:23 PM »

I've used several of the Russki caps and was impressed with the quality.   They are not as cheap as once, but nothing is these days but are still a bargain in the new cap marketplace.

73DG
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« Reply #4 on: March 19, 2011, 12:39:31 PM »

Mine have come from hamfests, eBay and broadcast gear.  I have never purchased the Russian or European ones (Comet?  from Sweden I think but I'm probably mistaken--some N. European country) having always gone for Jennings.  However, a friend who homebrews amps purchases the Russian vac. variables and doorknobs and says they are FB.  Yes, you will need a turns counter.  As far as I know, no one makes the ones like you find on vintage gear such as the BC-939.  If you want a Groth or similar you have to look for it like any other used part.  If you don't mind new, Palstar, MFJ, Kintronic sell them (Kintronic is very expensive).  B&W used to make them but I don't know if they still do or not.  But get the cap first.    
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« Reply #5 on: March 19, 2011, 12:43:10 PM »

I concur on Max Gain. Good guy, good company.
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« Reply #6 on: March 19, 2011, 04:54:23 PM »

What besides 600M requires a 1000pf input cap?


Thank you!  This is VERY good information.  To answer the question  Wink  Class E rigs on 160 meters - I need about 800 to 900 pF.  On 75  meters, about 450pF.  It's because the impedance of the circuit is lower than that of a tube circuit.  There is more capacitance and less inductance in the tank circuit.

Thanks and Regards,

Steve
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« Reply #7 on: March 19, 2011, 05:01:30 PM »

Steve,

You should seriously consider using some high current capacity padder caps rather than using the vacuum variable to supply all of the capacitance.  There are some very good quality Russian RF caps available on ebay.  A couple of added bonus advantages are lower total cost, faster band changing (with 500 pf of padding switched in for 160 switching it out puts you close to the proper amount for 75), and reduced wear and tear on the vacuum variable. 

The flexible bellows used on most vacuum variables has a finite life, rarely reached in normal use but in a lot of commercial applications normal use is set and forget. 
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Rodger WQ9E
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« Reply #8 on: March 19, 2011, 05:35:11 PM »

Steve, I have a Comet 1000 pf loading cap in my 75 meter class e final.
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steve_qix
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« Reply #9 on: March 19, 2011, 05:39:19 PM »

Steve,

You should seriously consider using some high current capacity padder caps rather than using the vacuum variable to supply all of the capacitance.  There are some very good quality Russian RF caps available on ebay.  A couple of added bonus advantages are lower total cost, faster band changing (with 500 pf of padding switched in for 160 switching it out puts you close to the proper amount for 75), and reduced wear and tear on the vacuum variable. 

The flexible bellows used on most vacuum variables has a finite life, rarely reached in normal use but in a lot of commercial applications normal use is set and forget. 
This is also a very good way to go.  I am currently using air capacitors with vacuum padder caps with other transmitters and it works just fine!  I don't have a big enough air cap lying around to do the job, else I would use it - just because of the lower cost!!!  I need a 500pF at LEAST 10kV and really probably more than 10kV capacitor.  Then, a 500pF padder would work and I'd still have enough range with the 500pF to cover whatever I might need.
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« Reply #10 on: March 19, 2011, 08:21:55 PM »

When you build a rig with vacuum variable caps it is a good idea to have spares.
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Bill, KD0HG
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« Reply #11 on: March 20, 2011, 11:53:11 AM »

OK, I've got two possibilities for you, Steve.

Jennings ceramic vacuum variables- 1000 pf @ 7.5 KV and 1000 pf @ 35 KV (one big 25# beast). Pulls from a working Continental AM transmitter. I *might* have turns counters if I can find them.

PM me if you're interested.

(I'm thinking the 35 KV guy might be useful in a tuna if they allow real power on any proposed low freq band)

Bill
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« Reply #12 on: April 02, 2011, 09:43:14 AM »

I want to thank EVERYONE for the input, offers and advise!!  I finally ended up getting 2 of the Russian 1000pF 10kV vacuum variables (they are not here yet - on the way).  Both new for $160 each, which I didn't think was too bad.  Of course, with the shipping they are approaching $200 each.

Anyway, I'm hoping these will work out.  Other people are using these Russian caps with good results.  Anything I should look out for?

Looks as if I'm going to have to make up some type of mounting system.

Thanks and Regards,

Steve


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« Reply #13 on: April 02, 2011, 12:57:13 PM »

Quote
Anyway, I'm hoping these will work out.  Other people are using these Russian caps with good results.  Anything I should look out for?  


Steve,

Yes. The Russian caps will work FB as long as you compensate for the random directional rotation. When the vac variable was made, the Russian machines used indexing letters with some printed backwards due to their alphabet. Thus when used with an American turns counter, you will have to think in reverse-Russian to find American  resonance. You can easily fix this by mounting both the cap and turns counter in reverse, accessible from the rear panel,  using clear Taylor-hybrid epoxy.

T


(just kidding... Wink
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« Reply #14 on: April 02, 2011, 02:26:16 PM »

I have not used any of the Russian vacuum variables but all of the RF parts I have gotten that were of Russian/USSR/Eastern Bloc origin have been top quality.  I am sure the vacuum caps will serve you well.

Some of the Chinese stuff is "interesting"  I bought one of the Chinese copies of the Angry 9 and it came with a soldering iron that looks like it started life as a small hatchet, to use you must first heat it in a fire.  The wood/metal screwdrivers look very much like what is sold by Harbor Freight today so they are using a tried and true design.  The set works fine but at least parts of it don't inspire the same level of confidence as that produced by the USSR.
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Rodger WQ9E
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« Reply #15 on: April 02, 2011, 02:32:24 PM »

I'll second / third the russkie caps.  They are BEASTS, and I've run rediculous amounts of power through them.

Their caps are good for blocking, too.  I use a pair of their 100Kv caps in the mobile amp.


--Shane
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