• You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.

Question about Air Variable Cap's

ElectronTubesRule

Active Member
Sep 6, 2011
257
16
28
Ok so I have never done an RF amp from scratch only repaired them and modified them. I know from past experince that 10m band often by the time all the math was done I needed something like 4-8pf but seldom could I find an air variable in that low of a range that would also fit the case. So you almost always sacrificed the best Q for what would fit the case.

So know that I am looking at building my own amp from scratch and am looking at some of the air variables on the market they all seem huge?!?! A lot of these are rated up to 4500V and and are in the 32-800pf range. So why on earth would anyone need that large of a pf range on the air variable???

From practical experience what range do you guys find to be the most likely to be used and the easiest to tune? I am trying to find reasonably priced multi gang set up's over these massive giant single gange beast's???

I have never had to repair or replace any of the parts in a multi-band HF amp's input tuning stage before so I have no clue. Considering the fact that coupling caps are used to join sections in the circuit I can not imagine one needs 240pf on either the tun or load side but that is just me making an assumption.

So some input would be welcome. Experience trumps most book knowledge any day of the week in my book!
 

4-8 pF seems really low for a plate tuning cap even on 10/11m. You must have a lot of stray capacitance or really high output capacitance in the tubes you are using.In multiband amateur amps the plate tuning cap is usually around 200-300 pF and the loading cap is between 1000 and 2500 pF.
 
Well living in Georgia at the time a lot of the amp's where really low end class D amps that Amateurs int he area used due mostly to cost. So it was really a 11m amp being used on 10m as is or with slight mods on 6 m unless it was one of the rare models that was multi-band. They things seldom needed to be replaced though it was usually new tubes, replace caps and carbon resistors that had blown etc..... I will double check I have two amps that while advertised as 10/6m amateur amps I am sure where truly built for black market class D use. I will check then out. They have the tiniest air variables I have seen. not much more then 1-2 inch's long no stops, the coil is pure silver wire very thin and not more then 1 inch long. I am not kidding.....To be honest I suspect that more voltage is going through those air variables then they are rated for!!!

When I get around to it I am going to use the software that came with the 2008 ARRL book. It has a nice exxel table that makes selecting values very simple!

I just kept seeing all theses 750pf 850pf 1200pf 2500pf and many of them are rated for voltage of 4500+++. I just could not wrap my head around why anyone would need that much capacitance on the load or tuning side. To me it seemed like an air variable that large would make tuning harder rather then easier.

I am going from memory but they just looked insanely huge! This is why I asked because I figured either my memory was wrong or I was seeing mostly unused product that was too large for most projects! I have a two stage model for the plate loading that is 240pf total. I am still shopping for loading cap which I knew would need to be much larger but I was not sure how much larger was the norm.
 
Ok so here the rub. When I find air variables above 1000pf they rated in the 1-1.5KV range when I look at the ones in the 3kv-4.5KV range they are all under the magical 1000++++pf range.....Even ones specifically listed by RF Parts and Palstar as load capacitors are usually under 1000pf but rated to 3-4.5KV.....So it has been interesting to say the least. So I am probably going to end up with something in the 820-1000pf range on the load side.


I did get a larger air variable for my old beautifully built 10m/6m wink wink wink so soon it will be a properly done true broad band HF amp. I am also going to add another coil unlike the original that is solid silver wire I will use copper and silver plate it. The original coil is so small the diameter of the wire is not even as thick as your run of the mill round wooden toothpick. I am slowly replacing all the old components with new ones.
 
You do not need a high voltage rating for the load capacitors. A kilowatt plus amplifier can use receiver plate spacing for the load capacitor. It only has to handle the normal RF voltage associated with the power output plus a little bit for SWR etc. A kilowatt is only about 220 volts into 50 ohms. Even the Kenwood TL-922 used a loading capacitor with plate spacing similar to what would be used in a receiver section.
 
Ok well then now it makes sense why I can not find a "load" unit rated that high but I can find tuning unit's rated very high. I posted a question a while back asking if the air variables had to handle the peak DC power in the system or the RF power in the system or what? I never got an answer. You would think that would be covered in the ARRL Handbook but I have read it more then once through and it leaves a lot of things unanswered. That is why I am here asking questions. None of the sights I have been to answer that question either.

I just do not want to make any mistakes or assume anything not with the voltages involved and the potential for eating up tubes and other expensive components!
 
Ok well then now it makes sense why I can not find a "load" unit rated that high but I can find tuning unit's rated very high. I posted a question a while back asking if the air variables had to handle the peak DC power in the system or the RF power in the system or what? I never got an answer. You would think that would be covered in the ARRL Handbook but I have read it more then once through and it leaves a lot of things unanswered. That is why I am here asking questions. None of the sights I have been to answer that question either.

I just do not want to make any mistakes or assume anything not with the voltages involved and the potential for eating up tubes and other expensive components!


I must have missed your previous question or I would have answered it then.The difference in voltage ratings is because the tuning cap has to handle all the high RF voltages due to the reactance of the tank circuit converting the high impedance of the tubes down to 50 ohms or whatever the antenna impedance is. The loading cap simply has to handle the "tuned state" RF voltages which rarely ever exceed a few hundred volts.
 
Is their a good explanation for why we use ceramic doorknob caps as the first choice for coupling parts in the tune and load sections? Even in those amps not using doorknobs they still always use some form of ceramic cap. I do not want to just copy cat designs I kind of want to learn why we do what we do as I build this beast. None of the electronic books I have read really explain why we choose various cap's for various jobs's in fact the only thing I really have gleaned is that electrolytic work great in filtering in power supplies and that is about it. Why we do not use them for instance in the tuning and load circuits is never explained. I have read quite a few books from the 1950 to 2008 by Amateurs and some by the ARRL and it is kind of like they expect you to just understand the why after covering the most basic of basic electronics?

It is kind of like someone gave you a 1000 piece puzzle but they are making you assembly the puzzle with the brown side up so you never get to see the big picture even after you have totally assembled the puzzle.

It is unfortunate that money does not grow on tree's and I do not have time in spades! I would love to take some electronic class's but my medical school schedule does not leave much room for any fun class's!!! I am sorry I ask so many questions I hate to be too needy but I do like to learn not just plug and chug and put it together by following the schematic with not idea why the heck I did what I did?!?!
 
Output network caps in the usual pi or pi-L will have a lot of voltage on the tune cap and a good amount of current on the load cap.

Doorknobs will work as padders for a load cap but each unit must be carefully choosen so as not to exceed it's current capability. The circuit might need 800 pf. In practice it might take 4 200 pf caps in parallel. If there is any sign of heating or power drift which can be fixed by retuning look for a marginal design.

Watch out for TV type doorknobs. They look like regular doorknobs but don't handle current very well.

Disc ceramics are great as bypass caps but should be avoided for high current situations. Lots have been tortured in cheesy CB amps.

Be careful of blanket statements read on hammy web sites. Using a receiver air variable for a load cap in a QRO situation requires careful inspection and thought. Many older air variables will not handle a few amperes of rotor current. The units which use large surface area and multiple connections to the rotating shaft will waste less power. Some even have braid between the shaft and chassis.
 

dxChat
Help Users
  • No one is chatting at the moment.
  • dxBot:
    Tucker442 has left the room.
  • @ BJ radionut:
    LIVE 10:00 AM EST :cool:
  • @ Charles Edwards:
    I'm looking for factory settings 1 through 59 for a AT 5555 n2 or AT500 M2 I only wrote down half the values feel like a idiot I need help will be appreciated