3 4-125a for a 2 meter amp

Discussion in 'Amateur Radio Amplifiers' started by AA7QQ, Jun 4, 2015.

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  1. AA7QQ

    AA7QQ Ham Member QRZ Page

    Would you possibly know of any tutorial for this neutralization process? Otherwise, what would the added parasitic reactance affect detrimentally? If it's not major, I would think the look (Remember I am going for that factor) of the Millen style capacitor would be nice.

    Thank you,
    Ed
    KG7SWP
     
  2. KA9JLM

    KA9JLM Ham Member QRZ Page



    I think that is called a Gimmick capacitor ?


    Sounds like it should work. Never tried that.


    I never had a problem neutralizing 1 tube. :p
     
  3. AA7QQ

    AA7QQ Ham Member QRZ Page

    This will be 2 tubes in Push Pull.... I remember something about a gimmick capacitor from back in the day, but never paid attention. I also notice that I can buy silver plated copper tubing of the correct diameters. Would it change either of the stripline lengths, or is that fixable with the adjustments?

    Thank you,
    Ed KG7SWP
     
    Last edited: Jun 7, 2015
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The process for neutralizing is the same for almost any kind of amplifier. Turn off screen voltage, apply a dummy load termination with an RF voltmeter or millivoltmeter across the output, apply RF drive, peak the grid circuit for max drive and then adjust neutralization for minimum RF feedthrough as measured by the RFVM -- the feedthrough can be so small it might take an RF mVm to get a meaningful indication. I usually use my old Boonton 92A or similar for this.

    The 'crossover wire stub' neutralization scheme was used in many PP VHF amps for several decades. Even the old Heathkit Seneca and old handbook designs from the 50s used it. It works very well and is easy to adjust.

    The issue I'd have with the Millen is it has a large frame supporting the disks, and that adds both series inductance and stray -- non-adjustable -- capacitance. Seems more appropriate for lower frequency amps, although I never actually tried this at 2m. Never needed to, since the stubs work so well and are very 'old school.'
     
  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    What ever you use you need to figure temperature change.

    Steve has it right, and you can not get any Oldschooler than that.


    I like new words. And the smell of hot tubes.
     
  6. KM1H

    KM1H Ham Member QRZ Page

    If you have drive to burn use a low value carbon comp resistor right at each grid. Voila, nice and "neutralized".
    I havent built any PP since the 829B 2M final in the TX I built in 1957 and that was cross neutralized with just a couple of sense probes; never did adjust them, just measured according to the article and used it as I had no way of measuring anyway.....we didnt worry about all that high tech stuff back then.

    Silver plating will have a very marginal, if any, effect at 2M. My 3CX1000A7 1500W+ 2M amp uses bare copper strap for the split anode line and a 4CX250B cavity amp I built many decades ago for 432 uses copper plumbing pipe. Both put out what is expected. The 8877 220 mHz amp has a lot of silver since I had it done for free at work. Looks purty but I doubt it helps any since it makes 1500W without even trying.
    None of those amps drift during use which was a big problem with some threaded rod tuning schemes used in the past; tune once and leave alone unless you change antennas.

    Carl
     
  7. AA7QQ

    AA7QQ Ham Member QRZ Page

    OK, a gimmicking I will go. I am just going by the "A high-powered tetrode rig for 144 Mc" ARRL article by W1HDQ. I am just going to use the PA section & not the tripler & driver part. It states in the article that you can just use the amplifier section. I will probably have to do some form of input tuning for it, as of course it was driven by tubes and may not be at 50 ohms.
    THe silver plated tubing isn;t that expensive & will probably get used in the future. I do want it to look purdy !!!

    Thank you,
    Ed
    KG7SWP
     
  8. WA9WVX

    WA9WVX Ham Member QRZ Page

    Ed,

    Was I mistaken that you want to have a window in the front of the 2 m RF Power Amplifier so people could see the tubes glow while in use? I would highly recommended not to do this feature especially at VHF or above because 20 years ago while I performing repairs on high power RF amplifiers, I learned a trick to check the ceramic tubes whether they were conducting or not. The RF amps had metal chassis with two metal doors and interlock switches. All that had to be done would be open one door, push in the interlock switch and look at the opposite tube as it would glow purple when conducting or no glow as if there was something wrong with tube or in the Grid box. This could accomplished for both tubes.

    Well, one day our corporate safety guy came into the repair shop with a RF Radiation Detector and pounced on all of us that worked RF amplifiers tubes and solid state. He was livid watching us troubleshoot the different problems with the metal shields off of the power amplifiers (I'm not exactly sure how to troubleshoot power amplifiers with the metal shield on them). The guy had some kind of special ball sensor on a stick and meter to read the radiation levels. I had a 1/4 KW RF Amplifier up on my bench for repair, everything was normal but he inquired how I troubleshot the amplifier and I showed him my trick for tube conduction ... he almost came unglued when I demonstrated it, shove the ball into the opening and it practically pinned his meter.

    I promptly got a lecture about that trick or idea as if I would melt or get tumors in my body. I listened and thought to myself, hmmm, where was this guy in the early 1970s while I and three other techs in the factory worked on these 1/4 KW stations everyday? I also worked on 450 MHz, 800 & 900 MHz tube Power Amplifiers but they only had a single tube and I could believe they would be a bit more dangerous because of the frequencies. If I were building a pair of 4-125A in Push-Pull RF Amplifier Design and wanted a window in the front of the RF box to show people what it looked like, I'd add a Metal Door that could be pulled up for a short period of time so the people could see the inside of the Power Amplifier but keep it shut 98% of the time to prevent any radiation damaged to their eyes.

    Dan
    WA9WVX
     
  9. AA7QQ

    AA7QQ Ham Member QRZ Page

    I was actually looking at this product & will call about it in the morning to see if it is stupid expensive, or if I might be able to get smalls to build the upper enclosure:
    http://www.ramayes.com/rf_shielded_glass.htm

    I do want it to be safe.

    Thank you,
    Ed
     
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I've used Invar for threaded tuning rods in VHF amps and they work well over a really wide temperature range.

    The Henry UHF amp had a thermal stability problem, and the LOAD control was through the top of the amplifier, which wasn't very convenient. I used to tune those after 30 second key down, and when transmitting from a cold start the amp would produce maybe 400W but after 30 secs would be about a kW and then not drift much more after that. That was a silly scheme, but I used it for years.:p
     

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