3 4-125a for a 2 meter amp

Discussion in 'Amateur Radio Amplifiers' started by WA4SIX, Jun 4, 2015.

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  1. KA9JLM

    KA9JLM Ham Member QRZ Page

    You are having some fun now.


  2. KM1H

    KM1H Ham Member QRZ Page

    Those bigger tubes can barely make it to 120 mHz much less 144; great on 6M however.

    I could always tell a 4CX250B amp on VHF since the IMD is horrible and the band noise very low; the crud wipes out all around the calling or DX frequencies. Now the crud from SS rigs and bricks has pretty much destroyed the bands.

  3. WA4SIX

    WA4SIX Ham Member QRZ Page

    OK, I like the stripline design. It'll be great when I get to that point. I will also take into account that the larger glass envelope tubes are better suited for 6 meters (My favorite band). Who knows, I might do a 6 meter plate modulated AM transmitter. I guess I just love the glow. Now, I have seen several 3-500Z amps in plexiglass / glass enclosures on HF. I am going to guess that would be bad for 2 meters. What about 6 meters? Not planning on having kids, anyways. Or, would the safety glass (Chicken wire impregnated) work? Possibly a light aluminum screen on the inside?
    I can get more Larcan RF decks to build any amp I like & put it in a box. For tubes, I would like something that impresses people & may get them interested in ham radio. I guess what I am trying to say is that looks are important with a tube amp for me. All of my life, I have been wary of the tubes. Now, I plan to jump right off the cliff & build functional artwork.
    Let's stay away from the ceramics for now....

    THank you,
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Yes, this, and the SS amps killing the bands with crud isn't so new, it started in the late 70s or so.

    Some of the new FET designs by Klitzing seem very clean, similar to triodes. But the bipolar bricks, holy cow.

    I use those only on FM.
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Six meter AM seems pretty dead, but maybe it will make more of a comeback. I still have a Gonset G-50 in the garage -- plate modulated AM at about 40W output -- and haven't used it in probably 15 years...but it would be fun to use it again.

    The glass tubes bigger than the 4-125A aren't as good on 2m as the 125A is. I don't know 'anyone' who has gotten 4-400As or bigger tubes to work on 2m.

    4CX250Bs and others in that family work great up to 450 MHz or so, but IMD can be a problem. If you live where nobody has strong signals, it may not be any kind of problem. The difference between a very clean amp and a pretty dirty one, for IMD, might only be 10-15 dB. When I was a new VHFer in the sixties, almost everybody used 4CX250Bs to run power on 2m, and we lived pretty close together. It wasn't much of a problem unless our beams were aimed at each other.

    That's one huge advantage VHF has over HF...you can notch somebody on 2m by 40-50 dB by swinging your beam...that's almost impossible on HF.:eek:
  6. WA4SIX

    WA4SIX Ham Member QRZ Page

    Well, looks like I have all of the parts except for the neutralizing capacitor & 4-Ohmite Z144s. Mouser stocks them.
    Now, as for the safety glass for an enclosure? If I cut it with an extra 3/4", then score the 3/4" line to make it correct & crush the outside strip of glass, then sandwich it between 2 pieces of angle aluminum for bonding of the chicken wire, would that be loosley acceptable for shielding on 2 meters? I wish I could source some of that "Transparent aluminum" from the Star Trek movie...

    Thank you,
    Last edited: Jun 7, 2015
  7. WA4SIX

    WA4SIX Ham Member QRZ Page

    The neutralizing capacitor is a Millen 15011, would anyone have any specs on it? I found them.... Darnitall, I had one of these 20 years ago & was just using it for HV experiments..... Gone now. I guess I'll have the machine shop build me a couple discs with rounded edges & build the frame, myself.

    Thank you,
  8. K9STH

    K9STH Ham Member QRZ Page

    "Fender washers" are available at most home improvement centers, in various diameters, that can easily be adapted to use as neutralization capacitors. It is easy to solder a flat head machine screw to the washer to make adjustments easy. The cost is, generally, less than $0.25 each and they are readily available.

    Glen, K9STH
  9. WA4SIX

    WA4SIX Ham Member QRZ Page

    Well, I guess that fixes it. I bet I could go to Waterman's (Welding/machine & aluminum boat manufacturer) & see what they have for holesaw plugs. Get some aluminum threaded rod & tig it together. Thank you for the inspiration. Do you think I would be better off with brass or aluminum? The original is all aluminum.
    Thank you,
    Ed KG7SWP
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The Millen cap is one I wouldn't use on 144 MHz, it has too much parasitic reactance the way it's made. I guess it can be made to work, but seems clunky to me.

    Its adjustment range was supposed to be less than 1 pF to about 10 pF or so I believe; very small values. For neutralizing PP 2m amps I just use crossover insulated wire stubs soldered beneath and sticking up through holes in the chassis near the tubes, and cut length to adjust neutralization by measuring the 'leak through' signal. Costs nothing and takes five minutes to adjust. Once adjusted, there's no reason to play with it again -- like, ever.

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