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Discussion in 'Amateur Radio Amplifiers' started by WA6MHZ, Jan 11, 2019.

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  1. KE0ZU

    KE0ZU Ham Member QRZ Page

    I remember that amplifier from years ago, maybe 40 or so, but don't remember where I saw it. Maybe Ham Radio mag, or Orr's west coast handbook??

    Looking at the deposits on the perforated metal at the far end of the exhaust stacks, it looks like both cylinders were running a little "rich".;) For replacements, go with whatever is cheapest or what you have the most of. Maybe Sue can donate a few to help offset the cost of some "new caps" :rolleyes:

    I think I'd test each of the caps, with some independent B+, for leakage at their "V" rating. Use a hundred K or so on the negative side as a limiter, and somewhere to measure the resultant voltage/current. If all is well, fire it up. It seems to have a pretty healthy bleeder.

    Congratulations on your new toy, and hope its an easy tune up.
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That's an awful lot of filter capacitance, a lot more than is needed at 120Hz even if Ip reached 1A, which it won't.

    I'd replace the caps with something considerably smaller. My own 2m kW (homebrew) power supply uses two 8uF 2500WVDC oil filled caps in parallel (16 uF total, no equalizing required) purchased from Fair Radio for about $10 each. At 600mA Ip the ripple is too small to modulate the carrier, which is really all I care about.

    My version of this amp can be seen in Hamnation episode 43 beginning at about 26:15 into the video: Turns out this version is somewhat easier to construct using cheaper tubing and no heavy soldering, and instead of the rotating disk to tune the lines, I just use a "flapper" which does the same thing and doesn't ever come into contact with the B+.

    With 2.2kV Ep (350V Eg2) and a pair of 4CX250Bs it can put out 1kW PEP or about 800W CW in AB1 with zero grid current and close to zero screen current.
    WA7PRC likes this.
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Change the filament transformer to make it more universal if you wish, but frankly 4CX250Fs are really easy to find and usually cheaper than Bs (probably because the Fs were used in so much USAF gear so there's tons of surplus ones out there.

    The screen bypasses in the sockets almost never fail unless cockpit error put full B+ across them, in which case the tube screens are vaporized and the whole amp can't possibly work. But at 350V, I've never seen these fail.

    I think you already determined a dropping resistor or two between the B+ and the screen regulators failed or opened. This is a really common problem with high-value (K ohms) power resistors that are not installed directly in a cooling airstream. If you remember the old Gonset 500W amps for 6m and 2m (912A, 913A or something like that), they only ran 1600v B+ and used gas tube regulators with a 50W WW resistor to feed the regulators and the reason about 99% of them failed was that resistor opening up. That doesn't hurt the tubes at all -- it just stops the amp from working!

    Back in the 1970s I repaired a lot of those. All I had to do was relocate that resistor from above the chassis where it got very hot and wasn't cooled to below the chassis, where I'd add a little 40mm cooling fan blowing across it. Bingo, then they last forever.
    WA7PRC likes this.
  4. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    It looks like the HV DC will be aound (1800VAC times 1.414) or 2500VDC
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Yeah, probably.

    That's 500V more than the tubes are rated for.:p I use mine at 2200Vdc "key up," which comes to about 2kVdc "key down" at full power and the tubes seem to tolerate that okay. I have no idea how long they'd last at 2.5kV.
  6. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    in the Failed 6M KW using 4CX250Bs I made. the PS used a Heathkit SB-220 stack at 220 each (8 Caps). I forget where I dropped the Screen V from, maybe like this a few caps from the bottom. Mid point would be 1300 so 2 down would be about 750V. The PS in that project worked great, it was the Grid circuit that didn't work so it wound up in the Museum of FUTILITY!

    So I could just get a Far Circuits board and construct a Cap bank on it (with Bleeders). I am going with ur suggestions that I try the HV and see if it work I am making an transformer extension cable so I can run the PS with the transformer on the floor
  7. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    not sure how to knock it down. I am fixed with the XFMR I have
    W4CHA likes this.
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    A variac would do it.

    But don't bother. Evidently it worked. 4CX250 family tubes are more rugged than their specs.

    I've never run one at 2500Vdc, but it might "take it" okay. Theoretically, the higher the B+, the less strain there is on the screens, and with those tubes the grid and screen are the fragile elements. The "delta" between plate voltage and screen voltage is important, and the higher that difference is, the less the screens will try to draw current, which is a good thing.

    Try it!
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Whoever built that went way overboard with the HV filter caps.

    Strains the rectifiers (surge), the on/off switch (if it doesn't use a relay or something), etc. But maybe he used what he had.

    With so much filter capacitance, if you remove the bleeders, there's probably enough stored to say "73 and see you later" after the power fails in a blackout.:p
    WA7PRC and K9ASE like this.
  10. N2EY

    N2EY XML Subscriber QRZ Page


    See post #4 in this thread for the source article.

    73 de Jim, N2EY
    WA9SVD likes this.

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