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Gonna Build a 2 Meter Tube Amp

Discussion in 'Amateur Radio Amplifiers' started by K2WH, Jul 4, 2011.

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  1. K2WH

    K2WH XML Subscriber Volunteer Moderator QRZ Page

    I have decided (after blowing out 2 solid state amps for 2 meters) to build a tried and true rugged single tube 2 meter amp. A single 4CX250 or 150 vacuum tube, can put out 300-400 watts easily and, very common to find at and; that power level is all I need or want.

    I used to build them in the 70's and 80's but as I got older, I sold most of them and never really got back into building. However, after the loss of these 2 amps, I decided to go back to what I know works reliably without constant monitoring for problems. Strangely, all of the stuff needed to build the amps in the 70's and 80's could be had at hamfests. No more, after perusing Ebay, just about everything and anything needed for the amp can be obtained there. Tubes, tube sockets, finger stock, copper stock, doorknob caps, feedthrough caps, variable caps, connectors, transformers, chassis's etc.

    Sorry state of affairs but very convenient.

    Last edited: Jul 4, 2011
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Actually, you can find most of these items "new" from RF Parts.

    Some other stuff you can get from Ameritron as "spare parts," they sell everything used in their amps for fairly reasonable prices.

    Fair Radio Sales in Ohio has a lot of stuff, too.

    My 2m kW amp uses a pair of 4CX250Bs in P-P. Built in 1979 and never serviced, still works fine with the same tubes. The only maintenance ever performed is to blow out the blower, sockets and tubes with an air hose every couple of years to get the dust out of them.

    If you want it for FM only, even easier since they'll be in cutoff and not draw any current at all until needed. For FM you really don't even need regulated bias or screen supplies, since linearity isn't required.

    GREAT project. A good circuit to emulate is the one used in the Gonset 903A amplifier, it's very efficient and easy to copy. Running Class C for FM, it hits about 70% efficiency and makes 250W output with a few watts drive.

    They silver plated the plate tank stripline, but I've made "copies" of this amp just using bare copper flashing and it didn't make any difference.
  3. K2WH

    K2WH XML Subscriber Volunteer Moderator QRZ Page

    Hey Steve. Actually I will be using the amp in SSB service so I will need to provide stiff regulation of the screen supply. As you say, many of the parts are available on line. When you think about it the most difficult part in building your own vacuum tube amp IMHO is the required power supply. Can't just hook it up to the 12vdc supply.

  4. KM1H

    KM1H Subscriber QRZ Page

    Using 70's circuitry you will be the scourge of the band on SSB in a crowded area as IMD is in the low 20's at best. Use a G3SEK tetrode board or a clone and upgrade with a Ruskie 4CX400A (600W per tube max vs 300 for the 250) which is a cleaner tube to start with or find the elusive 4CX350F. Many 150/250 pulls were from commercial FM and have likely developed screen emission which further destroys the IMD.

    The Europeans went on a 150/250 cleanup mission years ago due to population density and their wide availability which became a serious problem.

  5. W0BTU

    W0BTU Ham Member QRZ Page

    I didn't realize they were that dirty of a tube, Carl. I ran a single grid-driven 4X150A on 144 MHz SSB/CW many years ago with 275 volts regulated on the screen and 2 kV (IIRC) on the plate. Input and output circuits were stripline. I never thought to check how wide my signal was, and no one complained.
  6. K2WH

    K2WH XML Subscriber Volunteer Moderator QRZ Page

    Agreed. I built many 2 meter amps using 250's and 150's and never ever had a single complaint. Perhaps that was because of the stripline construction and the stripline filter on the output of the amp. Since the website references power and control schemes for tetrodes not amplifier design considerations, perhaps the use of these tubes on HF put out a bit more garbage than on VHF. I reviewed the kits and the theories etc. (thanks for the link), but I would rather roll my own. Its more fun and much more self satisfying.

  7. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I think the issue with the 4X150/4CX250 family is just moderately acceptable IMD performance, which hasn't much to do with the tank circuits.

    My P-P dual 4CX250 amp has been on the air in hundreds of contests using big antennas and lots of strong local signals and never had a complaint, either, even operating 5 kHz away from other big guns in the contest. I run 2200V Ep, 350V Eg2 (regulated with gas tubes!), and no grid current -- careful loading will achieve that. It can develop 800W PEP output with no grid current indicated at all, if it's loaded well.

    These things probably get "sloppier" when overdriven or underloaded so grid current starts showing up.
  8. W0BTU

    W0BTU Ham Member QRZ Page

    I said what I did just to show it had a tuned input. Stripline vs. coil isn't going to make much difference in IMD, but tuned input vs. no tuned input might.

    I was just wondering what Carl meant when he said 70's circuitry. I think he (and you too) knows more than I do about amplifiers. :)
  9. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    My 4CX250 amps were all grid driven and required such little drive I normally had a 6-10 dB 50 Ohm resistive pad between the exciter and the amp, so IMD would likely be the same no matter what the input circuit looked like. However, since they're push-pull and required phase splitting, I did use a tuned input circuit in all of them, at least to accomplish that. If I used high-Q "tuned lines" (1/4" copper tubing to make loaded 1/4-wave line sections) they would drive easier than with lower-Q "lumped constant" input networks, like a small coil and link, but since there was always too much drive available anyway, it hardly mattered.

    My "plumber's delight" 2m P-P amp can be driven to 600W output on FM using the output of a 2W handie-talkie to drive it, if I omit the 6 dB pad. I've demonstrated that a lot of times to visitors, who, if they grew up in the "post vacuum tube" era, were usually astonished.

    I usually drive it with an FT-736R followed by a 6 or 10 dB pad. With proper loading, it runs real AB1 (no grid current at all) and is still about 50% efficient with no grid current. I can push it into AB2 and achieve a bit more output and efficiency, but who cares? It runs plenty of power.
  10. W0BTU

    W0BTU Ham Member QRZ Page

    Oh yeah! I did have a grid swamping resistor on my amp, which I almost forgot about. I drove it with a Hamtronics transverter which put out 2 watts, still way too much without the resistor.
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