2M KILLERWATT

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

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  1. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    About 13K at 25W should do it if it's a wirewound air-core resistor that isn't blocked from air flow.

    I sure wouldn't do that. Two OB2s and one OA2 in series are fine for the screens just as they are for AB1 operation. To switch to Class C where Eg2 can be much lower, just use a switch to tap down one OB2, leaving one OB2 and the OA2 in series for the regulated tap but all three tubes "firing." Don't use a pot, that's silly and defeats the purpose of regulation.

    You can do that, but I never bothered and never burned up a tube yet. My dual 4CX250B 2m amp was built by me in 1979 and still has the original tubes in it, working the same as they did in 1979. It's been through thousands of hours of transmitting over all those years. With proper LOADing you can adjust key-down screen current to be about zero. With keying or modulation, it can swing slightly positive or slightly negative. I use a zero-center meter in the power supply to see this, it indicates +/- 30mA but when the amp is tuned and LOADed properly it barely wiggles. Key factor is loading. A possible hazard is a feedline or antenna failure, which obviously will destroy the loading and can cause excessive screen current. If I feared that, that's when a fail-safe fault detector would be worthwhile; but it's never happened to me.

    When I tune up (almost same circuit as yours, just with different pipes and flapper vane tuning instead of the disks), I monitor RF output power, grid current and screen current only. I do have a plate current meter but never look at it. Increase drive slowly and peak plate tuning and loading for max output power, then increase drive more, re-peak adjustments while also keeping an eye on grid and screen current. With about 5-6W drive, output should hit 600-800W and final adjustment is LOADing only, crank it up for zero grid current and watch screen current -- it shouldn't be more than a few mA +/-. You'll see the LOAD control can actually zero the screen current.

    After going through this a few times, tuning up takes about five seconds. Unless you bump the controls or change frequency quite a lot (or change antennas), it won't need to be re-tuned at all.
     
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Pat, if you're interested I wrote an article about building a single-tube 500W 2m amp, including designing and building the power supply from scratch, and it was published in the April 1994 issue of 73 magazine, pages 40-46. A few snippets from that are below, but the article's on line for free download.

    The power supply easily powered a single 4CX250B at 500W and was very compact, weighing maybe 20 lbs when finished -- it's shown with schematics and photographs in the article. It would not need to be much bigger or heavier to power a pair of tubes, as it can handle 500mA ICAS at 2kV B+ just fine; I did not even use screen regulation, but had a separate screen supply which was very "stiff" and provided a very low impedance source for the screen, so could sink +/- screen current. I think I measured screen voltage to the tube at +320Vdc and didn't change more than maybe 1V from key-up to key-down under various load conditions. Had to keep it all very simple, as this was intended for portable hilltopping with a small gas generator, and I used it in the 2m "Sprint" that year to win it, operating with a portable yagi from Saddle Peak (and FT-736R driving it, cranked down to 5W or so). We used it in a few other portable contest operations: Small, light, easy to carry, requires almost no re-tuning.

    2m amp article 73 magazine April 1994 (2).PNG 2m amp article 73 magazine April 1994.PNG 2m amp article 73 magazine April 1994 (3).PNG
     
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  3. W1BR

    W1BR Ham Member QRZ Page

    Screen current can go negative, depending on loading. There were meters that sat bit off the left of scale to permit screen current monitoring--Hallicrafters for one used them in a 4cx300 HT-33 amp they built.

    Screen current is very low, and it is critical to monitor for excessive current. Those screens will burn out quicker than dropping resistors. Also, if you do any changes, adding better screen voltage regulation is a good idea to control IMD. Those tubes require a very stiffly regulated screen supply. A guy in England sells tetrode control boards that might be a good investment for that amp. Just like big triode amps, screen current is as important as grid current when tuning.
     
  4. N2EY

    N2EY Premium Subscriber QRZ Page

    Great article, Steve! Thanks for sharing.

    But....1994 was 25 years ago.....and if building such an amp requires spending some cash, there may be better options.

    In QST for October, 2012, there's an article about a 1 kW (output) amp using an LDMOS device. RF section is quite simple, has a bunch of doo-dads for control and protection. The LDMOS device is $175 at Mouser - isn't that about the price of a new 3-500Z?

    I'd be interested in your opinion of that project.

    That's one solution - here's another: Install screen-to-ground resistor(s) of the correct size to draw about 5 mA. Then mark the screen current meter(s) so that 0 is actually at the 5 mA point.

    With such an arrangement, up to 5 mA of "wrong way" screen current can be accommodated.

    btw, the 1964 W1HDQ article notes the importance of being able to separately indicate the screen current on the tubes, as an indication of balance. It is cautioned that if this is not done, conditions can arise where one tube is drawing excessive screen current and the other practically none, and soon you have a rather expensive ceramic-metal doorknob - or two.

    73 de Jim, N2EY
     
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  5. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Well, my little 500W 2m amp "project" cost about $200 total including the RF deck (surplus Motorola paging amplifier) and building the power supply from scratch, so it was a good deal at the time.:p

    I am quite familiar with the new LDMOS high powered amplifiers and they are amazing. Jim W6PQL has them all over his website http://w6pql.com/1_kw_2m_ldmos_amplifier.htm and even sells complete or nearly complete kits of parts to build them. I think he may also sell some fully assembled, but haven't checked that lately. The LDMOS devices themselves are pretty cheap but of course require a machined flat copper heat spreader, big heatsink, cooling fans, etc. in addition to (usually) a power supply providing 50V at 40A or more. Luckily, 48V surplus Telco power supplies are available pretty inexpensively and probably work fine for this; but if you have to build one from scratch or buy a new one, that would cost a lot more than the LDMOS device.
    I did that in my very first dual 4X150 amp a long time ago but noted it was kind of a waste of effort, considering with proper loading, the final screen current for two tubes is very close to zero; so even if one was -5mA and the other was +5mA (resulting in zero total) it wouldn't really matter. The Johnson Thunderbolt 6n2 (only about 325 sold or something, it was an expensive beast for the day) used a pair of the same tubes and didn't monitor the screens independently; nor did the HF Thunderbolt (a lot more sold!) which used a pair of 4-400As grid-driven. I think in a reasonably "balanced" design, with proper loading, the worry isn't warranted.
     
  6. AI3V

    AI3V Ham Member QRZ Page

    Wow, 500 watts from a single 4cx250, unregulated screen supply, and you wrote its probably not even necessary to monitor screen current.

    I bet you were REAL popular on the air.

    And then you use a fuse! With over twice the fusing current as the screen grids themselves.

    Sorry Steve, just cause Wayne published it doesn't make it a good amp.

    This makes "cb garbage" look good.

    Rege
     
  7. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Rege, I suspect you haven't designed many amplifiers.

    I have.

    The "500W" is DC input power, not output power (although it can make about 350W output quite well) and the screen supply has such low dynamic impedance there's no reason to add more regulation to it. It's the screen source dynamic impedance that counts, no matter how it's achieved. If you had a source with Z = 0, would you add regulation?

    That amp has been on the air since late 1993 and has never failed in any way. It does have screen current metering, guess you didn't look at the schematic; the screen current meter is installed right on the front panel of the RF deck and is the most important thing to watch when tuning.
     
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  8. N2EY

    N2EY Premium Subscriber QRZ Page

    Sure - but that was 25+ years ago. $200 then inflates to about $350 now.

    And I don't think that was your first successful 2 meter amplifier.

    Of course a lot depends on what is already on hand or can be had for low prices. But, as you know, there's a HUGE difference between spending money on a sound design that has been carefully worked out, and simply throwing money at a Hammy Hambone project, trying to force it to work.

    Sure - but the fact that W1HDQ made a point of it is important.

    Consider that the 6N2 Tbolt was almost certainly carefully adjusted for balance at the factory, and that the Tbolt is a parallel design using tetrodes with much higher screen dissipation. (Eimac rates the 4-400A screen dissipation at 35 watts!)

    Good stuff all around.

    73 de Jim, N2EY
     
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  9. N2EY

    N2EY Premium Subscriber QRZ Page

    The 6N2 Thunderbolt is a parallel design as well - pair of 7034s in parallel, not push-pull.
     
  10. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Well, that's true! But I'd think one tube could still easily draw more current (grid, screen, anode) than the other one if they're not fairly matched.

    I owned a 6n2 T-bolt for a few years around 1969-1972 or so (sold it for more than I paid for it, and needed the money!) and it was an unusual design that basically had the 2m plate tank line in series with the 6m tank inductor, so when you selected "6" meters, the 2m tuned line was still in place; as such, if you had a 2m antenna connected and "drove" it on six meters, and adjusted the tuning for "max output," you had a powerful tripler! And it did that. There may have been a warning in the manual about this, I don't remember, but I watched it happen.:p

    Still, it was a cool thing to have. After I sold mine, I built separate 2m and 6m kW amps with a shared power supply and that was saner.:)
     

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