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Smaller tubes with a lot of glow?

Discussion in 'Amateur Radio Amplifiers' started by KC8QVO, Dec 26, 2019.

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  1. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'd think it shouldn't be, also; however it sure could be. Some tubes were actually designed for extremely long storage and operating life because they were intended to be used in places that were completely unaccessible for service, like those used as repeaters in the original Transatlantic cable. Special materials and techniques.

    I do have receiving tubes from the 1950s still working fine, so there's no question that some tubes last a really long time without specifically being designed for that. I don't know much about 1625s and the only ones I ever saw on the market, even in the 60s, were all WW2 surplus.

    I've never been involved in the vacuum (or gas) tube manufacturing industry, but did "run" a thickfilm power hybrid operation for a couple of years in the late 1980s and the hermetic packages also used preassembled Kovar pin seals with glass or ceramic insulation, kind of like what's used with tubes. Usually larger pins, as these had to handle serious current (often as much as 50A), but still the same materials, and some lower power hybrids running at only 1-2A used small pins.

    We had to gas leak test them 100% after they were assembled in a nitrogen environment (not a vacuum) and had plenty of rejects due to bad Kovar-glass seals. That's where they leaked, and if they did of course were rejected and never shipped.
  2. AH7I

    AH7I Ham Member QRZ Page

    I'm sorry, I think you missed some bits in the thread.

    1. My post presents an example to address the '4 x 807s can't make 200 W' comment.
    OP said he wanted 4 tubes. Perhaps this thread has provided some insight into compromise.

    The 807/1625 will run with 0.4 W drive per tube per the spec. I get ~30-40 W out of a single one with close to that and well under max plate voltage.
    Of course it's not as a pseudo triode running GG.

    2. Tuned input is likely to be required with any tube amplifier making the power he wants with the drive he has.

    3. The 'special' 1625s are not uncommon. National brand comes to mind.
    But, again, the OP is not likely to be running pseudo triode.
    Because, he wants 5 W drive.

    4. When was the goal post moved to include digital modes? The OP did not mention digital in his OP. What percentage of sold amplifiers will run their specified output with digital? My Tentec Hercules 2 comes close at 500 W for 15 minutes. It could be continuous but would require replacing a bunch of passive parts.

    I've said all along that aside from not meeting the 4 tube criteria, a 4-65 or 4-125 is the best option for light, power, and drive. The 4-250 or 4-125 is better because it will make the desired power at a lower voltage and is easier to find in functional condition at a reasonable price. Although for aesthetics, a pair of 4-65 are very attractive. That's a compromise. I've also suggested that for a beginner on his first amp, he bite off something simple like a single tube, single band amp.

    IMHO, encouraging a 160-6 four tetrode amp for a first project is encouraging failure. Maybe it's just me, but that's an awful lot for a first bite.

    73, -Bob
  3. N2EY

    N2EY XML Subscriber QRZ Page

    4 x 807 can make 200 watts Class C. I know because I've gotten 100 watts out of a pair for more than 40 years.

    Exactly. The problem is that four of them in parallel will have stability and matching problems in grounded cathode. Not that it's impossible, just not simple or easy.

    So the "modified 1625" idea is a non-starter. It needs 20 watts or more of drive. Plus it runs the tubes at 1200 volts and pushes them very hard.

    See post #41 in this thread.

    Which ones? Seems to me the biggest problems would be the power transformer.

    The main problem with the 4-125 or similar is that it's complicated. Needs a bias supply, screen supply, special socket, and the B+ is 1500 to 2500 volts. Contrast that with a single 811A or, better yet, a 572B. No bias supply, no screen supply, 1250 volt B+, 150 watts output, 4 pin socket. Needs about 6 watts of drive but that's doable.

    I agree 100%!
  4. AH7I

    AH7I Ham Member QRZ Page

    PP parallel might help.

    OP also added 6 m somewhere in the thread along with digital.
    Next thing, someone will suggest it capable on 600 m band :)
    ART 13 went from 200 kHz through 20 m so it's not a crazy idea.

    I agree.
    The single 811 or T160/572 is a good way for OP to get started.
    I'm still going to say single band is best for a first edition.
    It will offer less distraction from getting a nice power supply with
    sensible metering and useful TR switching done.

    73, -Bob ah7i
  5. W1BR

    W1BR Premium Subscriber QRZ Page

    Modify an old ARC-5 transmitter. The 1625 tubes can be driven easily with very little power. Simple enough to make it into a linear (pretty sure there
    were write ups in the old ham literature.) Only problem is you may be limited to one or two bands.

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