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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. KC8QVO

    KC8QVO Ham Member QRZ Page

    What are some smaller tubes that put off more light than most and are easy to come by? Preferably ones that use a pin base and socket, no top of the tube anode etc?

    I'd like to build an amp that will do about 150-200 watts out with 5 watts drive, and maybe up to 500 watts out with higher exciter power, that uses 4-6 tubes. If I did my math right that is in the 15dB gain range (add 3dB for every double in power - start with 5w, then 10, 20, 40, 80, 160w, right?).

    The primary goal is to have an amp for my Elecraft QRP rigs (KX3 and K2) that glows. I am not concerned with space and weight for the design, in fact I want to open up the specs for easy access, adjustment, and component selection (such as open air inductors).

    I am even thinking of building in a band pass filter section that works in TX and RX like what these are [] but home brew and switchable from the control panel, either by relays or mechanical switches.

    I'm half way thinking open air tube construction (tubes free above the box) would be ideal, but the other side of me says that is too vulnerable so maybe a solid base box and a mesh screen for the top? I'd like to keep cooling to a minimum - maybe if anything use a muffin fan to waft air around the tubes. I'd just like to avoid forced-air cooling.
  2. KH6AQ

    KH6AQ Ham Member QRZ Page

    Last edited: Dec 26, 2019
  3. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    If visual appearance of glowing is a key goal, take a tip from some of the audiophile amplifiers: Put LED's in the base/mount shining upward into the bottom of the tube!
    W1TRY likes this.
  4. N8CBX

    N8CBX Ham Member QRZ Page

    Be careful of tubes with long leads in HF operation. The 6146B is the way to go. You can buy two tube sockets from old Kenwood & Yaesu hybrid radios, as well as the transformer and so on.
    Jan N8CBX
  5. N8FVJ

    N8FVJ Ham Member QRZ Page

    Four 6CA7 (EL34) Use 500 volts on the plate. Four 807 would be better, but has top anode. 807 can use 700 volts on the plate. Either will provide 200-250 watts out.
  6. WA4SIX

    WA4SIX Ham Member QRZ Page

    If I recall right, 6AU6As were used as drivers. Might look into them for what you want to do. Might need a bunch, though.

  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Why not the top anode connection, and why use so many tubes when "one" would do this job?

    A 4-125A will boost 5W to 150W easily, all in one fairly small tube. The socket isn't expensive and as long as all the generated heat isn't completely trapped inside the enclosure, it really doesn't need any kind of fan. Most would use at least a low-noise fan, maybe a low speed 80mm or 120mm fan to keep air moving around, and running at reduced speed makes them very quiet.

    For that kind of gain, same with all tetrodes or pentodes, you need four power sources: Heater/filament; anode; screen; and grid. The screen supply can be derived from the plate supply (drop it, regulate it) but you still need a filament winding and a negative grid bias supply. But that would also be the case with a pair of 6146s or almost anything providing such gain.

    I like the 4-125 for this power level, and they're cheap. And just one tube will do this job.
  8. N8CBX

    N8CBX Ham Member QRZ Page

  9. KC8QVO

    KC8QVO Ham Member QRZ Page

    Aesthetics, thats all.

    I understand all the points you make and that a single tube would work. Multiple tubes have more going on. From a design/theory perspective it isn't that it is that much more complicated - it is the same theory, just more parts. That is also the same reason I mentioned that I wasn't wanting a compact amp. Without a constraint on design the possibilities open up.

    One idea that I want to to hold on to is multiple tubes in parallel. I assume the best way to go about it is to use "matched" tubes. In my high power design idea with the GS35b I am still wanting to figure out a way to pre-drive it to keep the load off the exciter. That is another subject for another thread and design, but in the low-power idea here I have a few base line ideas but a fresh slate for tube selection so from that starting point I have a lot of freedom. For the simplicity of it (compared to what will likely be a nightmare to resonate a pre-drive) putting multiple tubes together in parallel is appealing. Again, I realize one will work, but from an aesthetics perspective one tube just is too boring.

    I lucked out with one of my transformer scores back in college and have a wide range of voltages to play with. In the one I have (rectified, filtered DC) ~70, 389, 815, 1240, 1560, and 1630v. I have a few smaller transformers for the 12-50v range also. Depending on what form things take I can use what I have then try to streamline a power supply later.

    Looking through the tubes mentioned the 4-125 is a cool tube, but I think its bigger than I want to go and has a top connection. The 6L6GC, 6CA7, and 6AU6A are more along the lines of what I was thinking, size wise.

    As for the LED idea - interesting and I have seen a lot of LED lighting ideas for enclosures, not so much for tube illumination. However, for tube illumination I just want the filaments to do their job (and within spec, not running "hot" just for more light). So I suppose I will get what I get in that department to some extent.
  10. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    If you want light, look into older thoriated filament tubes. I’m not saying that the design is easy, but a 211 doesn’t have a top cap, runs on 1200 volts, and looks really impressive when lit.

    It would be good up to the 40m band.

    You can gain insight by obtaining an RCA transmitting tube manual and leafing through it. You will find a few tubes that fit your idea.
    Last edited: Dec 27, 2019
    K6LPM likes this.

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