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160 meter amplifier

Discussion in 'Homebrew and Kit Projects' started by K9STH, Dec 27, 2010.

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  1. K9STH

    K9STH Ham Member QRZ Page

    Unfortunately, the 3-500Z that I was planning on using for a 160 meter (only) amplifier (I have other linears for 80 through 10) is not too good! Has not had voltage applied for a while. Therefore, instead of obtaining another 3-500Z I decided to go with a pair of GI-7bT triodes. Only $35 each and they come from here in Dallas, Texas.

    With a key down rating of 400 mA at 2500 volts, that is 1 kW input and 600 mA under peak conditions which is 1500 watts input. The plate dissipation is 350 watts. Assuming 65 percent efficiency at 160-meters a pair should make the legal output peak limit on both SSB and CW without any problems! The only thing is that my present plate transformer is going to be really pushing it to make 800 mA key down, more like 600 mA to 650 mA without straining. I will just have to see if it can make the higher current, at least for SSB.

    Also, the tube is much easier to cool. Even if I get 1000 watts peak output that is a whole lot better than my present 80 watts peak output that I get from my Hammarlund HX-50A.

    Glen, K9STH
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Glen, I might have a somewhat bigger transformer for you if you need it.

    I have to move some stuff around in the garage (KIDS!!!!) to expose the wall shelving and do some searching. I'm pretty sure I had a 40 lb or so transformer there that would deliver about 1A ICAS, but I don't remember if it was 240V primary only or 120/240. It was a NOS transformer from Henry Radio, one used in their 8874 amps; but their transformer shop was local here iin So. Cal. and made very high quality stuff.

    I'll take a look if you're interested.
  3. WB3BEL

    WB3BEL Ham Member QRZ Page

    I think that your GI7bt are good tubes but you are overestimating how much power they will deliver long term.

    I'd say a pair with blower cooling they will reliably make about 800W RF output power. Perhaps a bit more if you hammer them.

    The radiator on them is not really that great thermodynamically. It looks better than it actually is. The interface between the cooler and the tube itself is a press fit sleeve. The quality of this thermal interface varies greatly from tube to tube as does the quality of the cooler which appears to be sand cast. Some of the tubes have porosity holes near this interface which makes for greater thermal resistance. If you are going to use it for SSB service you have more leeway.

    They are pretty good tubes if you get working ones...

    So I am guessing your power supply is probably a good match for these tubes.

    Make the amplifier and see how you like it. If you decide you need more power then you could switch to something like the GS35b which has a much higher power rating. But for that you would surely need a better power supply.
  4. K9STH

    K9STH Ham Member QRZ Page


    These are the bT version which do have a larger heat sink on the plate than the plain "b" version. However, what I was basing my figures on are the specifications of the tube. Even 800 watts is a considerable improvement from my now 80 watts!


    Go ahead and take a look. Then decide what you need to get for it and I'll see if I can afford it! The primary for 240 volts is no problem. I have 240 VAC available on my main operating console.

    Glen, K9STH
  5. WB3BEL

    WB3BEL Ham Member QRZ Page

    The BT version has the same cooler as the B version. BT has a larger ceramic body. It does not make all that much sense to me, but there must be some reason that they did that to make the BT (Tropical version). I think that these are for the most part interchangeable for amateur applications. Probably the airflow is better over the standard B version.

    If you truly want a better cooler the one which was made for the GI-46b will fit and has a much superior finned heatsink.

    These are a bit more expensive tubes and there are a lot of GI-46b tubes on ebay without coolers.

    But what you have should make a fine amp.
  6. AE1PT

    AE1PT Ham Member QRZ Page

    Using the GI-7 is a pretty common conversion for SB-200's that are being re-tasked for 160m or 6m. Or just modded to replace the 572B triodes. Any number of "sweep tube" type amps such as Yaesu, Dentron, and Drake. Also, it seems a pretty straightforward choice for homebrew such as yours. Charlie, VR2XMT did a series of really nice entries on his back in 2007. One has to page backward on the blog to find them:

    The GL-35 is pretty popular too. Sockets for either can be bought quite reasonably (and even as a tube/socket package) from either K4POZ or W4ZT. Links are shown at the bottom.

    I have two of these and the sockets that were bought a couple years back for some future project that has not yet been fully visualized! It may end up being the SB-200 when the Chinese 572B's become just too much rubbish. I have a lot of life left in my Cetron's though... :p
  7. AE1PT

    AE1PT Ham Member QRZ Page

    The wall is thicker as well--even more "ruggedized" according to the spec sheets. Supposedly for very high vibration/shock environments.
  8. WB3BEL

    WB3BEL Ham Member QRZ Page

    Yes, I've seen those comments regarding the ruggedness of the BT version.
    My point is that I don't think it really relates to thermal performance ie the tropics...

    I am sure that it is stronger to shock but what that has to do with the tropics is unclear...

    At any rate, I think they are basically interchangeable as far as amateur service.
  9. KM1H

    KM1H Subscriber QRZ Page

    Go with a GU-81M and a viewing window. They easily handle 4KV with a bit of air.

    The GI-7 is usually seriously overworked and undercooled in most ham conversions Ive seen but you never hear about the failures and rapidly going soft. Use 3-4 and run conservative.

    Another creative choice: Four 3CX100A5/7289 with water cooling jackets made from copper plumbing pipe. I run a pair on 1296 and 2304 for 30 years.

  10. K9STH

    K9STH Ham Member QRZ Page


    I looked at those websites before ordering the tubes. W4ZT died several years ago.

    One thing is that making a socket for the GI-7bT is easy for HF.

    Basically, all that I am wanting to do is to get a little more power on 160-meters with the minimum effort/expense. I already have almost full legal output on 80-meters through 10-meters including the WARC bands (1400 watts). The amplifier is going to be single band only.


    You can do all sorts of things with 2C39B / 3CX100A5 / 7289 for UHF. I have a number of those (ceramic versions), brand new, left over from when they were used in Motorola T44AAV- equipment. Those units used one as a tripler/driver into a 2nd one as a final amplifier on 450 MHz.

    I don't really have to worry about air flow. I have available from muffin fans to a high volume blower that Motorola used to cool a 4CX250B in a 100 watt output continuous duty amplifier. That blower is capable of easily cooling down a number of full legal output tubes.

    Glen, K9STH
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