First Homebrew HF power amp

Discussion in 'Amateur Radio Amplifiers' started by N7GTB, Jan 9, 2012.

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  1. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Last year I built a linear for just 160-meters using a pair of GI-7bT tubes. They are easy to build a socket for and are pretty "tame" as well. I am getting over 800 watts out with a pretty "puny" high voltage supply. If I were to use a better power supply, the tubes would make well over 1000 watts easy.

    Glen, K9STH
  2. N7GTB

    N7GTB Ham Member QRZ Page

    Those tubes are certainly attractive on cost... Which mode(s) do you use it with?
  3. WA7PRC

    WA7PRC Ham Member QRZ Page

    Check your PMs again, Vern.
  4. N7GTB

    N7GTB Ham Member QRZ Page

    Your turn...
  5. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

  6. KM1H

    KM1H Ham Member QRZ Page

    A search on Fleabay, for sale forums and hamfests will answer that. Ive yet to pay over $25 for NIB of either and thats without looking hard. The cost of a filament transformer is a non issue if you want power and a new 5VCT/30A is usually $50-75

    73 de Ken H>

    I recently paid $40 for 4 NIB 813's from a local and those will eventually replace the 4 well abused ones in one of the AM amps here. I see them often at $20-30 each.
    The only new 3-500's are Chinese of questionable quality and now pushing $200 a pop including shipping....probably cost $10 to build.
    Used or NOS Eimacs and Amperex are a gamble since gas is a very prevalent problem.

    Russian tubes have a reputation of poor IMD and Ive yet to see anybody say different with proof, they are fine for CB[​IMG]

    If you want a tube that will last forever go with a 3CX3000A7 or YC-156 known good pull. An easy 1500W at under 3000V and 100W drive.

    Or wait for LDMOS to mature.

  7. N7GTB

    N7GTB Ham Member QRZ Page

    Looking at the QEX article from a few years back I can see what you are talking about. Also, with four MOTs the author only specs a max load of about 1400 W DC depending on filter cap used. If I'm reading things correctly, the GS-35B would require about 2500 W DC input for 1500 W out... That might take a few xformers and event then I'm not sure what would happen to the cold side of the HV windings of the 'outer' units.

    I've got a few old 811s and 812s laying around that I might try to use with a MOT power supply, and buil an 'olde tyme' 500W CW transmitter (just for fun someday)...
  8. N7GTB

    N7GTB Ham Member QRZ Page

    Either I'm looking at the wrong times or I'm grossly misreading the above... *** I certainly mean no disrespect to anyone here***...but I'm just not finding this to be the case...

    I'm looking at that infamous auction site now, and for NOS/NIB 813's, the cheapest ones listed are a pair for $307 (U.S.) + $60 shipping from down-under... There are several of 'unknown' condition "as is" and a few of those are 'BIN' ranging from $25 to almost $50...

    If anyone here has a source of NOS or decent NIB 813 tubes for $50 a pop then PLEASE PLEASE put me down for four, and I'll start building my amp right now! :)
  9. WA7PRC

    WA7PRC Ham Member QRZ Page

    150W of filament power will keep the shack nice 'n warm. For Vern in Eastern WA, that's good in the Winter but bad in the Summer. The GS- Russian (and similar) tubes pull 1/3 of that.

    They're cheap for a reason.

    IMO, this is an overstated reliability issue. So far, my Chinese 3-500ZGs have been working fine for more than 5 years (and I'm not easy on them). And, on the many email reflectors I've been reading for the past several years, I haven't seen any significant trend of these tubes failing. I will agree about unused tubes tending to be gassy.

    I've read the opposite about Russian triodes & IMD3. I've also read that running any tube significantly below maximum POUT results in increased IMD3.
    At $600 to $700, they don't give away 3CX3000A7s (YC-156s go for about half that), and a 7.5V/50A filament xfmr is huge and not cheap.
    LDMOS has been around a few decades, and many ham equipment manufacturers (including the big names) have been using them for many years. Most people would call that mature. My former employer has been using them to drive CO2 Lasers well into the kilowatt output range (Lasers are awful loads). They didn't experience a big failure rate.
  10. AG6K

    AG6K Guest

     For a full 1000w in linear service, 6. 811As would be better since they have rather low anode dissipation capability and they are not very linear above 200mA. However, since each 811A has 0.6pf of C-feedback, you will be dealing with a total C-fb of 3.6pF - which means that you will need to use better than avg. - a.k.a., lower-Q - VHF suppressors to keep the squirrels away. With 6, 811As and minimal fil.-V your tubes should last a long time Vern.

    • Rich, ag6k.

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