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First Homebrew HF power amp

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

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  1. N7GTB

    N7GTB Ham Member QRZ Page


    It would appear that I've been severely bitten by the bug to build an HF tube amp. My requirements aren't too elaborate, just the basics: multi-band (80, 40, 20, 15 and maybe 10. 160m would also be nice) with power out of no more than 1kw...where 600w to 800w constitutes a comfortable operating level. My older beam is rated at a kw, with the rest of my station needing some upgrades to handle anything more... My dummy load is an oil filled paint can that will handle 1500w max.

    So I've been researching all sorts of designs, past and present, commercial and otherwise (i.e. from ARRL handbooks, Radio Handbook by Orr, internet designs, etc..). At first I had the fantastic notion of building an amp using one of the two 5CX1500A Eimac tubes that I've been sitting on for the last 30 years... But after doing some initial reading it's obvious that project should wait...!

    That said, I'm now looking at some older established designs from the 1950's, 60's and 70's that use the 811A, 813 and 572B. I've also considered other tubes, but to stay with established designs the tubes mentioned still seem to be available at reasonable cost. Four 811As in tandem appear to make a full gallon as far as I can tell, and at a cost far less than some others...

    Now, I've built tube gear (transmitters, receivers, power supplies and audio amps), which has given me an understanding of and a healthy respect for high voltages...but as I said this is my first hf linear amp...

    How am I doing so far? ...any helpful advice for the newbie?

    Thanks in advance.

    p.s. - I also see that some folks claim to have (successfully) used power transformers from microwave ovens to build HV plate supplies...would like to hear opinions on that as well...
  2. K9FV

    K9FV Ham Member QRZ Page

    Sounds like you are on the right track - and since you're well familair with tube equip, you are aware it is going to cost more to build the amp than to buy a SB-220 that will do the same thing, and the AL-80 series amp is in your ball park power range and has the advantage of covering 160 meters.

    Have you figured your budget to build yet? Any idea of budget? for a KW amp..... $800? less? more? All depending on how deep your junk box is. The HV transformer is a major cost, and the plate capacitor is fairly expensive if you have to buy that.

    just a side note: from the reading I've done the new 572 nor 811 are the best design tubes around for HF amp due to shape of tube and how internal configuration of parts..... I'm sure someone will chime in on this. The 3-500Z tube is very popular, but does require an expensive filament transformer.

    On the microwave transformer - I've read about that, and it "works" - BUT there are several drawbacks that I will let other's with MUCH more tech knowledge than I have comment on the "why" it's not the best idea around. The core is not the right type to consider re-winding to a new HV transformer - I think that is correct.

    have fun and good luck.

    73 de Ken H>
  3. KK4FUS

    KK4FUS Ham Member QRZ Page

    That's funny, a few days ago we had a discussion in FIDO7.SU.HARDW.OTHER (if anyone still remembers Fidonet) about using microwave oven transformers for high voltage power supplies.

    The consensus was that oven transformers are made with the minimum amount of materials (copper and iron) the manufacturer can get away with. Which means that those transformers have high idle current, generate a lot of heat and are not designed to work continuously. You could theoretically make a new primary winding, or connect two transformers in series, but it does not seem to be worth it.
  4. WA7PRC

    WA7PRC Ham Member QRZ Page

    Magnetizing current is high due to low primary reactance. One way around it is to resonate the primary winding with an appropriate value of C. I considered using such xfmrs but realized I'd need to use FOUR of them in order to get 3KVA... and then the secondary voltage would still be wrong.
  5. KM1H

    KM1H Ham Member QRZ Page

    A single or pair of 4-400A's or a pair of 4-250A's will be a lot more reliable than 811/572B's. Availablity of NOS surplus or good pulls is fairly steady and a regular Johnson ceramic socket with airflow directed at the pins and glass is a well accepted substitute for pressurized chassis, special sockets and chimneys.

    The old 813 is a great tube that is way underated in Pd but it would take 3 to obtain a clean 800-900W. NOS is still plentiful but pulls or "unknown" are highly questionable especially as the frequency increases.

  6. K9FV

    K9FV Ham Member QRZ Page

    Carl, are 4-400s or 4-250s that much less expensive than 3-500Z tubes? They all require the high current filament transformer.

    edit: ran off, thought 2nd half was starting.... it wasn't.

    Isn't the 813 fairly cheap and available? If not, the old standby 3-500Z is a good tube - just copy the AL-80 series amp..... or perhaps the Russian triodes... GS31b tube? Cheap, buy a couple for spares and never worry about running out.

    73 de Ken H>
  7. SM0NOR

    SM0NOR Ham Member QRZ Page

    Great that you have joined the group of amplifier builders! My story is similar; I had great plans with big amplifiers on the drawing board, but I decided I needed to start with simpler things to learn the craft. In my case I went for the 811A because I had some at hand and there are lots of designs available. But I agree with Carl above that the 811A/572 is not the best choice unless you are looking for that nostalgia look. I went for the latter and motivated the choice by making it into a retro design with visible glow. I'm very close to finishing, but I ran into some transformer issues which are not fully solved yet. My next project will be based on the Russian GU74. There are lots of discussions on the net about this tube. Some loves it, and others are skeptical. But it is fairly cheap (around €100) and used in many commercial designs such as Acom. Another, extremely cheap, option is the Russian Gi7B. Two of them will give you the power you are looking for. They are around €20 each. I built one of my really early (ugly) designs with one of these because of its low price and ruggedness. It is really not a linear amp tube, but it is very common among builders in the East because of price and availability. I don't recommend it for your "perfect" amplifier but for a very cheap, educational project it might be worth looking in to. Better burn one of these instead of a NIB 8877 for a $1000'+ :).

    Good luck and please share your experiences.

    And don't get intimidated by the "big" guys shouting at each other in the forums!

    Ulf SM0NOR
  8. W8JI

    W8JI Ham Member QRZ Page

    Four 811's are a kilowatt INPUT power with short duty cycle operation, not a kilowatt output. They are about 600 watts output, for short duty cycle modes.

    Personally, I think 811's or 572's are OK if you are willing to sort through tubes. New tubes do have a very high defect rate, but they are probably less overall hassle than anything else when cost and availability is factored in. Plus they work with lower voltage, and that is a plus for tank components.

    They can work, but they often have magnetic shunts and operate at saturation to provide voltage regulation as the power line changes. They also often have the secondary grounded to the core. This makes them poor for Ham use, although they can be used.

    73 Tom
  9. N7GTB

    N7GTB Ham Member QRZ Page

    Thanks everyone. Lot's to think about here, and I'll be looking at the tubes mentioned. I'll post again once I've started building...

  10. WA7PRC

    WA7PRC Ham Member QRZ Page

    Check your PMs.
  11. K9STH

    K9STH Ham Member 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
  12. N7GTB

    N7GTB Ham Member QRZ Page

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

    WA7PRC Ham Member QRZ Page

    Check your PMs again, Vern.
  14. N7GTB

    N7GTB Ham Member QRZ Page

    Your turn...
  15. K9STH

    K9STH Ham Member QRZ Page

  16. 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.

  17. 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)...
  18. 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! :)
  19. 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 P[SUB]OUT[/SUB] 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 CO[SUB]2[/SUB] Lasers well into the kilowatt output range (Lasers are awful loads). They didn't experience a big failure rate.
  20. AG6K

    AG6K Ham Member QRZ Page

     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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