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 Premium Subscriber 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 Subscriber 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 Premium Subscriber QRZ Page

    Check your PMs.
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