First Homebrew HF power amp

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

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  1. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I see 3CX3000A7s around here go for much, much less than 8877s. Not sure why, maybe they were commonly used in broadcast applications by some local manufacturers who went away. But they sure are common at the local swap meets, often in sealed Eimac cartons.

    I wouldn't be searching "on line" for 813s and such; the bidding audience is too large, drives prices up to crazy. At the TRW Swap Meet it's common to find someone selling NOS 813s for $25-$30 each. I never bought any, but they're out there.
  2. TF3T

    TF3T Ham Member QRZ Page

    I'm in the process of building a HF amp now, and I decided to use a Russian GS-35b tube. my design premises where:
    - Contest use - max power of 1500w
    - Cost - keep cost down hence cheap tube.
    - Use available components.

    I decided to buy a big plate transformer from Peter Dahl, - it was expensive (I had to get it to Iceland!) - but It's going to pay off. Main reason for buying a big plate transformer is to be able to power my 2m and 6m amplifiers as well. I'm now moving my 2m amp into a smaller box - as it was a 2 man job to move him when I had the HV psu internally.

    So I have been doing extensive planning for the last month, visiting ebay daily to find good parts for reasonable price and reading forums and handbooks.

    If you decide to build a tetrode amplifier - I would highly recommend the Triode boards from GM3SEK - I used it with my 4cx1000 2m amplifier and it has worked wonders. It has such a good manual that explains everything you need to get a triode amplifier working.

    I was tempted to use a Tetrode / dual tetrodes for my amplifier now - but I could not look past the fact the GS-35b costs around $130 on Ebay and seems to be a good tube to use. It's fairly easy to build the socket from scratch.

    pictures will follow

    good luck with your planning - I'm pretty new to this amplifier building business- but I think these projects are the most fun projects I have embarked on!

    73, Benni TF3CY
  3. TF3G

    TF3G Ham Member QRZ Page

    Hi Benni. If you build an Amp with a tetrode wouldn´t you then use a tetrode board from GM3SEK or are they the same ?

    Gisli TF3G
  4. N7GTB

    N7GTB Ham Member QRZ Page

    Hi Benni,

    Thanks for the good wishes on my amp project, and good luck to you as well! I too am spending much of my available time reading. I have much to learn about the current state of this aspect of the hobby... I have very few parts on hand, and as I discovered yesterday all of the 'large' power transformers I had in my 'junk box' are simply too small for the power requirements I'll need. So I'm on the hunt for one I can adapt/re-wind, etc...

    -Vern N7GTB
  5. TF3T

    TF3T Ham Member QRZ Page

    As Gisli pointed out !
    Of course it's the Tetrode boards for a Tetrode ;)

    - Benni
  6. N7GTB

    N7GTB Ham Member QRZ Page

    BTW, you're the second person to mention/recommend using a control board... I've not had a chance yet to fully read up and understand their benefit. In a nutshell, what function do they serve?
  7. TF3T

    TF3T Ham Member QRZ Page

    I think it depends.

    With ground driven tetrode - you will have two control voltages to worry about - grid/screen - The tetrode board from GM3SEK is a regulated supply with trip-reset functions. You need to watch out that the grid of the tetrode is only allowed to dissipate few watts at most. It does the bias switching HV control (cut out HV when tripped) PTT control/sequencing.

    With the triode - they seem to be easyer to control, just a simple bias switch to cut off the triode. But with a expensive piece like the 8877 - (that I'we heard is fragile) I would like to have good control. There are two solutions available that I know of, Paul WD7S with the Triode board, and Ian GM3SEK with Triode and Tetrode boards.

    Only think I don't like about the GM3SEK board is the relays used for bias switching - I would welcome a SSR or a transistor switch of some sort.

    Read up on GM3SEK control boars, there is a lot of good theory of operation in each of the manuals. (

    73, Benni TF3CY
  8. KM1H

    KM1H Ham Member QRZ Page


    Of course; they were in production for about 50 years in the US, tens of thousand NOS were surplused after WW2 and Korea and the tube manufacturers hyped the bigger stuff real cheap and then kept jacking the price. Now its time to retaliate.

    You must be on different forums plus you dont get the repair volume that comes thru here. If I believed my own experience there would never be a need for a 1986 LK-500ZC still has the original tubes and thats included many DX and WPX contest records, 9BDXCC with several over 300, 8BWAZ and more.

    All talk, no proof, as Ive yet to see any Russian tube plots. The ones Ive heard ID'd on the air sound like CBers and that includes several owners from various forums always bragging about their cheap and wunnerful Ruskie crap.
    Point.....even MFJ doesnt use them:)

    You can buy brand new Chinese for that and good pulls are $100-200. The $90 transformer can be prorated over a lifetime of use, you wont hurt it with a 100W rig no matter how you try. An unskilled creative person can rewind a suitable size core in a few hours. Yes, YC-156 pulls at full output are $300 and no socket required.

    By mature I dont mean industrial service.
    Im referring to 1000-1500W capability per device with IMd 3 in the -40's at HF and without having to run them at half power. Most of the linear effort is UHF and microwave.

  9. WA7PRC

    WA7PRC Ham Member QRZ Page

    Without admitting error, I'm not going to get into yet another peeing war with you, Carl. I just don't care enough. You're not right but, you win.
  10. KM1H

    KM1H Ham Member QRZ Page

    I dont consider a simple discussion a peeing war but if youre not able to offer more than that so be it.

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