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Thread: First Homebrew HF power amp

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  1. #21

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    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.
    A government which robs Peter to pay Paul can always depend on the support of Paul.

    -- George Bernard Shaw

  2. #22

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    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. #23
    Join Date
    Jan 2009
    Location
    Reykjavik
    Posts
    13

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    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. #24

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    Quote Originally Posted by TF3CY View Post
    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
    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...

    73,
    -Vern N7GTB

  5. #25

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    As Gisli pointed out !
    Quote Originally Posted by TF3CY View Post

    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.
    Of course it's the Tetrode boards for a Tetrode


    - Benni

  6. #26

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    Quote Originally Posted by TF3CY View Post
    I would highly recommend the Triode boards from GM3SEK -
    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. #27

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    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. (http://www.ifwtech.co.uk/g3sek/)

    73, Benni TF3CY

  8. #28

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    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.


    They're cheap for a reason.
    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.



    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.


    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.

    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 replacement....my 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.



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


    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.

    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




    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.
    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.



    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.
    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.

    Carl

  9. #29

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    Quote Originally Posted by KM1H View Post
    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 replacement....my 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.

    Carl
    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. #30

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

    Carl

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