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Brand New Sweep Tube Amp

Discussion in 'Amplifiers' started by 443, Jan 10, 2012.

  1. 443

    443 W9WDX Amateur Radio Club Member

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    Anyone build one? The tubes used for Guitar amps are common, cheap, not going anywhere, and typically require similar plate transformers to the old days sweep tube amps.

    I got a pair of these may modify an old BlackCat JB-150 (originally 2 8417 tubes) to use these Svetlana SV6550C tubes ($55 matched pair).



    SV6550C datasheet and application note, data sheet, circuit, pdf, cross reference, pinout | Datasheet Archive
     

  2. office888

    office888 Active Member

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    Why not just build a proper amp with 3-500z, 813, or the widely available surplus Soviet radar tubes? They're actually designed for RF TX applications.
     
  3. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    Take the old D&A phantoms, install some 811A's, cheap tube ,cheap socket, throw some bias on the cathode, make a doubler circuit for the B+ and have a sweet 300 watt amp with a pair of 811A's.
     
  4. office888

    office888 Active Member

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    I'd push for a fairly conservative number out of 811As. I hear they don't do so "hot" when pushed to the max with AM service. Plus they're only rated for 30 mhz I believe?

    They are dirt cheap though!
     
  5. 443

    443 W9WDX Amateur Radio Club Member

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    I know you could build a "proper" amp with 3-500Z etc, but plate transformer a lot smaller/lighter for a couple 6550's/KT88's, etc. And 400-800V vs the 2000-3000V you would need for real RF TX tubes.

    Just saying, it would be "cool" to have a brand new sweep tube amp. You know, some people like them (even though it's really not "logical" ;-)
     
  6. office888

    office888 Active Member

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    It's false economy to save $100 on a power supply, then eat that up in tube costs later.
     
    2 people like this.
  7. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    300watt PEP is not bad for the 811A.

    I just got finished with repair of a home brew 811A two holer, 1800VDC unloaded B+, 70 PEP watts of drive shows 300 watts on the Autek WM1 Meter, scope shows clean. Big string of diodes for cathode bias keeps the IP at reasonable amount under key down, tubes barely turn any color.

    Take that same tube put it in class C no Cathode bias and watch it melt in AM use if the carrier input is not controlled.

    There are better tubes to build out of, Russian ceramics, but for the price of the 811A, use them while they are still making them.

    Ameritron is still manufacturing amps using them and the ops drive the SH*T out of them on the bands. Of course SSB mode.
     
  8. 443

    443 W9WDX Amateur Radio Club Member

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    :)
     
  9. KM3F

    KM3F Active Member

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    Several things.
    Those Guitar tubes are meant for audio only as noted in the specs.
    They have too much inter electrode capacity and not enough plate dissapation to use for HF application.
    They are Tetrodes and require good screen voltage regulation.
    To get the same power equivelent from lower plate voltage, the plate current has to go up using more tubes.
    This makes things worse yet for RF application using those tubes.
    For example the higher the number of tubes the lower the plate impedence that has to be matched to the 50 ohm output and the lower the max frequency that can be used before huge losses take over.
    Example: at 3000 volts on the plate of a 3-500, the plate current rating is 500 ma = 3000 x .5 = 1500 watts plate dissapation.
    How many tubes would it take to equal that dissapation using the audio tubes even on 160 meters?
    At even 500 volts on their plates it would take 3 amps of current.
    Finding a transformer to do that is harder than using a 3000 volt supply at a half amp.

    You can't get around it in any manner. Either HV lower current, low voltage higher current or very low voltage much higher current (for solid state amplifiers), for the same relitive power levels.
    This is the perspective you seem to be missing.
    Good luck.
     
    1 person likes this.
  10. 443

    443 W9WDX Amateur Radio Club Member

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    Nah, I wasn't missing ANYTHING, I had noticed alot of people running those "guitar amp" tubes in their old school amplifiers with great results. Perhaps the actual application pans out better than the theory as things do come out that way sometimes.
     
  11. 443

    443 W9WDX Amateur Radio Club Member

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    I think some of the tubes are good for 150-200 watts each BTW. As you know, 500 watts to 1000 watts is not that much difference on the receiving end, so getting 500 watts from 4 tubes that can be had as low as $19 each is not a bad deal I think, of course you'd need to find the right plate transformer, but seems like I have seen lots of them out there and you can always run multiple transformers too if they are all too small in current output.
     
  12. 443

    443 W9WDX Amateur Radio Club Member

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    I don't think they were Tetrodes either, I think they are Beam Power Pentodes.
     

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