Swan 1200-X

Discussion in 'Amateur Radio Amplifiers' started by N8SAN, Jun 28, 2017.

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  1. WA7PRC

    WA7PRC Ham Member QRZ Page

    Nah.

    However, it's best to know when you're about to go too far. Sweep toobs are increasingly unobtanium, and aren't the most linear power device. But, sometimes, with minimal time/money investment, they can be worthwhile. :)
     
  2. N8SAN

    N8SAN Ham Member QRZ Page

    You get it! That's just it! I have so little cash in this thing, and as you said as they become unobtanium, if I can make a set of tubes work easily, I'm going to do it.
    Plus, my knowledge of tubes has quadrupled because of trying to figure it out!
     
    WA7PRC likes this.
  3. W0SNL

    W0SNL Ham Member QRZ Page

    Interesting thread! There's a misnomer here that is part of the confusion. The 8950 is indeed intended for RF transmitting, unlike all the sweep tubes that have been mentioned here. That's one reason why it has multiple pins going to the cathode. It provides a lower capacitance connection, making the tube easier to use at RF frequencies.

    So other than the rewiring needed to get the proper heater voltage, the other thing to pay attention to is pin 6. If there are any connections to pin 6, they are relying on the 8950's internal connection to the cathode. Just jumper pins 2 and 6 to fix that problem.

    Other than that and the heater wiring the 6LF6/6LX6 should be a drop in replacement.

    If 5 and 6 are tied together, then the amp is probably using screen drive. Just a guess, need to see a schematic.
     
    Last edited: Jul 11, 2017
  4. N8SAN

    N8SAN Ham Member QRZ Page

    Glad you think so! :D

    Interesting. T'he manual I found with a schematic shows 6LQ6 tubes, which aren't even close to the animals discussed here.
    "Screen drive" another tube terminology I'll have to explore.

    So would I then tie in the 2,5,6 pins together. Where 5 and 6 are already tied.

    So to all the other people that have dropped in replacements and only changed the heater voltage, and it works, were they running the tubes inefficiently? Or what is the effect of not jumping 2 to 6?
     
    Last edited: Jul 11, 2017
  5. WA7PRC

    WA7PRC Ham Member QRZ Page

    No. Less series reactance (XL). The only capacitance is shunt inter-electrode, which also affects high frequency response.
    The 8950 is listed here: http://frank.pocnet.net/sheets/084/8/8950.pdf
     
  6. W0SNL

    W0SNL Ham Member QRZ Page

    Yes, less series reactance. That term works for me.

    Ryan, if this amplifier has connections to pin 6 and a sweep tube is substituted that does not have pin 6 internally connected to the cathode, then yes, they screwed up. If my guess is right, and the screen is being driven, then the cathode/control grid is probably grounded (at least at RF), meaning the circuit could be more forgiving of not connecting pin 6 to pin 2. If that were my amp and I had no access to good 8950's but I had 6LF6's, I would rewire the heaters, tie pin 6 to pin 2, fire it up, and neutralize it.

    Another reason why it might work without the jumper is pin 9, depending on where it is connected (pins 5 and 9 are connected internally).
     
    Last edited: Jul 11, 2017
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    In the Swan, like most "sweep tube amps," all the grids are grounded, the cathode is driven.

    It would be a cool project to ditch all the sweep tubes and their sockets and wiring and re-wire for one of the inexpensive Russian triodes which will still work with low B+ around 900V. That requires a lot of mechanical work, adding a cooling system, etc. but in the end you have an amp with a tube that would probably last forever and be loafing along at 500W output power. Those tubes are all NOS surplus as far as I know but there seems to be an endless supply of them and they're not expensive at all. Probably very stable operation at HF since they're all VHF-UHF tubes and in my experience "good VHF tubes" are incredibly stable at HF and don't require neutralization or have any tendency towards parasitic oscillations: One reason why the 8877 is such a good tube on HF.
     
    WA7PRC likes this.
  8. W0SNL

    W0SNL Ham Member QRZ Page

    How can that be? According to Ryan the cathode and control grid are tied together.

    The 8950 is not a sweep tube. It is a RF final or driver tube, designed for that purpose. Good to 30 Mhz.
     
    Last edited: Jul 11, 2017
  9. N8SAN

    N8SAN Ham Member QRZ Page

    pin 5,6.jpg For reference, this is what I'm seeing.
     
  10. W0SNL

    W0SNL Ham Member QRZ Page

    Perfect. Makes sense. Pin 9 is tied to pin 2. That makes the jumper from 5 to 6 irrelevant when dropping in a 6LF6. Just rewire the heaters, neutralize, and you should be good to go.
     

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