Question: SB 200 GI-7B Conversion

Discussion in 'Amateur Radio Amplifiers' started by N7WR, Feb 1, 2020.

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  1. N7WR

    N7WR Subscriber QRZ Page

    I have an opportunity to pick up a restored SB 200 with the following upgrades
    NEW GI-7B tube and a NEW spare unopened
    3. Harbach power supply rebuild board
    4. Harbach soft-start
    5. Soft-key mod (8VDC at 14ma keying)
    6. LED meter lamp and diode meter protection
    7. W4ZT bias board--plate idle current set to approx 20ma
    8. SO-239 RF input connector

    Offered at a reasonable price and supposedly capable of a conservative 700 watts output. Over the years I have used "stock" SB 200's but never one with the Russian tube conversion. This one comes with a new spare tube as well. Looking at the schematic I already plan to add a standby/operate switch.

    Anyone here have experience with this particular modification that could provide feedback/tips? This modification was done by K4POZ.

    Thanks in advance for any info
  2. KM1H

    KM1H Ham Member QRZ Page

  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I have no new information, but don't all those tubes require 120 seconds warm-up (not instant on like 572Bs)?

    That always bugged me about 8874s, 8877s, etc. A little bit.

    Most commercial amps with "warm up required" employ a timer circuit that prohibits transmitting until a certain time has passed. If a conversion amp doesn't add such a timer for the relay(s), I'd want to add one.
    WA7PRC and N7WR like this.
  4. AH7I

    AH7I Ham Member QRZ Page


    How/where does one see IMD in those curves?

    73, -bob
    N2EY likes this.
  5. AH7I

    AH7I Ham Member QRZ Page

    For the project I am working on, I've different
    cooling scheme to move enough air (spec 24 CFM)
    for the tube at 600 W average output. Code/SSB duty
    will not need nearly as much air. The amp at the link can
    not move anywhere near spec air mass the way it is set up.
    If it's working at 700 W PEP output and the tube is not
    overheating... then it's moving enough air for that :)
    I'd want to measure temp before feeling comfortablem expecting
    at 700 W CW (if it can make that) with the small cross flow fan.

    73, -bob
  6. K6CLS

    K6CLS Ham Member QRZ Page

    Can the transformer in the SB200 deliver enough current (anode)?
  7. N8CBX

    N8CBX Ham Member QRZ Page

    Complete BS, in my opinion. I suspect the grid structure will be blown or melted at that power level.
    Jan N8CBX
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Never used that tube in anything.

    But the SB-200 power supply might be good enough for 700W PEP output but I sure wouldn't push that for RTTY or any mode with a longer duty cycle.

    At least, not without a good smoke detector and maybe fire extinguisher handy.:)

    Of course, really same goes for a 30L-1.
  9. MM0IMC

    MM0IMC Ham Member QRZ Page

  10. AH7I

    AH7I Ham Member QRZ Page

    Off the cuff I questioned that small fan. Thinking about it some more...

    Will tube protect supply or will supply will protect tube?

    Heath specs input to final by duty cycle as:
    1200 W PEP, 1000 W (50% duty cycle and key down max of 5 min) CW.

    Peter Dahl SB-200 replacement spec is 800 V 500 ma.

    The transformer guy I've talked to says the spec is for rise to 80 degree C.

    The tube dissipation spec is 350 W at 24 CFM (sea level?).

    The anode mass is tiny compared to the transformer core.

    At 700 W output, key down, the tube is going to be dissipating at least
    300 W.

    5 min at 300 W is ~21,500 gram calories. (21.5 kcal, the kind you see on the food label). The heat sink is 160 grams. Lets give another 90 grams to the anode structure. In 5 minutes, with no dissipation, the temp will rise by 106 degrees C. Spec says seals are good to at least 160 degrees.

    Will the fan move enough air to take away 21,500 calories in 5 minutes of key up?
    Will anyone key 5 min on on then 5 min off at 50 % duty cycle enough times to reach the tubes maximum seal temp?

    Unless I've messed up with math or input, it looks like one GI-7B with a small fan is OK for replacing 2 x 572B in a SB-200.

    Based on a very rudimentary understanding of how the tube data translates
    into what it can do, for 700 W, I think you'll need more than 2100 V.
    It looks like 2500 V at 400 mA is maximum spec for the tube for continuous
    use. Since it's a pulse tube, there should be plenty of reserve for higher peak at lower duty cycle.
    Then you'll need more air.

    73, -bob
    Last edited: Feb 3, 2020

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