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SB-200 Tube Replacement Procedure

Discussion in 'Amplifiers' started by tba02, Sep 24, 2016.

  1. tba02

    tba02 WOOF

    Jan 26, 2014
    Likes Received:
    I would like to replace the tubes in my SB-200, but I want to make sure I do it safely.
    I have done the reading and know there are lethal voltages etc., but I have not been able to find walk through anywhere on the internet.

    The amp has a Harbach PM-200 power supply module installed. It also has the soft-start and soft-key installed.

    Here's my understanding of the process.
    • Disconnect from power and ensure it can't be connected to power until completed.
    • Allow voltage to dissipate, understanding that the HV reading on the meter may or may not be accurate and voltage may still remain even though it reads zero.
    • Open case, remove cage to allow access to the internals.
    • At this point all steps are a one handed operation with the other hand in back pocket to avoid creating a circuit that could lead to a very bad day.
    • Using a chicken/jesus/oh-sh*t/capacitor discharge stick*, bleed off any potential high voltage from the power supply capacitors (and any other large caps?).
    • Remove anode clips.
    • Remove tubes.
    • Install tubes, anode caps, and reverse the rest of the above steps.

    Assuming the above is correct, questions:
    Looking at the original schematics and construction manual it would appear that the HV plate wire is a large blue wire (which is I confirmed is still as originally built) and connects to "H" on the Harbach PS module (diagram attached).
    • I would use the chicken stick to dissipate HV at connection point "H"
      • - are there any other areas or capacitors that I should be concerned about?
    • Should I then remove the anode clips using insulated handle pliers, or is it better to remove the RF chokes from the door knob capacitor and then remove the clips?
    • Gently remove and replace tubes (soft cloth, no greasy fingers etc.)

    Moving on, I have two options for tubes, some used, tested, and been sitting for unknown period Cetrons and also new Taylor/RF Parts tubes.
    Once the new tubes are in place what is the best next steps as I have seen conflicting information (gettering).
    • Do I not connect the anode clips, bring power to the tuibes and run them for 8-12 hours?
    • Do I connect the anode clips, re-assemble and use a variac (I really would prefer not to have to find a variac)?
    • Do I re-assemble completely and let it run for a while before transmitting?
    • Does it matter that one set is new as of 6 months ago and the other set has been on the shelf for xx years?

    So what am I missing? Where am I lacking? Comments or criticisms?

    *When building a chicken stick, is a resistor recommended or not? I was thinking a single ~40k 3-5 watt resistor would be in the ball park were I to use one.


    Attached Files:

  2. BBB

    BBB Well-Known Member

    Jan 17, 2012
    Likes Received:
    This instructional video shows how to prep the tubes prior to removal. You want to make sure the t00bs have definitely failed. Notice the tech uses the back of his finger to tap the t00bs, not the front, so as to keep his finger prints intact in case the glass envelope shatters exposing the high voltage internal components within the t00b.

    *Note: For Non-Educational Comedic use only.
    #2 BBB, Sep 25, 2016
    Last edited: Sep 25, 2016
    222DBFL likes this.
  3. tba02

    tba02 WOOF

    Jan 26, 2014
    Likes Received:
    Dang, please don't make me go to QRZ/EHAM.
  4. Captain Kilowatt

    Captain Kilowatt Professional Amateur
    Staff Member

    Apr 6, 2005
    Likes Received:

    Those tubes are not simply low on emissions but rather look like they have a grid/plate short happening.

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