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AT100 Resistor mystery?

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  • AT100 Resistor mystery?

    Anybody care to take a stab at this would make my day!
    This is from a B52 AT100 that I bought recently - Power tube V8 was blown.. after a new set of tubes, I was checking the bias when V8 burned blue and I smelled something dubious. After shutting it down, I found that esistor "Rx" was popped, but I don't see it listed on the schematic - or a value anywhere..?? Am I losing the rest of my sanity here?
    Attached Files

  • #2
    I took a couple reference photos of the AT100 I just worked on recently. Best I can tell it is a 10 ohm (Brown, black, black). I believe that nearby connector with red wires goes to the pilot light? What do the terminals at each end of Rx connect to?


    • #3
      red wires to pilot is correct.. as far as the connection, it's hard to tell w/o looking at the other side and I was in no mood to pull that mess of a board last night!


      • #4
        even though the resistor is toast, you should be able to use your DMM and check for continuity from each end (after cutting the resistor in half with a pair of dikes) to some of the components in the vicinity to see what it connect to, without pulling the PCB out.


        • #5
          It looks like it might be one of the 100ohm resistors on the filament winding to ground.
          I've repaired a few of these, I can double check next time I get one in.


          • #6
            I believe Billy nailed the value - common components: pilot wires @ .1 ohm, gnd @ 52, tube pins 2 & 7 @ 42, tube pins 1 & 8 @ 161 - most others 1k up.
            readings are the same from either end of the resistor (removed).


            • #7
              replace w/ 10 ohm - measured current @ 150 ma - rebiased @ 120 seems to play fine! I appreciate the input!


              • #8
                hi bay4hotrod
                Well I can't see RX on the schematic either !
                Need some more info ..can you trace where RX terminates ? ie do either end connect to the tube socket anywhere and if so what pin ?
                I notice that the body of RX looks brown compared to the surrounding resistors.

                This could have been caused by the fault and subsequent "burn out" but could also suggest a higher wattage resistor. Are there any others near the other tube sockets that look similar ?

                The two red wires that terminate in a plug/socket they look a bit light for heater supply do they come from the bias supply ?

                I notice with switch 5 a more complex than usual selection of HT and bias supply.
                One of the selections is cathode bias (D8 and R95) so there are a few more points to possibly go wrong than a traditional single style of bias.

                The "Blue" glow that appears in a tube is not a major problem me its just impurities in the vacuum fluorescing in the same way as deliberate impurities in neon tubes produce different colours.
                Sometimes exact resistor values cannot be determined until the amp is built have seen this in other amps usually its a fixed value for bias which you can't tell 'till you measure the current flow in the tubes .
                Perhaps the stencil was made before the schematic was finalized .

                Following that theory for a moment , check and see if you can find all the resistors in the bias circuit ie R125 , R128 , (adjustable bias) and more suspect R126 ,and R127 which are switched in as I understand it when the cathode bias setting is being used (R95,D8).

                Also check all those diodes with a DMM set to diode test - D6A,D7A and D8.
                The fact that there are 2 x D7's ! suggests to me there may have been a problem with component reference numbering.

                I'm not really qualified to comment on your sanity but hopefully after you have done some investigation some will be restored !


                • #9

                  This resistor is unique in it's placement hence the bafflement (is that a word ) - The plug-in runs directly to the pilot and is closely related to the resistor.

                  Good point about the stencil- I own an auto shop as a trade and am not green to tube ckts - I'm fluorescent lime green.. give me a security module or pcm!

                  Replaced blown resistor w/ .5 watt 10 ohm and seems to be fine after toning the bias down about 30ma (total). Didn't perform poorly before though.

                  I though that the 'blue' might signify overloading and that promted me to shut it down - then I smelled that "fried" smell.

                  Most people on here talk a couple of stories above my head, but I enjoy reading the banter - makes me want to go tear something up!

                  Going to run it hard this weekend and see...


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