Well, I’m back… with 1 Output Tube Arcing – Modified FISHER X202B

Discussion in 'Fisher' started by Cdemeo1, Nov 12, 2017.

  1. Cdemeo1

    Cdemeo1 AK Subscriber Subscriber

    Messages:
    148
    Location:
    San Francisco, CA
    Hi everyone!

    So, after a successful coupling cap replacement (found here) with major help from larryderouin and dcgillespie, I’ve now found a new problem. One of my ElectroHarmonix Output tubs is arcing inside the tube. It looks like lightning because, well, it is.

    Anyway, I’m hoping it’s just a bad tube. The amp was previously serviced by AEA here in he Bay Area (they’re an advertiser in this site). I am mostly pleased with the work they did. Below is a list of the work they performed on this X202B:

    • Signal Capacitor Replacement
    • Inrush Current Limiter
    • Transformer Blowout Protection
    • Individual Output Tube Bias Mod
    • Regulated Power Supply Mod
    • Phono Preamp Mod
    • Power Supply Capacitor Replacement/Upgrade
    • 4 x matched new Electro Harmonix Output Tubes

    With the work listed above, what do you think I’m dealing with here? I’m hoping it’s just a bad tube. I’m worried it could be something else. Attached is a pic of the tube that’s arcing and a pic of the underside of the amp (taken before I replaced the bad coupling caps from the last lost).

    I must admit that this issue is certainly something that I may have caused. Carelessly, I had the RED speaker cable on the Left Channel hooked up to the “8” Ohm terminal instead of the “4” Ohm terminal. My speakers are 4 Ohm ADS L710s. I must have mistakenly hooked it up incorrectly after repairing the coupling caps. On my bench, the amp was hooked up to 8 Ohm speakers. So when I returned the amp upstairs I hooked it up fast and wrong. I’m a dummy. It’s true.

    However, I’m pretty sure that the arcing was happening before I replaced the coupling caps. Pretty sure…

    Help?
     

    Attached Files:

  2. Patrice B

    Patrice B AK Subscriber Subscriber

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    I vote for a bad tube. If you swap the tubes, does the same tube arc again? Speakers on 4 or 8 ohms is not the source of your problem.
     
  3. Cdemeo1

    Cdemeo1 AK Subscriber Subscriber

    Messages:
    148
    Location:
    San Francisco, CA
    Thanks for the reply, Patrice! I’m hoping that’s the case. I haven’t switched any any tubes yet. Will try that in the AM. Thanks!

    In all honesty, I’m very new to tubes. Seeing as how there was an individual bias circuit added to each Output tube socket, will that have any sort of effect on wether I will see arcing in a specific tube or not? Tryin’ ta learn.
     
  4. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    C -- A couple of comments if I may:

    1. I'm confused that they said they installed an "Individual Output Tube Bias Mod", as the Fisher X-202B already includes chassis top bias controls from the factory that allow the current draw of each output tube to be optimized. You might question them on this as to what they specifically did regarding this modification.

    2. I'm assuming that the "Transformer Blowout Protection" is likely their language for installing individual (typically) 10Ω cathode resistors that when sized correctly will invariably burn open in the event of an output tube arc event so as to protect the output transformer primary winding. The fact that your tube continues to arc is a concern as it indicates that this resistor didn't burn out, allowing the tube to still arc, and put the transformer primary winding in danger.

    3. Within the list, I do not see any indication that they installed Screen Stability resistors on the output tubes. These resistors are quite important and really mandatory when the power supply caps are changed out for modern components. Modern power supply caps have a much lower ESR value (Effective Series Resistance), which in certain cases can set up what's known as a "tank" circuit in the screen grid circuit of the output stages. This tank circuit can store up significant amounts of energy, to the point that it actually promotes output tube arcing. The cure is to install Screen Stability resistors which prevents the tank action from happening. This is a subject I wrote about over a decade ago now wherein I first proposed the installation of Screen Stability resistors as a cure for this problem. Their inclusion will not prevent a defective tube from arcing, but is will prevent the circuit from promoting such events, which is the usual cause of them. The installation of Screen Stability resistors has been shown literally world wide now as a successful fix for output tube arcing issues that are circuit generated, and is virtually standard practice for the techs who frequent and contribute to at least the AK Fisher forum.

    4. Not sure what their Regulated Power Supply Mod might be, or what power supply points it regulates. Regulator circuits that operate at high voltage however are typically not easy/simple modifications to make, and require significant planning and engineering to produce a demonstrable benefit. It would be interesting to know what this entails.

    Understand that I am hardly wanting or trying to throw stones at the tech you used indicating the work done you listed. I'm simply reacting to the list itself and in particular, any items listed (or not) that could contribute to your current concern. Without a doubt however, connecting 4Ω speakers to the 8Ω output tap did not cause the output tube issue you are having.

    I hope this helps!

    Dave
     
  5. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    If it's still within his warranty period take it back and make him fix it, including replacing the tube. And ASK QUESTIONS as Dave suggested above, TAKE NOTES.
     
  6. Cdemeo1

    Cdemeo1 AK Subscriber Subscriber

    Messages:
    148
    Location:
    San Francisco, CA
    Hi Larry & Dave. This is super super helpful info. I didn’t think my 4Ω/8Ω speaker hookup would have caused this but felt I should mention it just to be sure.

    Unfortunately, I’m well out of the warranty period for the work that was done. I too, do not wish to disparage AEA but I must say that ultimately, I would prefer to find another tech to fix this issue. They haven’t been very responsive when I’ve reached out to them about the coupling cap issue and I would just prefer to go somewhere else.

    Do either of you (or anyone else reading) know of someone in the Bay Area – maybe another AKer – who could fix my amp? I know a few good techs on the East Coast since I lived in both DC and Baltimore but I know of no one really out here in SF who I am comfortable giving this to. Also, I would prefer not to ship it if I can avoid it.

    All the best,
    C
     
  7. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    PM AK-er RUFLERUF who's in Oakland. He might know of someone other than AEA. Also PM ferninando IIRC he's down near Sunnyvale or thereabout's.
     
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  8. Cdemeo1

    Cdemeo1 AK Subscriber Subscriber

    Messages:
    148
    Location:
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    Thanks for pointing me in the right direction. You guys are the greatest.
     

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