1987X Bias

Discussion in 'The Workbench' started by Roger Hundley, Nov 21, 2019.

  1. Roger Hundley

    Roger Hundley New Member

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    Hi guys,
    First post. Looking for some advice on my beloved amp.

    Just replaced the original power tubes in my year old amp with a set of matched Mullards.

    Plate voltage at 454 vdc. Wanted to get them to 38 ma but best I could get them was 39/35 ma. Would you guys accept that or get another set of tubes? The were supposed to be matched. Boxes marked 33.4 and 33.3.

    The amp sounds fine to me.

    TIA for any input.
     
  2. pedecamp

    pedecamp Well-Known Member

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    I'd contact the vendor and explain to them and get another set.

    Was there anything wrong with the original set, a year old they should be good.
     
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  3. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    First get a good electronics contact cleaner/lubricator and apply it to the socket prongs and tube pins.
    You can do the same with the preamp tubes, all jacks and cord plugs.

    Anyway clean everything and reseat the power tubes. You can also try swapping sides/sockets with them to see which gives the closest to matched readings for idle bias points.

    Please post back with final idle bias measurements on the power tubes.
     
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  4. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Welcome to the Marshall amp forum Roger :cheers:.

    The output tube screen resistors should be tested. If these resistors are out of spec it will make the output imbalanced.
    This might explain the 39 / 35 current from matched tubes.
    Instead of the tubes being mismatched it could be that your screen resistors are causing the imbalance.
    Therefore you need to measure those resistors before concluding that the problem is the tubes (all the information is not known yet)

    These 2X resistors are 470 ohm 5 watt, and they should both be measured with power off / the power tubes removed.
    (Use an ohm meter.)
    The resistors should both read almost exactly the same if you want matched current from 2 output tubes.

    The other explanation is that the tubes are not really matched that close.
    But without checking those screen resistors the facts are not in yet.
    (don't blame the tubes until you know that your amp is all up to spec)

    1987Xscreens.png
     
    Last edited: Nov 22, 2019
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  5. Roger Hundley

    Roger Hundley New Member

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    Thanks for the input guys. Maybe a little more history will help.

    I've had the amp for a year. Bought new from Sweetwater. Over the course of a year it has blown the H.T. Fuse randomly. Probably 6 times total. Last weekend while playing, it fizzled out and there was evident smoke and smell this time. I pulled the back for the first time and looked at the tubes. One appeared to be cooked vs the other so I ordered a new set of tubes. No testing just got new Mullards and replaced them to get where I am now.

    Picture of the tube attached.
     

    Attached Files:

  6. Roger Hundley

    Roger Hundley New Member

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    This makes sense. I am not a master electronics guy but can get through anything with enough info. The attached picture. Are the film resistors the big white rectangular you can see beside the leads I have attached in this photo? Can you measure the resistance across them while in the circuit? Assuming you can.
     

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  7. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    UN-plug power from the amplifier first.
    Remove the output tubes to measure the resistors.
    These are the screen resistors.
    These are 2X white block ceramic resistors soldered to the tube sockets.
    Yes when the tube is pulled you can measure it OK in the circuit.

    1987xscreenresistors.png
     
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  8. pedecamp

    pedecamp Well-Known Member

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    Still, I would contact the vendor and explain you cant bias them to 38 like you want and send them back for another pair, but definitely test the screen resistors just to make sure. :yesway:
     
  9. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Uhm, he said a new 1987X. So that has 6CA7/EL34 with 1k screen resistors.

    Again clean and move the power tubes around. Recheck bias. Tell us what you get.

    It is good to check things when a tube blows. But do not just stop at the screen resistors. Check all voltages around the power tubes for proper levels and might as well check the OT primaries DC resistance as well.

    With power OFF, can you get output transformer primary resistance readings from the center tap to each power tube plate (pin #3)?
    The center tap would connect to the B+ rail at that HT fuse (F2) out, filter capacitor(s) and choke coil area.

    schematic
    https://drtube.com/schematics/marshall/1987-01-60-02_iss4.pdf
     
    Last edited: Nov 22, 2019
  10. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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  11. Roger Hundley

    Roger Hundley New Member

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    You guys are great! Schematics and everything! Looks like I have my Friday evening mapped out. I'm going to do the cleaning, swapping tubes around, check the screen resistors, anything else I can figure out from the input and report back. If it gets to above my pay grade it still is under warranty and a shop not far away. I'd rather not go there but I am a layman with this. :fingersx:
     
  12. Roger Hundley

    Roger Hundley New Member

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    I will once I determine as best I can it isn't the amp. Viva Tubes from Amazon if that means anything.
     
  13. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    How heavily did you use the first set of tubes, hours per year?
    How much of that was cranked?
     
  14. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Every once in a while we get users with new sets and low bias readings. They are selling weak tubes at regular prices. Bad. :mad:
     
  15. Roger Hundley

    Roger Hundley New Member

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    In my home. An hour or so most nights practicing. 3-4 hours jamming at home on weekends. So ~750 hrs... probably less. I run an attenuator and volumes at about 50%.
     
  16. Roger Hundley

    Roger Hundley New Member

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  17. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Eh, you should get more than that on a nice set of tubes if nothing goes bonkers but change at your discretion.
     
  18. Roger Hundley

    Roger Hundley New Member

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    Some Results.
    Cleaned the PT Sockets
    Left Screen Resistor - 993 ohms
    Right Screen Resistor - 991 ohms

    Left Socket OT resistance - 48
    Right Socket OT resistance - 48.6

    Plate Voltage - 456 vdc

    Left Tube Bias - 39.8
    Right Tube Bias - 36.3

    Left Tube Bias after swap - 36.0
    Right Tube Bias after swap - 41.0
     
  19. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    All that looks good except for the matching of the power tubes. It looks like 3.5mA to 5mA out. That is like 9% or 10% out on matching.

    Their ad questions/answers say they are supposed to be matched to 1mA at 400 plate volts. It also mentions 5% which would be like 2mA.
    It is safe to say those do not seem to be.
    The numbers appear to be following the tubes.

    One thing to check though is VDC negative bias voltage on each power tube socket at pins #5.
    While at it also measure both power tubes for VDC at plates pins #3 and screens pins #4 just to make sure all is normal on both sides.
    It is good to know these and have for reference.

    Measure all that and give us the numbers to which power tube.
     
    Last edited: Nov 22, 2019
  20. Roger Hundley

    Roger Hundley New Member

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    I put the old tubes back after all this and they biased to 38 both sockets. Don't know what caused the original fuse blown and smoke but played it for a couple hrs last night and no issue.

    I contacted Viva tubes and asked for replacement or refund.

    Thanks for all the help guys.
     

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