2550 Jubilee bias settings

Discussion in 'Amps and Cabs' started by guitarjb44, Mar 1, 2012.

  1. guitarjb44

    guitarjb44 Supporting Member

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    My recently acquired Marshall Silver Jubilee 50-watt head (2550 model) sounded great when I got it a few weeks ago. Last week, after a few rehearsals, I decided to check the bias. I don't remember what the exact reading was, but it was very high- somewhere around 50mV or maybe higher.

    I checked the plate voltage at the HT fuse, and it read 470vdc. Based on this reading, and using a Bias Probe, I tried adjusting the bias to 37.23 mV (70% of 25 watts per tube EL34 tube).

    My problem is:
    At the lowest setting, the bias on the amp only takes it down to 41mV. The tubes are not perfectly matched; the other tube reads around 45mV at the lowest bias setting.

    But even more important:
    Rehearsal earlier tonight is the first time I played it at volume after lowering the bias reading- and it sounded weak, lifeless, and had too much treble. I don't know how old the tubes are, but as I mentioned: before I messed with the bias setting the amp sounded great.

    So...am I doing something wrong? Are my calculations off? Or are the tubes toast and just happened to sound good at a too-high bias setting because they are about to go south entirely?

    Next time I am at the rehearsal space, I will just try a different pair of tubes (new ones which are perfectly matched). But I thought I would throw this out there, ask if anyone thinks I am overlooking something.

    Thank you in advance for any opinions or advice.

    -Joe
     
  2. guitarjb44

    guitarjb44 Supporting Member

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  3. zenas

    zenas Member

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    Were the tubes redplating at the higher setting? If not I'd just run it that way.
     
  4. guitarjb44

    guitarjb44 Supporting Member

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    Yeah: I guess if there is no red-plating, it shouldn't be a problem with adjusting the bias by what sounds best.

    Thanks for the advice.

    Joe
     
  5. Buckeyedog

    Buckeyedog Supporting Member

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    Tubes prob have been running real hard at those settings for a while. They most likely are worn out. So you could just prob keep running them hot, but be prepared for a new set soon I would imagine. So I'd probably get a new set, bias them correctly, then use the old set as a backup.
     
  6. GearHeadFred

    GearHeadFred Member

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    Something is not right. 50 ma @ 470 = 23.5 watts, or 94% @ idle.

    Are you measuring the 470 with all tubes in, while the amp is running?
    I've never been inside a Jub.. Is the HT fuse really the plate voltage (no dropping inbetween)?

    Lastly, how are you measuring the idle current? You use the term "Mv", so I assume you have 1 ohm resistors on the cathodes? Are they accurate?
     
  7. guitarjb44

    guitarjb44 Supporting Member

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    I am using a Groove Tubes Bias Probe to measure the idle current. I suppose it is possible the bias probe is faulty and no longer taking accurate readings. I will look into that as well.

    I will also re-check the plate voltage at pin three.

    -Joe
     
  8. zenas

    zenas Member

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    Shot in the dark here. Is the 50ma for two tubes?
     
  9. wyatt

    wyatt Member

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    Ideally, you don't want to exceed 80% of dissipation in a fixed-bias set-up. Red-plating starts around 160% of dissipation. That leaves a big "danger zone".

    Measuring across the HT fuse gives you the B+ voltage. To measure the plate voltage, measure across pins 3 (anode) and 8 (cathode). IIRC, a 2550 plate voltage should be more like 430VDC.
     
  10. guitarjb44

    guitarjb44 Supporting Member

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    Well then it looks like that's where I blew it- reading at the HT fuse instead of pin 3. I'll check on that and try again. I thought I remember reading somewhere years ago that the HT fuse was reading from the HT fuse was sufficient. Oops...

    -Joe
     
  11. sickboy79

    sickboy79 Member

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    I'd grab a new set of EL34s. My 2553 head (small box 50w - same as yours) usually runs about 450 on the plates. Easier to get a new set of tubes than change the bias resistor, IMO.
     
  12. guitarjb44

    guitarjb44 Supporting Member

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    Earlier tonight at rehearsal, I tried a new matched pair of EL-34s. Nothing changed: plate voltage measured from pin 3 is 470 to 473 volts, so bias for 70% should be set to 36 or 37mV. But the bias pot won't adjust that low.

    As someone mentioned earlier- my Groove Tubes bias probe tool must be faulty. When I check he resistance between the plus of the bias probe, the reading is .5 ohms,. Shouldn't that be exactly 1ohm, or am I checking that wrong?

    -Joe
     

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