Question about 100 watt Marshall... heat!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by JeffMN, Jun 23, 2006.


  1. JeffMN

    JeffMN Member

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    I'm getting ready to gig with my Marshall SL-X 2100 this Saturday. I've worked out a couple of kinks with it - scratchy pots and a bad power tube - and all seems fine now. I put a new matched set of tubes in it tonight. Unfortunately, it appears that my DMM (a Fluke 16) can't measure any more than 200mA current, so other than measuring the negative grid voltage that I have applied (-42v) I have no way of telling if these things are running too hot... or do I? I'm using the Torres book "Inside Tube Amps." I'm trying to measure plate current via the "Transformer Shunt" method, hooking one lead to the center tap and the other to the plates. My multimeter can't measure the current of 2 tubes it appears, it just displays overload. This is my first time trying to adjust bias for a set of tubes. Is there another way of measuring the current?

    All I can say is, maybe it's just because I've never owned a 100 watt head prior to this... man, do those things give off some HEAT! :eek: It's kinda scary, and makes me second guess the bias setup! They're not glowing or anything like that (other than the heaters) and the original tubes got plenty hot too. I have used the Marshall Artist 3203 for a few years now as a power amp and it's 30 watts, and only 2 EL34s. Yes, it's half the amp... but it's nowhere near as hot as this thing gets. Is this normal for a 100 watt head to run this hot?
     
  2. Rex Nomad

    Rex Nomad Member

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  3. JeffMN

    JeffMN Member

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    I had measured the plate voltage. IIRC, it was 428V. I'm using that Torres book, and it tells you to measure the current that is running through the plates to the OT. One of the methods to use (that seems to make sense to me) is the "Transformer Shunt" method. Hook one end of the DMM to the center tap of the OT primary, the other to one of the plates on any tube. This should give you the current draw for one side (2 tubes) of the OT.

    Does that make sense? This book, while it has some good parts, is an interesting read... :)
     
  4. TubeAmpNut

    TubeAmpNut Member

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    I'd first suggest you not use the shunt method unless you have a lot of experience inside the amp.

    But if you must....

    In a 4 tube amp, you are measureing the plate current through two tubes. So you need to half the current reading on the meter. So, if you read 80mA, then each tube is getting 40mA. This is about what you want at ~430V or so. I wouldn't go much higher than this, but an acceptable range would be ~35mA to 42mA. You plate voltage will vary with idle current.

    Secondly, if you are reading over 200mA, you are doing something wrong. either your measurement is incorrect or you are about to ruin your tubes. Since your bias voltage is at -42, i seriously doubt your idle current is over 200mA for both tubes. Remotely possible, but unlikely. Did you take the amp off standby before you meaured? Are you leads in the appropriate position on the DMM?

    Again, unless your are very experience, the shunt method is dangerous. One slip of your probe can spell disaster for your OT.


    BK
     
  5. JeffMN

    JeffMN Member

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    Well, admittedly, I've never biased an amplifier before. I've built a few stompboxes in the past, and also built the HiOctane project at AX84. I'm no expert, that's for sure. But, the transformer shunt method sounded the 'quickest' for this type of measurement. Using the shunt method came with bold warnings from the book to be careful, and I am being careful! Isn't plate current what I'm most concerned about in the end - the amount of current from the plates to the OT?

    I stand corrected from my earlier post... I went out to the bench and looked at my notes from last night. There is 482VDC on the plates. I've got -42V on the grid.

    I'm using a Fluke 16 multimeter. I think the reason why it wasn't measuring properly (and overloading the DMM) is it's measuring in microamps (uA) - that would make sense to me. There doesn't seem to be a setting on it to measure milliamps, at least that I could find last night.

    Is there a different way to measure plate current with a DMM? Or do I just say 'good enough' knowing what voltage is on the plates and what's on the grid? I know I could buy a BiasRite kit, but I have to believe that I can accomplish what they're doing with a DMM. Am I wrong?

    I appreciate your response. Believe me, I'm not doing this with a beer in one hand, or on 2 hours sleep. I know there's much danger in there, and one hand is always in my pocket.
     
  6. donnyjaguar

    donnyjaguar Member

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    That meter can measure to 10A. :)
     
  7. Dave C

    Dave C Gold Supporting Member

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    The Fluke 16 only measures up to 200 micro amps max, what you're trying to measure is higher than that. I'd suggest getting a bias probe or a different DMM that will read milliamps.
    Dave C
     
  8. JeffMN

    JeffMN Member

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    Well, I checked the manual, and I don't see it... it says 200 milliamps max.
     
  9. JeffMN

    JeffMN Member

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    That's the same conclusion I think I've come to... this DMM can't measure the current I'm trying to. I think I'd rather just get a better DMM as I can use it for other things as well.
     
  10. TubeAmpNut

    TubeAmpNut Member

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    Yup,

    get yourself a bias probe. It's not as fast as the shunt method, but you minimize the risk of shorting your OT through your DMM lead when you slip and make contact with the chassis. You also read mV with the probe, not mA so you won't have to buy a new meter.

    BK
     
  11. Dave C

    Dave C Gold Supporting Member

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    You could also install a 1 ohm resister on each cathode (pin 8) and read the voltage drop across the resister which when divided by the resistance equals amps. So if you read .031v....divide by 1 (ohm)=.031 amps or31 milliamps. You'll need a very accurate meter and a very close tolerance 1 ohm resister.
    There is yet another method courtesy of John P (also on this board) where you would measure the resistance of the OT from center tap to pin 3 of each side , measure the voltage drop of same and use your resistance measurements of each side of the OT in the calculation similar to above. I haven't tried this yet personally but have no doubt it'll work well. Hope this helps.
    Dave C
     
  12. donnyjaguar

    donnyjaguar Member

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    My apologies, the meter won't measure 10A.
     
  13. JeffMN

    JeffMN Member

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    I've read this in many places, it seems like a common practice. Why wouldn't they do this at the factory then? Confusing.

    I think I'll try that. If I get some 1 ohm resistors for this, what wattage rating should they be? I've seen many references to this method, but the only one that specified a wattage rating said to use a 3 watt resistor. Seems simple enough to do, and I'll probably do it tonight or tomorrow, assuming I can get the parts locally here. I don't suppose Radio Shack would have these on hand? There's another shop I can hit for it, it's just not right 'down the street.'

    Any thoughts on the heat this thing gives off - is a 100 watt 4 tube EL34 amp normally roaring hot? I'm just surprised it's as hot as it is after awhile... maybe that's the way it should be, I dunno. It sounds great, and everything looks okay, I just don't have it up a performance levels yet. I'm playing an outdoor show on Saturday, so it'll get a workout there.
     
  14. Dave C

    Dave C Gold Supporting Member

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    I usually use 1/2 watt so anything higher would be fine just as long as they are as close to 1 ohm as possible....2% or better. Yeah, Marshall 100 generates lotsa heat...ya might want to leave the back grill off and rig up a fan too.
    Dave C
     
  15. donnyjaguar

    donnyjaguar Member

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    Well let's see. 10Watts for each filament and about 18Watts power dissapation = 28Watts x 4 = 112Watts. Add in the receiving tubes say 8W and you have 120Watts.

    Given the average efficiency of 5%, have you tried to hold a 126Watt lightbulb lately? :)

    DJ
     
  16. JeffMN

    JeffMN Member

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    Laugh - you make a good point! :rotflmao
     
  17. JeffMN

    JeffMN Member

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    Cool. Thanks for the info! I'll be stopping by the store on the way home.

    :BEER
     
  18. V846

    V846 Member

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  19. Dave C

    Dave C Gold Supporting Member

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    Good link David(V846).....much better than I could explain it. Thanks.
    Dave C
     
  20. V846

    V846 Member

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    Hey Dave, I didn't even try :D

    db
     

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