Test measure of a bias probe?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Knekor, Mar 15, 2006.

  1. Knekor

    Knekor Member

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    Hi
    Well, I think that my bias probe isn't messuring my bias right.

    I've measured two diffrent amps with it and no matter how much I turn the bias it doesn't show that the amp goes over more than 30mA.

    Is there a way to check if the bias probe is ok?
     
  2. John Phillips

    John Phillips Member

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    What amp is it? 30mA can either be too little, or too much, depending on the tubes and the plate voltage.

    (eg, if it's something like a Fender Deluxe with 6V6s and 420V plate voltage, 30mA would be too hot.)

    If the bias probe is producing any reading at all it's likely to be OK.
     
  3. Knekor

    Knekor Member

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    Well, i've tried two amps. But the one I'm having in front of me now is a Marshall 25/50 silver jubilee

    Plate i've messured to 420V and the tubes are Svetlana El34
     
  4. John Phillips

    John Phillips Member

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    I've come across a lot of Svetlanas that run cold... it's probably the tubes. Have you any others to try?

    I wouldn't worry too much about it as long as the amp sounds good. Marshalls seem to sound better (to me) quite a lot below the 70% maximum anyway - usually more like 50-55% - and are less hard on the tubes like that. If you can get it to anything over 30mA it's probably fine.
     
  5. Knekor

    Knekor Member

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    Oh okay
    I have a Mesa Boogie Single Rectifer as well. And I get the reading 21mA on 6l6's, isn't that way too cold too? The plate on that one is 400V.


    I can walk you trough how Im going about the whole thing, maybee im doing it wrong.

    1. I put the probe in one of the powertube sockets
    2. I put a powertube in the probe(mV version, active pin: 8)
    3. I put the other powertube in the other socket on the amp (i only have 2 powertubes)
    4. I put the red wire on "V(ohm)mA" and the black wire on "COM" on the DMM and set it to read DCV200m
    5. I put the amp on standby to warm the tubes
    6. I take the standby switch off
    7 I read at most 30mV
    8 Me=:NUTS
     
  6. pfrischmann

    pfrischmann Member

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    420 volts for a Marshall. Is that what the modern ones show? I would have figured 450-500.
     
  7. John Phillips

    John Phillips Member

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    That sounds correct to me. I should say though that I've never used one because I'm familiar with working on dangerous voltages and just meter across the OT primary inside the amp, which would be a way of confirming the bias probe readings - but DON'T try this unless you understand the risks and what to do.

    Mesas are set very cool from the factory, that sounds not unusual to me. Many people don't like this... which is why they're often criticised for having no bias adjusters.

    One thing you can do is to try running it on the EL34 setting - yes, I know it says you have to set the bias to match the tube type! EL34s require less bias supply voltage, so switching to 'EL34' with 6L6s in will run the amp much hotter. You MUST check the bias immediately if you're going to do this. At 400V (which seems surprisingly low, but I can't remember for sure) you can run standard 6L6s at up to about 38mA or 6L6GCs at up to about 50mA, although I'd be inclined to keep it lower if possible (ie 35/45).


    But given that the plate voltage reading looks low too (and I think Jubilees are also higher than 420V... I could be wrong but I think the last one I worked on was about 470V - it was only a couple of weeks ago but I'm getting old :)), could there be something wrong with your voltmeter? Or is your supply voltage below normal? If your meter has an AC voltage range over 120V you should be able to check this, but BE CAREFUL.
     
  8. Knekor

    Knekor Member

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    Well...my DMM is a very VERY cheap one, I'll try another one as soon as I can.

    How do I messure that AC voltage?

    Also, I live in a really REALLY old house, is it possible that the amp isn't getting enough power from the outlet?
     
  9. John Phillips

    John Phillips Member

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    Yes, that's why I suggested to measure it. You should be able to just put the probes into the wall outlet (not sure exactly what your outlets are like in the US, in the UK you have to put something in the ground pin hole first to push the shutters on the other two back) - but BE VERY CAREFUL. Make sure the meter is set to AC volts, at least 120V range obviously, and that you don't slip and touch anything you shouldn't while it's connected. If the outlet is switched, switch off before inserting the probes, put them in, switch on to take the reading, then back off again.

    The only problem is that if the meter is faulty it will probably give you a too-low reading there as well, so it still won't tell you for sure where the problem is. If it reads wildly low (like under 100V) the chances are that it's the meter though, line losses in the supply are usually not that great.

    If any of this sounds too risky for you, get a tech to check the amp and/or an electrician to check the house. Electricity can kill you.
     
  10. Knekor

    Knekor Member

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    Okay!
    I think I'll let my uncle do that for me, he's a proffesional...not worth getting killed over.

    I'll try another DMM too just in case...I'll get back too you,

    Thanks so much for all your help so far.
     
  11. Dave C

    Dave C Gold Supporting Member

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    You can easily check your probe by plugging it into your meter and without a tube plugged into the probe and the probe not plugged into the amp turn your meter to resistance (ohms scale) and you should read 1 ohm exactly or very very close. If you read anything else, either the meter or the resister is at fault , and I'd try another meter before condemning the probe.
    Dave C
     
  12. Knekor

    Knekor Member

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    Allright, so this is what i've done

    I bought myself a really good meter, and it basicly shows the same amount as my previous one does.

    I took all my stuff including my amplifier to my friends apartment and got the exact same readings over there. So I guess the voltage in my house is fine.

    The only thing that differs with my new meter it says that the plate is 430V instead of the 420v that I had at first. (on the marshall)

    What would happen if the 1ohm resistor wasn't in there, or if it's broken? I'll try to ohm measure it with the meter tomorrow
     
  13. John Phillips

    John Phillips Member

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    How far away is your friend's apartment? Is it on the same local transformer? It would be a good idea to measure the supply voltage, still. If it's substantially low it could be contributing to the problem.

    The amp wouldn't work properly with the probe in, and the readings would be way out (depending on the resistance of the meter), not just the slightly out you appear to have... which are just about within the right range if the supply voltage is too low.
     
  14. Knekor

    Knekor Member

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    Well, his apartment is 10-15 minutes away and not likely to be on the same transformer since I live in a house and he's in an apartment.


    Probably a stupid question but would the preamp tubes affect anything?
     
  15. Dave C

    Dave C Gold Supporting Member

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    Assuming the line voltage at your house is ok , your new meter is working properly , your bias probe is functioning correctly, your Svetlanas are running cold , maybe the range on your bias control needs to be adjusted to go lower. This involves changing a resister in the bias supply circuit , I don't know how involved you want to get with this.......it would be real simple for your tech to do. The other alternative would be to try another pair of power tubes that run hotter.
    Dave C
     
  16. Knekor

    Knekor Member

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    Allright so now I tried some new tubes. I think these measured 37mA at most. But they were glowing pretty hard, I'm not really sure how much they should be glowing. Is there a picture of how they should look when healthy? Otherwise, i'll try to take a picture of mine and you tell me.

    I think I'll take my amp to a tech and let them measure it up, then we can compare results.
     
  17. John Phillips

    John Phillips Member

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    37mA is plenty at 430V - that's around 60%, probably even including the screen current. Assuming your readings are correct you don't need to mod the amp or change anything else, if it sounds good like that (I might even reduce it a bit). I still suspect you may have slightly low supply voltage, since even 430V is on the low side for that amp I think... but no more than 10% and possibly only 5%.

    The brightness of the tubes isn't an indicator - they just vary. If you're seeing orange glow from inside the plate structure, that's filament glow and possibly screen glow. If the screen glow gets drastically brighter when you play the amp at high power, that's not a great sign but it isn't uncommon either and doesn't always cause trouble.

    What to worry about is if the plates themselves start to glow red on the outside (easiest to see in a dark room unless it's very serious - it's quite a different color, easy to tell apart); it shows the tube is running too hot - some people recommend using it as a visual bias indicator, by setting just below the point it starts to occur, but this is way too hot and will definitely give shorter tube life and a high chance of sudden failure.
     
  18. jbltwin1

    jbltwin1 Member

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    If nothing else, interupt the cathode flow to ground(or cathode resistor if it's cathode biased) and put your own 1 ohm resistor in there and get a VOLTAGE reading from it. It will show your adapter good or bad. That's how I usually do them anyway. Mike.
     
  19. Knekor

    Knekor Member

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    I'm still not sure that the measurement I'm getting from my amp is correct. I went and measured a Jcm800 at my friends studio, and that measured 420V plate too. From what I hear that's to little.

    I'm gonna let my tech take a look at my amp and then we can compare results. Maybee I can lend him my probes and he can check them out aswell.

    I'll let you know how it turns out

    Thanks
     
  20. Knekor

    Knekor Member

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    Okay, so I checked the outlet in my house and it measures 230v...or 229v i think it was.

    I also measured the probes I have and they seem fine, 1ohm on them.

    Sooooo can it be that the power inside of the amplifier itself isn't feeding enough power to the tubes?
     

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