Measuring bias with only a multimeter

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by slhguitar, Mar 13, 2006.

  1. slhguitar

    slhguitar Member

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    What pins do you connect the terminals to for 6L6 tubes? I am not going to be doing any adjustment, I just want to know how hot my HRDx is running, and what the appropriate bias is when I buy tubes. Thanks in advance for the help.
     
  2. VaughnC

    VaughnC Supporting Member

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    In order to use a multimeter to measure bias current, you need to purchase output tube adapters that insert 1 ohm resistors between the output tubes cathode and ground. You'd probably be better off just buying a bias meter as they are already equipped with the necessary tube adapters.
     
  3. slhguitar

    slhguitar Member

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    I will likely end up buying the actual Bias probe kit, but in the meantime, I would still be interested in how to do without it. Could I simply put a resitor in series with the pin? Thanks for the advice.
     
  4. brad347

    brad347 Member

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    you can put a 1 ohm resistor soldered between the pin and chassis and test there. Goes without saying but be sure to discharge the caps first and to use a really hot iron to solder to the chassis to avoid a cold joint.
     
  5. Matt H

    Matt H Guest

    you can also measure the dc resistance of each side of the primary (wrt the ct) and then measure the voltage drop across each half to figure out the plate current.

    this involves working with high voltages- high and LETHAL voltages. don't do this unless you understand exactly what you're doing.
     
  6. John Phillips

    John Phillips Member

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    +1. This is actually the best and most accurate method, since it avoids including the screen current and does not alter the circuit in any way. But it is DANGEROUS since you're connecting to two points which are both at over 400V relative to ground.

    You must measure the resistance of the primaries (best to check both sides since they are not always exactly the same) with the amp off, then it's best to clip the test probes to the points you need (OT center-tap feed and tube pin 3, plate connection) and only then power up. The voltage you are expecting to read is in the few volts at most, even though the voltage to ground is much higher.

    Then divide the voltage by the resistance to get the current.

    But DON'T try this unless you're absolutely sure you know what you're doing.


    There is an even quicker and more dangerous method which involves using the meter as a current meter across the OT primary ('transformer shunt method'), but I really don't recommend this as not only is it equally dangerous to you, it's more dangerous to the amp (and the meter) if you get anything wrong, since the meter is then nearly a dead short across the amp's HT supply... expect fireworks - I did once destroy a quite nice little multimeter doing this, simply by slipping with one of the probes. It's also not quite as accurate since bypassing the OT does change the circuit very slightly and increase the measured current.
     
  7. tonezoneonline

    tonezoneonline Member

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    If HRDx stands for Fender Hot Rod Deluxe you only need a DMM.They have bias points on the small pcb that holds the tube sockets.Check the schematic.
     
  8. slhguitar

    slhguitar Member

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    It does indeed stand for hot rod deluxe, and the schematic will be consulted. Thanks for all the help, and I think I am going to avoid all of this dangerous measurement, because as much as I like to think I know what I'm doing, I really don't. My question has indeed been asnwered, and I think the bias probe attachment is what I will buy. Thanks again for clearing this up.
     
  9. tybone

    tybone Member

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    You want dangerous? Try the transformer shunt method. When the voltage is high enough the arch is this strange colour of green.
     
  10. donnyjaguar

    donnyjaguar Member

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    I see some guys like to measure the plate (anode) current and screen grid current separately. What's the benefit of this? I usually just measure the collective current through the cathode. My line of thinking is that if the screen grid resistor(s) are okay (easily confirmed when amp is off) then you should be fine. The added benefit of putting a shunt in (I use 10R myself 'cuz I have a cache of 1%, 10R resistors) the cathode is that the danger of getting a nasty jolt is basically nil.

    DJ
     
  11. John Phillips

    John Phillips Member

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    I think the benefit is that you don't have to modify the amp. Almost no vintage amps and by no means all modern ones have resistors in the cathode connection. Why fit them? It's just as easy, and only slightly more dangerous, to measure it correctly across the OT - poking about near the tube sockets IS still quite dangerous even if you're aiming for pin 8. If you're not sure you're safe enough to take a plate connection reading you really shouldn't be in there at all, IMO.

    The problem with including screen current in the cathode current reading is that you don't know what it is - and occasionally bad tubes can draw excess screen current and throw things off. If you measure them separately you can spot it immediately - whereas if you don't, you may be tempted just to think the tubes aren't very well matched. But a 10% difference in cathode current can be caused by a 100% difference in screen current.


    Tybone - I know that particular color of green you mean, and that's why I don't use the transformer-shunt method any more ;). And I'm pretty careful too... just that if you do it often enough, one day you may get unlucky.
     
  12. Matt H

    Matt H Guest

    you dont' necessarily need to measure the voltage "across" the OT primary-
    (assuming you've already measured the DC resistance of each half of the OT)

    if you use an alligator clip lead, connect that to ground.

    measure the voltage at your OT's center tap.
    measure the voltage at plate1, measure voltage at plate 2.

    make sure you wrote all those numbers down.

    it's taking three high voltage readings, but you only have to have one hand in the amp while doing it (i.e. the safe way).
     
  13. John Phillips

    John Phillips Member

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    You won't get very useful readings that way. You'll get three readings of 400-and-something +/- 1 or 2 volts, and the precision of the difference (which is what's important for the current measurement) will be very low - you'll need to be on the highest voltage scale and you'll be lucky to read it to better than 1 volt on most voltmeters (3 significant figures), which is about a 50% error on your current calculation if the true figure is between 1 and 2V (which it most likely will be with a lot of common amps).

    The best method IMO is to clip one meter lead to the OT center tap connection and either clip - or if you're careful and have steady hands, probe - to the plate connections. Then you can set your meter on a much lower scale and you should be able to read the voltage to about +/- 0.01V at the same 3-significant-figure accuracy - which is actually way more than you need, 5% is good enough to make sure you're in the safe bias range and 1% is definitely plenty.
     
  14. donnyjaguar

    donnyjaguar Member

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    Thanks for your reply JP, quite informative and I can see the logic is sound (no pun intended). Years ago I tried measuring the current through the primary windings but I found it was very difficult. As the amplifier warmed up it would change and basically left me frustrated. I got the idea of the cathode resistor from some hi-fi designs. Then of course with ultra-linear designs you have the added confusion of the Sgrid voltages being higher than the plates.
     
  15. Wakarusa

    Wakarusa Member

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    'Course if you're handy with a soldering iron and really want an accurate answer, it's only a moment's work to desolder the OT lead from the socket and connect your DMM (set to read current) in line. It also helps to have several meters :)

    However, I freely admit to using a tube socket with a 0.1 ohm 1% resistor in the cathode circuit. For most work the cathode current yields results that sound good and are well within spec for the tube, and the method is fast and relatively safe.

    John's right about a funny screen current throwing things off, but I also cheat and have a test rig that all new tubes go through to check for this and other sins.
     
  16. avwalker

    avwalker Member

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    I've never heard that the tranformer shunt method was that dangerous.....i use it all the time......i know that using an ammeter is the equivalent of holding a live wire.......but I have a fluke mulitimeter with 1000V test leads......i really haven't been to shy about doing it this way.....i still always keep one hand in my back pocket, though.
     
  17. doctorx

    doctorx Member

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    I built my own dual socket bias checker with parts from Doug Hoffman for under 20 bucks. You can build it with the 1 ohm resistor or have it break the cathode and measure millaamps directly. I used a plastic case and a couple of switches to go back and forth between the two tubes.
     
  18. Matt H

    Matt H Guest

    john- you're totally right about the sig figs- brainfart on my part. :)
     
  19. John Phillips

    John Phillips Member

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    Bad idea (the bit in bold). Feeding the tube current out of the amp and through the meter can make the amp unstable. It's always better to use the resistor and only 'sample' the current by means of a voltage measurement. The only difference in cost is the resistor, which is a few cents even for a decent high-precision one.
     
  20. doctorx

    doctorx Member

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    Which amps? I've used this method on Fenders, Traynors, and Marshalls with no problems. Not that the 1 ohm method is bad, I use that too.
     

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