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Bias probe newbie question?

Discussion in 'Amp Tech Center' started by Ryebread70, Jan 23, 2017.

  1. Ryebread70

    Ryebread70 TDPRI Member

    Age:
    48
    93
    May 8, 2016
    SC
    So I need to bias my H&K Edition 20w combo amp. It has 2 el84's. This is the only amp I have that needs biasing, so i don't want to spend a ton of money on some sophisticated contraption. I have a cheap multimeter and I'm looking at the Eurotube 9 pin bias probe ($50). I know, "in general", how to check with just the multimeter but I'm not comfortable doing it that way. Is the Eurotube 9 pin probe my best option?
     
  2. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    I'm assuming by just a meter you're referring to the transformer shunt method? Yeah, I'm not a fan of doing it that way either, TBH. I've got a good meter and the recommended safety gear, but still. There's lower risk methods that work just as well.

    I've got their 8 pin version, and it's gotten quite a bit of use. It's simple, accurate, and it's not like you'll quit needing it unless you sell that amp.
     
  3. Ryebread70

    Ryebread70 TDPRI Member

    Age:
    48
    93
    May 8, 2016
    SC
    Thanks! So far it looks like the one from Eurotube is my best option. Looks like their were other companies like "Amp-Head" that I see referred to in old forum posts (not necessarily tdpri) but they don't seem to be in business anymore? Plus Eddie at Eurotube is very helpful on the phone!
     
  4. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    I don't remember if Hoffman or Weber sell their probes in 9 pin models, but they're also quite excellent. I just prefer the direct reading of the Eurotubes model. It's nice to have a second meter handy when using it, BTW. Plate voltage will drop a little bit as bias gets hotter, and that has an effect on your calcuations. Not usually enough to have an undesired effect unless you're really pushing the limits on idle, but it does happen.
     
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  5. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
  6. Silverface

    Silverface Poster Extraordinaire Ad Free Member

    Age:
    66
    Mar 2, 2003
    Lawndale CA
    I think he was saying he's not comfortable working around high voltages, so that wouldn't help him.

    As far as the bias probe, to use one properly you have to be able to measure the plate voltage - so if the probe does not have that option on it it's not enough to do it properly.

    Weber makes one with plate voltage measurement.
     
  7. Ryebread70

    Ryebread70 TDPRI Member

    Age:
    48
    93
    May 8, 2016
    SC
    From Eurotube website:

    "Our Noval 9 Pin bias probe is a single head unit that must be used in conjunction with a multimeter. This probe takes a direct DC plate current measurement by breaking the plate connection (pin #7). It does not use a reference resistor and it is not in the cathode connection. This means you do not have to compensate for screen current or any other variables."
    http://www.eurotubes.com/store/pc/catalog/9PinProbe.jpg
    I'm not exactly sure what all that means? But hopefully that's what I need.
     
  8. dan40

    dan40 Tele-Holic Silver Supporter

    955
    Aug 19, 2015
    Richmond Va
    From reading that, it does not appear that it has the capability to also measure plate voltage. If you are trying to avoid taking voltage readings at the socket, you will want to find a probe that also reads plate voltage. Like silverface mentioned, Weber sells a probe that switches between a current reading and the plate voltage reading.
     
  9. Silverface

    Silverface Poster Extraordinaire Ad Free Member

    Age:
    66
    Mar 2, 2003
    Lawndale CA
    It definitely doesn't read plate voltage - it's just a socket with a couple resistors attached to a 2 multimeter probes. Good basic units at least have a box with a meter attached to a socket; upgrade ones include a switch and 2nd meter or digital display to read plate voltage.

    It's costs about $6 to build one like the one shown.

    I don't ever recommend these. As you still have to read plate voltage you have to know safety rules - and you might as well not bother with the thing and spend an extra 2 minutes reading the bias using the transformer shunt method.

    This is important: these bias tools are useless if you don't have a good multimeter; actually, checking voltages or bias is a waste of time if you don't have a decent multimeter. An example is the $7 one they sometimes give away for free at Harbor Freight - it's not even within 20% tolerance. I tried a few just for fun (and more than one to verify what I found) and ended up throwing them away - didn't matter what I was reading, nothing was close to what I got with my Flukes.
     
  10. Ryebread70

    Ryebread70 TDPRI Member

    Age:
    48
    93
    May 8, 2016
    SC
    Jeez! This is getting complicated and just so happens the MM I have is the freebie from HF.

    So happens, I have a real need of a Fluke for work purposes and I am willing to buy one especially if it serves dual purpose. So let's just say I get the Fluke; now what else do I need? Or am I back to pulling connections and testing without an adapter/plug?

    Compu-Bias seems to be popular. But it's $230 plus shipping for a single purpose, dual noval probe.

    I guess this is why people take their amps into GC or somewhere to have the tubes swapped.

    My experience is, almost without fail, if I take the time to learn, educate, experiment, tweak, etc... with any project; I end up with far superior results than leaving it in the hands of a so called "professional?" I've learned a lot after picking up "repair/set up" work done by paid so-called professionals. Only to end up fixing their mess!

    Sorry, I've gone off on a tangent! So should I get the Fluke and if so what kind of bias probe? Or is the Weber Bias-Rite, Compu-Bias or some other single purpose contraption what I "must" have?
     
  11. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    If I were in your shoes, I'd get this Eurotubes bias probe for noval tubes, including EL84's. I have the version for octal tubes, and it is really handy. It gives both plate current and plate voltage simultaneously. Yes, it's expensive, but I think about it as an investment that keeps you from having to pay a tech to bias your amp. And, as you say, the folks at Eurotubes are really helpful and kind. I had an issue with mine at first (one of the digital read-outs short-circuited, I think), and they took care of it immediately and without hassle.

    http://www.eurotubes.com/store/pc/viewPrd.asp?idproduct=145&idcategory=6

    This is a single probe, so you'll have to move it back and forth between tubes if you're ever trying to match them yourself. If you know you have a professionally matched set, you can just worry about one of them. And if biasing is all you're going to do, you won't have to worry about getting a high-end multimeter until you decide you want to go further with amp repair or the like.
     
    Last edited: Jan 24, 2017
    Ryebread70 likes this.
  12. Ryebread70

    Ryebread70 TDPRI Member

    Age:
    48
    93
    May 8, 2016
    SC
    I'm gonna take your advice. That bias tool is the more expensive one, I didn't really look at it before. And the guys at Eurotube are very helpful. Thanks!
     
  13. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Sure! I do think you'll be happy with it. It's just so easy. And if there is ever an issue with it, Eddie and the others at Eurotubes will absolutely take care of it.
     
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  14. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    If the bias probe doesn't allow you to measure plate voltage then you might as well use the "output transformer resistance method" (posted above) to measure and calculate bias since all you have to do is take one other voltage measurement.

    I prefer this method over the "transformer shunt" method because the shunt method is more dangerous. With the multi meter in milliamp mode as soon as one meter probe contacts the high voltage the voltage travels through the meter to the other probe. It's very easy to accidentally make contact with something else in the chassis and cause sparks and molten metal to fly. If you do use the shunt method you must wear safety glasses and be very aware of both meter probes.
     
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  15. Silverface

    Silverface Poster Extraordinaire Ad Free Member

    Age:
    66
    Mar 2, 2003
    Lawndale CA
    Well - first, just throw that POS multimeter away!

    To use the transformer shunt method you don't need to pull anything. You do need to access the chassis, but other than the fact you are dealing with high voltages and need a steady hand (only ONE in the amp at a time, ever!) it's the fastest method there is - and you can check other voltages, lube pots and jacks etc while the chassis is open.

    I agree transformer shunt is more dangerous - but it's also more accurate, as you are reading the actual current instead of taking different measurements and then calculating it; transformer resistance is also more of a "ballpark" setting and generally not as consistently accurate. My point - if you're going to have the chassis open you are exposed to high voltages no matter what. Might as well use the most accurate method.

    I agree the shunt method is more dangerous, but setting bias voltage isn't as consistently accurate, it's a "rougher" setting and may not reflect what you still have to calculate as the actual current.

    I didn't look at the other Eurotubes unit, but Weber makes a bias tool that also checks plate voltage. It's a good unit if you don't want to open the chassis.
     
    Last edited: Jan 25, 2017
    Wally likes this.
  16. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    61
    Jul 18, 2010
    Western Connecticut
    I use the method Rob suggests. It's accurate, and safe. Easy to do one-handed.
     
  17. SaintSilas

    SaintSilas Tele-Meister

    Do you why my test pin readings with multimeter is higher than the eurotubes 8 pin meter? By about 10 mA!
     
  18. wanderin kind

    wanderin kind Tele-Meister

    Age:
    63
    278
    Apr 24, 2018
    Olympia WA
    you can buy 1 ohm resistors for about 10 cents a piece and solder them from cathode to ground, this might not be doable on a PC board amp,

    33 ma x 1 = 33 mv etc

    there is a no meter method that you can get good at, put your hand near the tubes and see how much heat they radiate, sounds like a joke but you can get a feel for this over the years, after the heat check, do a sound check, if it sounds good, then you are in business,

    to calibrate your hands, try the above method and then check ma with a real meter.

    put your amp on standby and feel the heat if you want to know what really cold bias feels like,
     
    Last edited: Aug 10, 2018
  19. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    And the video demo has a pinup girl riding a KT88, how can you not buy it! :p

    It looks like a nice device for those who want an all-in-one solution and don’t want to be inside the amp, and don’t already have multiple meters around.
    For those who already have nice meters, the cheaper $25 socket breakout one would work fine.
     
    Last edited: Aug 10, 2018
  20. Dacious

    Dacious Poster Extraordinaire

    Mar 16, 2003
    Godzone
    +1. I built a bias adaptor using Hoffman's instructions and they work, but you still have to measure plate voltage anyway.

    Measuring resistance of the transformer halves and noting them for future ref, then measuring the current draw across them. Easy and fast, dead accurate and you don't have to fudge for grid consumption. Swapping tubes, adjust to same bias, job'sj good.
     
    Last edited: Aug 10, 2018
    moosie likes this.
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