Amp Bias Tool Recommendations

Discussion in 'Amp Tech Center' started by OldPup, Oct 2, 2019.

  1. trobbins

    trobbins TDPRI Member

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    VintageSG - thanx for the link - I saw your part on ebay but had thought you were referring to a breakout of multiple pins.

    As a general comment, safety is quite a concern if using that style of breakout in a plate current measurement setup, as I doubt the wire insulation and 4mm plugs are rated for plate voltage levels, and the meter itself needs to be of a certain capability.

    With respect to the Eurotubes Pro One, do you know if the LCD meters have an extra clear cover over the LCD's own front cover? As far as I can tell, the internal circuitry and meters would be sitting at plate voltage, as they appear to be directly measuring plate current. So I am a little skeptical about the safety rating if there is just the LCD module thin glass cover sheet providing isolation.
     
  2. rolandson

    rolandson Tele-Meister

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    Then you would be very wise to stay out of the inside of your amp. A bias probe like those mentioned already will do everything you need, including help keep you this side of the dirt.

    Then, take some classes, read some books, maybe buy a kit for a low watt amp...all to teach yourself the how and why of them. It isn't hard, but it does require discipline and attention to detail, the kind of thing that comes with experience gained in baby steps.
     
  3. Milspec

    Milspec Friend of Leo's

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    I understand it is an idle reading, but not having to disconnect and reconnect every time while you dial it in is a benefit that I prefer.
     
  4. 1300 E Valencia

    1300 E Valencia Friend of Leo's

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  5. moosie

    moosie Doctor of Teleocity Ad Free Member

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    Yes, me too. I just do it with a meter. :)
     
  6. Milspec

    Milspec Friend of Leo's

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  7. Wally

    Wally Telefied Ad Free Member

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    Gus, can you explain how the 1 ohm resistor is placed in the plate circuit? That is a new one for me and is something I have never seen or heard of before?
     
  8. gusfinley

    gusfinley Tele-Holic

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    The 1 ohm resistor is placed between the Plate connection on the tube socket and the plate connection on the output transformer. With V=I x R, we can rearrange for I = V / R to get plate current and where R = 1, I = V. In other words, mV across the 1 ohm resistor translates to mV of plate current.

    Also, measuring from one of the Tip Jacks to ground gives you plate voltage in a fixed bias amplifier.

    When I do this, I like to place tip jacks ( these fit multimeter leads ) on the back panel and place the 1 ohm resistor between the terminal on the tip jacks.

    Additionally, placing the bias potentiometer on the chassis ( either screwdriver type or with a shaft-locking nut ) makes it possible to bias without the chassis upside down.

    Below you can see the added Tip Jacks and the added bias adjustment pot with locking nut ( just in front of the output transformer )

    [​IMG]
     
    Last edited: Oct 4, 2019
  9. gusfinley

    gusfinley Tele-Holic

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    This method allows you to no only see the effect of each tube on power output, but also to see the effect of each output transformer winding on power output.

    Using this insight, you can place your tube with the lowest plate current draw in-line with the highest resistance winding and the highest draw tube in-line with the lowest resistance winding to increase balance in the output stage - IF that is what you are going for (many class AB amps have inbalanced designed into them to increase harmonics).
     
  10. trobbins

    trobbins TDPRI Member

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    For anode measurement, the tip jack would have to be at least 1kVdc rated I'd assume, not some small signal voltage test point, and have some kind of hazard label if others were to use it.
     
  11. gusfinley

    gusfinley Tele-Holic

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    Yes, actually this week I have been trying to find a "safer" way to do this. One way would be to install a cover over the test tip jacks. I can't think of any other better connectors to use.

    By the way, the lowest rated tip jacks I have seen are 1500V! Keystone's are 5700V!
     
  12. OldPup

    OldPup TDPRI Member

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    Lots of great input here. I have only begun to dig into some of it. Someone recommended a Weber bias tool. I almost bought the Reverb listing, but then read online that Weber actually halted production on their tools because they were gaining a notorious reputation for being unreliable. Eurotube bias tools look to be all in one and very simple to use. They're sold out until late September (wait a minute...) but I do not need it right away, so I may hang tight until they are available again.

    The other option is to buy a multimeter. It has other uses around the house. And it seems easy/safe enough to use with the tube tester socket (I have gathered that just using pins could lead to some unpleasantness if one doesn't keep careful track of where the ends of both pins are at all times. I am capable in time of achieving this, but I can also just get a safer alternative). What multimeter(s) should I look into, which should I avoid? Assuming I go the multimeter route...
     
  13. trobbins

    trobbins TDPRI Member

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    Perhaps if you don't buy a multimeter that is rated for mains AC testing at the moment!

    There are a few quite cheap, but accurate and useful small multimeters available nowadays. They are not rugged, but that can give some incentive to treat them carefully. I have two Aneng AN8009 for bench use - there are others discussed in some detail at eevblog.com

    Note that biasing, and almost everything else related to valves, can allow a broad tolerance to operating conditions of a valve. As such, some informed leniency can usually be allowed - and I'd suggest that measuring cathode, instead of anode, current is one good example especially if that removes or mitigates safety related risk.
     
  14. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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    The cheap meters with the clicky dials and transistor testers on the front are adequate for the task. I believe they are available from your Harbour Freight shops for $5 or free with certain purchases.Rugged thay ain't, cheap and good enough they are.

    Don't buy the super cheap ones with fixed leads.

    The Aneng models are quite good. I use a pair of the cheap meters for the current and an Aneng ( rebadged as a RichMeter ) for the voltage.

    The cheap type :

    cheapmeter.png

    An Aneng, the most basic model. Costs around £/$10. Seems good value for money :

    aneng8001.jpg

    I would heartily recommend buying/making some pairs of croc clip leads for them too. The connectors are pennies. Keeps your hands free. Use clips with a flexible silicone sheath so the nose isn't bare.
     
  15. gusfinley

    gusfinley Tele-Holic

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    What voltages are you expecting to measure with your meter? This is VERY important.

    I burned out a 600V rated meter testing across the secondaries of an unknown transformer.

    I would recommend one with 1000V rating.
     
  16. Milspec

    Milspec Friend of Leo's

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    When I purchased my first motorcycle helmet, the advice was to buy the helmet based on how much you valued your head....similar thing goes for meters in my mind. You will be exposing yourself to high voltage, so buy a meter that matches how much you value your health. There are several good medium grade meters to do that, but I suggest a Fluke meter. Not to be homer for Fluke, but they are built like tanks and contain a lot of internal safety features (large fuses, blast shielding, etc.) to protect you from becoming a light bulb. The 177, 179, 87, and 187 models are all very good choices for a DIY person looking to work on amplifiers.
     
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  17. trobbins

    trobbins TDPRI Member

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    A meter only 'needs' blast shielding etc if working on mains AC or high energy secondary supplies. Many base level Fluke's are focused on AC mains circuits, and do not have sufficient low mV or mA capability for valve amps where most preamp stages operate under 1mA. Care is needed if circuitry has DC and AC levels - a 115 will range to 6Vac if DC level is over 600mVdc. And unless you buy multiple Fluke's then risk is added when shifting probes around.
     
  18. Guitarteach

    Guitarteach Poster Extraordinaire

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    I use one of these with a multimeter.

    Non-invasive and OK for the most basic adjustments/experimenting. About £15/$20

    AFB58799-57C9-4CCC-8ACB-48BD287A91FA.jpeg
     
  19. avspecialist

    avspecialist TDPRI Member

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    I have a tube socket meter, made by a tech. I was told it was only good for 6L6 tubes. If I wanted to test EL 34 tubes I need different probes. I’m not sure if this is actually true. Anybody know if it is?
     
  20. Milspec

    Milspec Friend of Leo's

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    I have seen people get hurt working with amp voltage using cheap meters designed to check disposable batteries and 12v voltages. Open up a cheap MM and see if you feel safe working around 600v in an amp. Yes, many Flukes are designed around AC mains usage, but the models I listed are capable of covering the mA ranges necessary for electronics work as well.

    When you can get a used Fluke in good working condition for around $125, I don't see any reason to pass on them when your safety is at stake.
     
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