Amp Bias Tool Recommendations

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

  1. OldPup

    OldPup TDPRI Member

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    Can anyone make specific model recommendations for the tool(s) necessary to rebias tube amps? I saw my amp tech do it a couple of times on some HRDs and it seems pretty simple. If I can avoid it I don't want to get a tool that will do more than I need it to. I also don't want to get a tool that does half the job. Any help is appreciated!
     
  2. Wally

    Wally Telefied Ad Free Member

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    I have no experience with such tools since I use a meter, but any unit you buy would best measure not just the current draw but also the plat voltage since both of those numbers are necessary to arrive at the plate dissipation number, which is the final number that informs you as to how the tubes are working in any one certain circuit. The Weber Bias Rite is such a unit. There are units sold on eBay that are very inexpensive that perform both functions, too. Fwiw, one can drill an access hole for the plate voltage measurement using a multimeter. I advised a member here quite a number years ago on how to go about this, and he made the simple mod to his unit to make it a better tool that gave complete info.
    Perhaps this bump will get you some direct user response.
     
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  3. schmee

    schmee Poster Extraordinaire

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    I use the Weber unit. Most any should work well though, but I would want:
    -measures both tubes/ 2 tubes at least at once. Going back and forth is a pita.
    -has a switch for plate voltage as well as bias milliamps.

    Nothing magic about the Weber unit, there are others like that too..
     
    Last edited: Oct 2, 2019
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  4. Milspec

    Milspec Friend of Leo's

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    Eurotube Pro One out of the UK.

    I just purchased a unit a couple of months ago and it doesn't get any easier than that tool.
     
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  5. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    I have the VHT, which works fine for basic things (it can do 2 power tubes, it shows mA, is accurate and can be calibrated).

    If (for example), I see 23-ish mA on a Princeton Reverb with the VHT unit and the amp sounds good to me and the player, good, we’re done. Quick job, buy me a beer later. It took 5 minutes.

    But if there’s any more to it, I hardly ever use the VHT checker, because on most of the amps I work on I need to to yank the chassis to take plateV readings, do calcs, make internal bias adjustments, inspect for problems, and probably monkey with something else. So I use a meter for all that, when the chassis is out.
     
  6. rolandson

    rolandson Tele-Meister

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    What's your skill level?

    If you know what you are doing, a multimeter is about all you need. If you want a little extra, download a signal generator and pick up an oscilloscope online somewhere and watch it live...!

    But, if you don't want that level, look into these...
    https://www.eurotubes.com/store/pc/bias probes.htm
     
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  7. h4ck.b0x7

    h4ck.b0x7 Tele-Holic

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    This. Sure, it’s beyond simple to do it with a multimeter. On the other hand, it’s REALLY nice to just plug this in and do some remedial math.
     
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  8. gusfinley

    gusfinley Tele-Holic

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  9. moosie

    moosie Doctor of Teleocity Ad Free Member

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  10. Milspec

    Milspec Friend of Leo's

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    The best feature of the Eurotube gizmo is that you can leave it connected while playing the amp, then adjust from there. Try and do that with a multimeter. Biasing an amp is like setting the timing on an old car with a distributor, setting it to factory specs is just the starting point, you need to adjust to performance from there. With timing, you go to factory setting and then advance until you notice spark knock...then retard a little until it is gone, bingo. I feel the same can be done with the Eurotube device, you can leave it connected while you play and make adjustments. Any other method requires you to probe, set the bias, disconnect probes, play, probe, set bias again, play, etc. until you are happy with the results.

    Several ways to reach the same destination, but I like the safety and convenience of the Eurotube Pro meters.
     
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  11. gusfinley

    gusfinley Tele-Holic

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    Actually, my preferred method is 1 ohm resistors in the plate circuit. This translates mA directly to mV so that it can easily be read with a multimeter. Add some test points to the back of the amp and you can measure and adjust on the fly.
     
  12. moosie

    moosie Doctor of Teleocity Ad Free Member

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    I'm not sure you understand the process with a meter, using the method I referred to above. It's not a shunt, so there's no current running through the meter. It's trivial to clip the leads, and leave them hooked up while testing. Why would I not...

    Thing is, bias is the measure of idle dissipation, not under load. So, one needs to stop playing before taking a reading anyway.
     
  13. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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    I bought two octal and two noval 'break-out' adaptors from Ebay, similar to the bias probes linked to above.

    Using one per valve + a multimeter to keep an eye on the voltage makes for easy setup and monitoring. If you have a bunch of meters lying around, or don't mind buying a few £/$5 cheapies ( they're good enough ), it doesn't get much simpler or quicker. I do heartily recommend mantling some croc clip leads for the voltage so you can work one-handed in there.
     
  14. trobbins

    trobbins TDPRI Member

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    Do you have a link to the break out adaptors?
     
  15. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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  16. rolandson

    rolandson Tele-Meister

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    Sorry to be a broken record...

    This is all about the OP's skill set. If he or she or they possess the knowledge to open up the chassis, then yeah, a multimeter.

    But @OldPup , the current inside that thing can kill you. If your skill set doesn't include working with lethal voltage...Don't.

    Get one of those that plug into the tube socket without opening the amp up. One that provides guidance into how to use it and what the numbers mean.

    I happen to like the "Pro One" offered by Eurotubes for simplicity's sake. It does all the heavy lifting.
     
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  17. Wally

    Wally Telefied Ad Free Member

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    Okay, I’ll suggest the easiest way to bias an amp....since biasing without knowing the plate voltage is about the same thing. All you need is the amp, a guitar and a tool to turn the bias pot. Play the guitar and listen. Then, turn the bias pot a small bit and play it to find out which direction one should turn the pot to increase or decrease the current draw. Increasing the draw will bring on a bigger, richer Sonic. Once you know that, gradually turn the bias adjustment in that direction until the plates just begin to show some redplating. Back away from that point and play while watching the plates. If the plates do not go into redplate condition while playing,
    The amp is safe to play. If redplate begins while you are playing, decrease the draw very slightly until,playing does not bring on redplate. Note: in fix3d bias, current draw increases while signal is being process as compare to current draw at idle. The amp is safe to play at any setting from there on ‘down’...decreased current draw will cause the amp’s Sonics to change....decreased harmonic content. Lower current draw is easier on the tubes. At some point and if the bias pot allows it, you will cause the amp to incur crossover distortion...things get harsh and imho unmusical. , long before that the amp is not sounding as I want it to sound. Within the boundaries of crossover distortion and imho redplating, one can find what they feel is a good sound for them. The advantage of this system is that it brings the player’s ears into the equation...and that is the ultimate point, correct....sound. My ears will put the amp somewhere in the 55-65% of max plate dissipation range, ime.
    The problem with this ‘biasing with eyes and ears’ is the same problem with biasing without knowing the plate voltage....one does not really know where the tubes are operating. But...the amp will work and the sound that is created may please the player’s ears. A more experienced player might even be able to set the bias by ear and have it somewhere very close to some known range of plate dissipation. I have used this method over the years enough to know what my results will be with my ears. However, I did this only as an exercise and have always checked the numbers with a meter.
    I use the transformer shunt method. It works, and as robrob notes in his comparison of the shunt method to the method that measures OT resistance, the results are within 1 percentage point of each other. I do not suggest that others use the shunt method, but then if one does not have a bias probe setup that reads plate voltage one is going to be measuring live circuit voltage anyway...and I do not suggest that people do that either unless they are experienced and comfortable working with live circuits. Hence my thought that a bias probe with plate voltage measurement is a complete tool. One that does not measure plate voltage is an incomplete tool, and I can do as well with only my ears and eyes.
    Do not try this eyes and ears method unless you understand it well...and one has to know what redplating is. Fwiw, redplating will not occur until plate dissipation is well above the 70% ‘rule’ that many use as a target or limit. Some use a higher plate dissipation point and understand that tubes wear faster that way....but they like the sounds they get with that hotter bias.
     
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  18. Wally

    Wally Telefied Ad Free Member

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    The 1 ohm resistors go between the cathode pin 8 and ground.

    This morning I watched a YouTube video of a fellow who built a socket arrangement to measure the current draw via the transformer shunt method. Are all of these bias tools using the shunt method? I thought they were using the 1 ohm resistor between the cathode and ground method??? I had never thought about it, but it would be difficult to get to ground from a resistor inside that probe’s socket arrangement while separating the cathode from ground, wouldn’t it? Lol.....Duh on me, I guess, for never considering something that I don’t need anyway?????
    He left the connections for the plate ‘available’ to a probe to get the plate voltage measurement, too. That would be a bare wire with live voltage on it. He did not include a disclaimer or warning, iirc. (8^0
     
  19. OldPup

    OldPup TDPRI Member

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    Awesome responses, all! You all have dangled quite a few threads for me to pull at to learn more. Someone asked about my experience level. It's pretty much zero as far as amp tech work goes. I can swap out preamp tubes (hold your applause, please). I plan on learning what I need to as I go along. I'd love to understand more about how amps work, how to service my own to swap tubes, speakers, etc.
     
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  20. gusfinley

    gusfinley Tele-Holic

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    That is if you want to read cathode current. You can always insulate any exposed leads of a resistor.

    As I said, I like to tun test points to the back panel and put a pot near the tubes so I don't even need to remove the chassis to bias.
     
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