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Good Bias Tool

Discussion in 'Shock Brother's DIY Amps' started by NTC, Nov 1, 2020.

  1. NTC

    NTC Tele-Meister

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    It seems there are bias tools at many different price points that have different levels of features. I would like to see what you folks like. I want to keep it under $100 (preferable way under) ad I have extra DMM's to read the voltages. This would be for two-tube fixed-bias outputs.

    I have always used the Gerald Weber "short the ammeter across the OT" method, but I don't trust it anymore. If/when I build anything again, i will put in the 1 ohm resistors...
     
  2. dogmeat

    dogmeat Friend of Leo's

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    Bias Scout, it's a Tube Depot kit. 20 bucks per unit. if you want to watch 2 tubes at the same time, get two. but then you'll need two meters or make a switching thing. I get by fine with one. I measure both power tubes and trim using the higher one as the sample

    [​IMG]
     
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  3. NTC

    NTC Tele-Meister

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    Thanks!
     
  4. wabashslim

    wabashslim Friend of Leo's

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    The beginning of wisdom. I have been "surprised" (ahem) at some of the things this Mr. Weber has put into print. Please do not confuse this Gerald Weber with the late Mr. Ted Weber of Weber speaker fame. Two entirely, unrelated, different men of differing levels of integrity, skill, and "probably" IQ.
     
    Last edited: Nov 2, 2020
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  5. NTC

    NTC Tele-Meister

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    I definitely know the difference. I used to go to their boards when Ted was still alive. Been about ten years...
     
  6. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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  7. Wally

    Wally Telefied Ad Free Member

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    Whatever tool you buy, get one that reads plate voltage in addition to the current draw. Anything else is incomplete, imho, because one has to know the plate voltage at the particular current draw in order to know what the plate dissipation.
    As for the transformer shunt method mentioned above, that is not an invention of Gerald Weber. It is accurate, and it is quick. I am not suggesting that anyone use the method, but it is what I use and will continue to use. Ommv. If you buy an inexpensive bias tool that does not measure plate voltage, you are going to have to take a plate voltage reading on the live circuit anyway....and also with the 1-ohm resistor fro: cathode to ground method.
     
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  8. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Excellent points, Wally. I never use my 'simple' bias tool any more (I’ve installed 1% 1R resistors) and I’d forgotten I still had to open up the amp *plus* pull/replace the tubes for every measurement....
     
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  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    No opinion on a bias tool. Like Wally said it would be better to have a plate voltage feature.

    I measure the resistance directly across the OT... The *Output Transformer resistance method*. I haven't felt a need for a *Bias Tool* or 1ohm resistors for that matter. IMO the money would be better spent on a couple more multi-meters but that is not necessary.

    I don't mind opening the chassis. It provides an opportunity to visually inspect and clean the moving bits every so often. General maintenance.

    *Output Transformer resistance method* is described on the Robinette *Bias Calculator* site.
     
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  10. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Good point, and the OT resistance method is a lot safer than the OT shunt method.

    On amps I build, especially adjustable (fixed) bias tray-style amps, where access takes time, I find 1%1R resistors, a few tip-jack test points, and an externally-accessed Fender-style bias pot make checking and fine-tuning bias by ear a 1-2 minute process. And that means I can do it whenever I want, eg, if I think my sound changed, or if I want to try a different rectifier tube, or when I switch the supply voltage with my 'vintage voltage adapter.' Blue is the bias pot; the others are the TPs...

    TPs.png
     
  11. SacDAve

    SacDAve Poster Extraordinaire

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    Those are a little to put together
     
  12. fishermike

    fishermike Tele-Meister

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    In my old age, I initially read the thread title as "God Bless Tool". I was thinking, well, I like Tool as much as the next guy, but where's the OP going with this one??? :lol:

     
  13. dogmeat

    dogmeat Friend of Leo's

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    guessing you mean they are a bit tricky to put together. yeah, maybe. I thought the instructions were pretty good, but yes... a bit of fiddling around involved. go to their site and pull up the build instructions. look at the photos, etc. it's simple parts, all you are doing is jumpering a socket to a base with a couple resistors in there. but yes, it can be confusing... to me it seemed like they are almost saying too much

    yes, Bias Scout reads plate voltage... the red plug is volts. there is a 100 ohm load resistor, and in parallel with that, the meter connects through a 1Meg dropping resistor so there is only mV on the red plug and not full plate voltage. it reads proportional

    the white plug "reads" current by volt drop on a 1 ohm resistor

    I thought about putting a switch in the system since both readings use the mV scale but its easy enough to just plug/unplug
     
  14. NTC

    NTC Tele-Meister

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    That is a new one to me. If one can get an accurate ot resistance reading, this should be accurate.

    I don't like the shunt method because you have resistances in parallel that may (granted, probably won't) affect each other and the reading. Plus I have blown up a DMM that way. Also, I don't have a problem measuring plate voltage.

    One could put banana jacks on one of those bias kits connected to the plate and cathode to connect a volt meter, but then you have exposed the high voltage to errant fingers. Probably no worse than poking around the chassis.
     
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  15. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I feel it is a pretty accurate way to measure the bias.
    It doesn't require using the delicate ampere function of the multi-meter.
    You do have to follow safety protocols just like any other time when dealing with a live circuit. It can be done *hands free* by using clips on the meter leads.
    It only measures the plate current so the screen currant does not need to be measured or estimated.
    You get to use a calculator to find the amperes.

    I believe external ports can be added in a safe manner. It does have to be done in a thoughtful manner though.
     
  16. Wally

    Wally Telefied Ad Free Member

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    Iirc, Rob has done the comparison between the shunt and the OT resistance methods....negligible difference if any at all. again...I am not advocating any method, and always advise that there can be danger at any time with these guitar amps.
     
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  17. schmee

    schmee Doctor of Teleocity

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    As Wally said, get one that reads plate voltage in addition to the current draw. Makes things easy. Also, if you can, get one that works for 2 tubes at once so you can compare the bias of each tube without rearranging things. Weber used to make some, others are out there. You should be able to land one for $100 I would think. Used anyway.
     
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  18. schmee

    schmee Doctor of Teleocity

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    I'm not seeing a lot around that do both Voltage and Ma. Stew Mac and GC have the VHT that only does Ma for $100. Here's one like mine for $75, I'd buy it fast: https://reverb.com/item/882163-weber-bias-rite-br-2-bias-your-own-amp-free-shipping
     
  19. Wally

    Wally Telefied Ad Free Member

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    No experience with it, no affiliation, first search....


    Eurotubes Pro One is a single socket probe that reads both current and voltage.
    wuite some time back, a member had purchased a current draw read only bias probe. He asked what to do about reading plate voltage. I told him that it was very likely that one could carefully drill a hole through the side of the tube base in order to allow a meter probe to contact the plate. One could then take the plate voltage in that manner. He did it, and it worked. I would surmise he was good with careful measurements for placement of the hole, had a sharp drill bit with a variable speed drill, and had some skill with the tool.
     
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  20. NTC

    NTC Tele-Meister

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    If I bought one of these tools, that is a possible way to do it.

    I am going to try the resistance method before I buy anything. Most of my amps require that they be opened in order to adjust the bias anyhow - tweeds and Traynors, a Bandmaster Reverb and a Sunn bass head...
     
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