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inexpensive ammeter for biasing tubes?

Discussion in 'Amps and Cabs [BG]' started by jasonbraatz, Dec 29, 2001.


  1. jasonbraatz

    jasonbraatz

    Oct 18, 2000
    Oakland, CA
    well...xmas kinda put a hold on me attempting to learn how to bias the tubes in my iod, but now i have the time to do it. so...i need an ammeter? any suggestions?

    jason
     
  2. notduane

    notduane

    Nov 24, 2000
    Location
  3. jasonbraatz

    jasonbraatz

    Oct 18, 2000
    Oakland, CA
    so...something like this would work?


    jason
     
  4. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    I'd get something rated at at least 10,000 ohms/volt. Radio Shack probably has a few.
     
  5. Jerry J

    Jerry J Supporting Member

    Mar 27, 2000
    P-town, OR
    I may be out of it, here, but are we talking about biasing preamp tubes?
     
  6. jasonbraatz

    jasonbraatz

    Oct 18, 2000
    Oakland, CA
    nope, the interstellar overdrive has a dual EL84 output stage complete with transformer.


    jason
     
  7. jasonbraatz

    jasonbraatz

    Oct 18, 2000
    Oakland, CA
    um....for sure?


    cos there are a few people on this board who HAVE biased their IOD's, and even SWR literature says you're supposed to rebias if you change tubes.



    jason
     
  8. Could any of your guys explain to me what it means to bias your tubes?
     
  9. Jerry J

    Jerry J Supporting Member

    Mar 27, 2000
    P-town, OR

    "Bias" in this context refers to the amount of voltage held on the grids of the output power tubes. This controls the amount of current the output tube(s) conduct exclusive of the signal current, or, looking at it another way, the amount of overlap where both tubes are conducting simultaneously.

    I hope this makes sense.:)
     
  10. Jerry J

    Jerry J Supporting Member

    Mar 27, 2000
    P-town, OR
    Just a note: biasing an amp is one of the most dangerous things you can do around an amp. The caps will hold LEATHAL voltage even with the amp turned off and unplugged. :eek:

    Poking around in a tube amp will kill you instantly or sure ruin your day. Please seek help on doing this if you've never done this. An amp tech may be willing to show you just how to do this if you ask nicely.

    I'm not trying to be condescending. I'm just hoping to impress upon you the serious nature of the internals of amps. I'm sure that PBG will agree with me on this.:)
     
  11. jasonbraatz

    jasonbraatz

    Oct 18, 2000
    Oakland, CA
    but i DID change tubes...that's the thing.

    the Ei's it shipped with were great tonally, but were very crackly and noisy. so i got a set of groove tubes el84's for now, but i plan on replacing it with another set of Ei's, this time from lord valve so i know they'll work.


    i can't really tell if it's cherryplating because it's in my rack right now, but i'll pull it and let it warm up and see for sure. i'm just concerned it's getting too much current because it seems to be getting VERY warm. (alot warmer than my old B15 did)


    jason
     
  12. mthoople

    mthoople

    Nov 1, 2001
    Denver, CO
    PBC, while some people DO repeat misinformation, previous discussions on biasing of the IOD involve first-hand information.

    The SWR IOD IS fixed-biased, I have the schematic, and I HAVE biased mine.

    While most amps running EL84's are cathode-biased, the IOD is not.

    Jason, I recently bought the Wavetek 23XT from Radio Shack for $40. It WILL go on sale again.

    Matt
     
  13. neptoon

    neptoon

    Jul 25, 2000
    summerville, sc
    nothing is better than a simpson 260, imo
     
  14. Tapp

    Tapp

    Aug 29, 2001
    USA, Mississippi
    They're right Psycho, the SWR IOD is fixed-biased with an adjustable trim pot to set the bias feed voltage. I received a schematic from SWR. I've personally set the bias in mine using the tranny-shunt method; works like a charm. BEWARE of using this method if you don't know how!!!!

    The only thing that is strange about the IOD is the lack of screen grid resistors. I know Psycho that you know (or know of) Bruce C. (Mission Amps). Bruce recommended I install screen grid resistors. I may try that in the near future but the plates are running fairly low voltage that it may not matter much. Will defitiely make it more reliable though.

    Tapp
     
  15. I'm a big fan of the Simpson 260 referenced above, but I do love a good autoranging meter. Try checking your local pawn shops. There's usually a few Flukes in my local pawn shops, and even a Fluke 87 for $70 every once in a while. That's a $300 meter when new!

    Seriously, though, autoranging is a nice feature. All you have to do is set the meter to AC Volts, or DC Volts, etc., and it automatically adjusts the scale for readings from 100mV to 600V.

    The tubes will only be pulling like 40mA or so, so just make sure the DC current measurement range is at least 200mA or so. Usually it's either 2 Amps or 10 Amps on most meters with Ammeter settings. Please know what you are doing, because if you hook the meter up wrong, it will be fried, and if you touch the wrong thing, you will be fried. So many meters in meter-Heaven due to the probe being left in the "mA" port and then used to measure 450VDC. Poof goes the fuse, if you're lucky.

    Chris
     
  16. neptoon

    neptoon

    Jul 25, 2000
    summerville, sc
    flukes are great, too...i've messed around with 5 or 6 different models, and they're great when a more precise reading is desired...if you know you're going to have 0 or infinity...or a reading or not, i always go with the simpson.
     
  17. Yep, you can't beat a needle swinging over to 0 and then drifting back to infinity to check a cap. Also, sometimes when measuring the resistance of an inductor, the autoranging meter won't settle down. It can't deal with the inductance. The Simpson and my other favorite, the Triplett 660(?), will swing from infinite over to the true DC resistance.

    Can't beat old technology.

    Chris
     
  18. jasonbraatz

    jasonbraatz

    Oct 18, 2000
    Oakland, CA
    so...will this work? i don't need awesome, i just need functional.


    jason
     
  19. The link doesn't work. Can you post a model number we can look up on the Home Depot site?

    Chris
     
  20. jasonbraatz

    jasonbraatz

    Oct 18, 2000
    Oakland, CA
    the SKU# is 276539