Help me bias my Two Rock Ruby...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Riscchip, Sep 25, 2004.

  1. Riscchip

    Riscchip Supporting Member

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    Trying to put some 6V6 tubes in my Two Rock Ruby...never biased an amp before, but I think I can pull this off with maybe a hand or two. A few questions:

    1) There is a bias test point next to the bias pot...the positive probe goes there, where does the negative probe go?
    2) How exactly do a measure "ma" on my multimeter? I'm not seeing any obvious setting for that.

    Thanks for any help.
     
  2. Stevo57

    Stevo57 Supporting Member

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    The red lead from the multimeter in the pot. The black lead to ground which could be anywhere on the chassis. I always poked at a jack nut on the back of the head. The multi meter I have was simple to figure out milli amps or MA. You may have to consult the directions of your meter I'm afraid. The reading will be the sum of the two tubes. So it will read 90 for 45 ma on each tube for example. Timster is the guy I asked all this same info of when I had my Ruby. That was my first time too.
     
  3. Hipster Dofus

    Hipster Dofus Member

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    I have a nice Fluke with a set of Clamp on probes. I use the black clamp on the Tranny lug that is close by, Red goes in the Test point, You can do it backward as well, it will read -. If you have no clamps, just touch the black probe to some part of the metal that is not painted, like the speaker jack lockring.
    set the meter to MV, milliVolt, mVolt, or what ever looks like DC MilliVolts....
    Make sure the amp is on, and a speaker load in pluged in. You prolly will not hear anything as you take the measurement, unless you set the meter to the wrong setting.
    devide by 2 and you will be done. seems like 80-to 100 mv is what most use for EL34's.
    If you swap many diff brands you might want to back the Bais off before sticking a diff set in as 90mv on one brand might be 130mv on another! They will mostly be close though.

    Enjoy that Loud Mutha
     
  4. Riscchip

    Riscchip Supporting Member

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    Thanks guys!

    Seriously...can't thank you enough for the help. I've found millivolts on my multimeter and I'm getting some stable readings from the ruby. I especially appreciate that comment about touching the non-painted bits on the chassis. I was really pulling my hair out over that--couldn't figure out why my readings wouldn't stabilize.

    With the stock EL34s it was measuring 45...for both tubes. That suggests to me that the bias pot moved a bit in shipping. Should be biased to around 35 per tube, right? So it should read around 90 for the EL34s, I'd assume.?

    My hunting on the Two Rock forum brought up 25 as the number for 6V6s. That sound right? I should adjust it so my multimeter comes back with a reading of 50?

    I popped in some 6V6s and set the bias to 50--just as a quick test. Wow, big change in tone. Sounds great, but different. Much darker and looser. I was suprised the change was so noticable. Much more significant than going from an EL34 to a 6V6 in single ended amps I've owned like the univalve, bivalve and Peters 3CP1.

    Thanks again for the help, guys!
     
  5. renico00

    renico00 Member

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    Assuming the Ruby has about the same plate voltage as real Express amps - ~408vdc, then your 6V6's should be ~25ma. The 34's should have been at ~45ma or more. That may seem high, but the plate voltage is pretty low compared to close to ~500vdc you'd see in the "classic" amps.

    So, I'd assume you should set it so your meter reads 50mv, since you don't have separate bias probes (not measuring each tube separately).

    Jeff
     
  6. Stevo57

    Stevo57 Supporting Member

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    Yes I was wondering too, if that 45 reading was both together or the sum of the two. I biased the EL34's in mine at 95-100 and it was very sweet. I tried some 7581's in it and they were really high, but the 34's were the ticket for me.
     
  7. Riscchip

    Riscchip Supporting Member

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    Thanks for the note. So 50 mv is the way to go it sounds for the 6V6s.

    The 34s were definitely set way too low for some reason. That reading of 45 was for both tubes at once (I'm assuming, since there is only one test point). The bias pot isn't super tight (doesn't require a screwdriver for me to turn it), so I'm gonna guess that it just moved during shipping. Glad I checked it anyway. I'm going to pop the 34s in there and set the bias correctly, see if it sounds better / different than it did when I got it. Not that it sounded bad...I bought this amp mostly as a curiosity (never had a Two Rock before), but it's blowing my mind it sounds so great.

    I also ordered up some yellowjackets, so I'll be sampling it's tone with the 84s, too. So far, nothing I can do with this amp makes it sound bad. Killer amp.
     
  8. Hipster Dofus

    Hipster Dofus Member

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    Mine was set to 35mv when I got it!:eek:

    It sounded fine, but seems to emjoy the 80-100mv as well.

    If you play a strat, stick a 5751 in V1 ans set the gain to 9:00 or 10:00. Clean to mean.
     
  9. renico00

    renico00 Member

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    Riscchip

    You say this is your first Two Rock. Be aware, this amp (the Ruby) is extremely different from everything else they produce. All their other amps are channel-swithcers which have the smooth overdrive and spanky cleans.

    Jeff
     

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