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Anyone biased a Rivera?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by justonwo, Mar 25, 2005.

  1. justonwo

    justonwo Member

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    I think it's time to replace my power tubes on my Rivera Quiana 212 since I saw a bright blue flash out of one of the tubes and heard some unusual crackling. Plus, the tubes are five years old. Since I've never done this before, I've been reading up on different methods of biasing, plate voltage, current, dissipation, etc. I have a few questions for those of you who've worked with a Rivera.

    1) I purchased a Weber Bias Rite with the plate voltage option. Rivera recommends unplugging the speaker when biasing using their method. Do I leave the speaker plugged in with the Bias Rite?

    2) It also seems that I need to turn both masters and volumes all the way down and leave the input jacks empty. Is this correct?

    3) Do I have to remove the chassis cover to access the bias pot? Is there no external access hole?

    4) In general, should the bias be turned all the way down when starting with a new set of tubes (in case they're significantly different from the previous set)? In that case, which way is all the way down? Clockwise? Counter-clockwise?

    5) Lastly, I just want to double-check my methodology. It seems the rule of thumb is 60-70% plate dissipation. For a 30 Watt 6L6, I would need 42 mA at 430 Volts to get 60% power dissipation. Correct?

    6) One more thing . . . any general safety rules for opening up the chassis?

    Anyone who can enlighten me, your help would be much appreciated. I want to make sure I have everything correct to prevent ruining a pair of $80 tubes.
     
  2. aleclee

    aleclee TGP Tech Wrangler Staff Member

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    1. I found that I had to have the speaker plugged in on my Rake.

    2. It's probably a good idea.

    3. You have to pull the chassis.

    4. Starting at the midpoint is probably good enough. Counter-clockwise is lower.

    5. A number of folks bias things much colder and are thrilled with the results. My approach these days is to start low and increase 'til it stops sounding better. Check the bias to make sure it's not too hot and then it's done.

    6. All the usual stuff. Non-conductive screwdriver. Keep one hand in your pocket at all times. Turn off the amp without putting it on standby to help discharge the caps in the power supply.
     
  3. justonwo

    justonwo Member

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    Thanks for the help, alclee. That's clears up most of my questions. One more: is there any reason to go through the process of trying to bleed the caps before attempting to bias? This seems superfluous since you have to turn the amp on to bias anyway. Non-conductive screwdriver: I assume any screwdriver with a plastic handle will work?
     
  4. Kevan

    Kevan Member

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    One other thing I'd do: Call Rivera.

    When it came time to bias my TBR-2SL, I was lucky enough to have Paul Rivera Sr. pick up the phone. He walked me thru the "real way" and his "quick way" to bias the TBR monsters. Kinda nice to get that info from the source.

    The folks at Rivera also keep great records of the amps they build, and all the tech specs on them. So much so, that if you wanted, you could order pre-biased tubes from them that will drop right into your amp...all ready to go (factory numbers, of course).

    Their cust. svc. is one of the best I've encountered. Well worth the call.
     
  5. fjs1962

    fjs1962 Silver Supporting Member

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    The instructions for biasing used to be in the FAQ on the Rivera site. The way they are designed makes them very easy to bias and all you need is a screwdriver and a decent multimeter. They even give you the factory mA number to shoot for.
     
  6. justonwo

    justonwo Member

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    The only problem with Rivera's method is they don't instruct you how to measure the plate voltage, which can vary, and thus make their current value inaccurate. With the modified Bias Rite tool, you get both the current and voltage so that you can calculate the proper plate dissipation.
     
  7. fjs1962

    fjs1962 Silver Supporting Member

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    That's true but for someone who doesn't understand the relationship between the plate voltage and current draw the Rivera instructions will get you in the ballpark. If you need the plate voltage you can always read pin 3 of the power tubes to get it. I've got a Weber Bias Rite but to be honest I only use it on amps that have external bias pots (so I don't have to pull the chassis) and you have to pull the chassis to get to the Rivera pot anyway.

    All that said, I guess you could make the argument that someone who doesn't understand the relationship between the measurements and how they relate to the bias shouldn't be messing with it anyway. ;)
     
  8. justonwo

    justonwo Member

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    Yeah, it's a shame the bias pot isn't accessible from the outside of the amp. I can't imagine why it would be set up this way. Perhaps so is doesn't get adjusted by someone who doesn't know what they're doing?
     
  9. Scott Peterson

    Scott Peterson Staff Member

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    :AOK That is *exactly* why.
     
  10. justonwo

    justonwo Member

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    Of course the penalty for not knowing what you're doing and adjusting the bias pot from *inside* the amp is heart-stopping electrocution. :D
     
  11. planetal

    planetal Member

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    I have a couple of Riveras and also have the Bias rite tool...
    The Bias Rite, with the voltage option, makes it really easy to do the bias on just about any amp.

    This was mentioned before, but I'll restate it.. The speakers do need to be plugged in.. and nothing should be plugged into the amp. If there is a reverb tank, you need to disconnect it. Also keep the boosts off and volume settings low. Get a long thin insulated screwdriver and lower the bias before you put in the new tubes.

    On my R-55 I can bias without totally removing the head from the cab, it pulls out to a degree and is stable. Ialso have a Suprema and a K55, on those I do have to totally remove the head from the cabinet to bias.

    Once you have done the bias on your amp, you will feel a lot more comfortable.

    Richard Johnsons Rivera Web site (used to be Scrobo) has alot of additional info.. the link to that is

    home.bellsouth.net/p/s/community.dll?ep=16&groupid=218298

    Al
     
  12. justonwo

    justonwo Member

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    Hey planetal, what does an insulated screwdriver look like? Is the metal shank covered in plastic all the way up to the tip or something like that?
     
  13. planetal

    planetal Member

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    What I use is a relatively thin and longish screwdriver with a small thin tip (for little screws)...

    The shaft is metal.. but the good deal of the handle is covered in plastic. The plastic acts as an insulator. I got it at a hardware store for like a buck or so..works quite well.

    It is intimadating the first time you do it, but if you are careful, once you've done it you realize just how easy it is.

    Just an additional note.. When you put the new tubes and set the bias.. after a few hours (3-4).. the tube bias may drift a little.. usually lower.. that's normal.. If you buy the tubes from Lord Valve, the drift tends to be minimal

    Hope that helps
    Al
     
  14. Greggy

    Greggy Member

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    FWIW, and for those who have no way of measuring plate volatge, my M60 runs at 430VDC. I suppose most other M60s will be close. The recommended ballpark current (from Rivera) is 40-42 ma. EL 34 tubes. So that's at 17.2 to 18.1 watts dissipation, or around 70%. Kinda hot. But I've found my M60 sound best at 40ma, so hot is good!
     
  15. justonwo

    justonwo Member

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    Thanks everyone for the replies. Armed with my insulated screwdriver and thick rubber gloves, I biased my Quiana this evening at 42 mA on each 6L6. Also, for what it's worth, the plate voltage was 430 volts according to my Bias Rite.

    The old tubes were clearly shot since one had an intermittent very bright flash of blue light. Somehow, the current on the old tubes was 58 mA or so. These things are 5 years old or so . . . does current start to drift up when tubes start to go bad?
     

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