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Thread: CL/50 Bias

  1. #1
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    Default CL/50 Bias

    So I dropped my CL/50 head off at my local music store for biasing (I just installed new tubes myself).. the tech called me back and said there is no bias adjustment, plus he doesn't know what the settings should be, so he can't even tell me if the current bias setting is correct. So besides finding a competant tech, what can I tell this guy? Does anyone know where the bias adjustment is and what the setting should be? Maybe I should just cut my losses and pick up my amp, I don't have a warm fuzzy feeling about this guy.

    Thanks!

  2. #2

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    Be smart, go pick up your amp and find somebody else to work on it. My buddy has been having non-stop problems with his Diezel VH-4, pretty much because there are incompetent techs working on it. The last thing you need is to have a tech doing experimental troubleshooting. From what I know there is nothing too complicated inside these babies. Pick it up, find another tech. Just my 2 cents.

  3. #3
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    Yeah I think you're right .. not worth the risk. I'll go and pick it up straight after work. Thanks for the quick reply!

  4. #4

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    Quote Originally Posted by Andrew
    So I dropped my CL/50 head off at my local music store for biasing (I just installed new tubes myself).. the tech called me back and said there is no bias adjustment, plus he doesn't know what the settings should be, so he can't even tell me if the current bias setting is correct. So besides finding a competant tech, what can I tell this guy? Does anyone know where the bias adjustment is and what the setting should be? Maybe I should just cut my losses and pick up my amp, I don't have a warm fuzzy feeling about this guy.

    Thanks!
    I would tell him that you'll pick it up and take it elsewhere. From what he's saying, he doesn't have any business working on your amplifier.

    Seriously, if this "tech" can't figure out where the bias trimpot is on the 'board, AND doesn't know how to figure the bias measurement he has no right touching your amp. For example, figuring the bias measurement should be simple for a tech. Without getting into detail, you can approximate the desired bias measurement with a few measurements and calculations.

    Technically, I think the "true" method of biasing a tube amplifier requires a dummy load, tube characteristic guides, and an oscilliscope to visually see it moving out of crossover distortion...but common joes have been doing alright with the calculation technique, especially since the advent of Ted Weber's Bias Rite. However, there is suspicion his Bias Rite isn't all that accurate, especially in unexperienced hands.

    Anywho, I'd strongly recommend you take your amp elsewhere. Sorry to go off on a tangent, but I'm just trying to illustrate he's not the right guy for your amplifier. Best of luck, hopefully you'll get it adjusted without further heartache.

  5. #5
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    Thanks Kleinm.. I went and picked up the amp after work. No charge, I was expecting a fight over that. Anyway, I got home and the amp seems to be working fine, so no harm done, just some wasted time and gas. I'm tempted to order one of those Bias Rite things and try it myself. At least then I'll know that it hasn't been abused by any clueless techs (except me, ).

    If I do try it myself, do you have a link to a site explaining the calculation method you mentioned?

  6. #6

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    No, but I can try to explain it. To calculate approximate bias, you'll need a Weber Bias Rite BR-A2. Since you're running a CL/50, I assume you're running a matched pair of EL34s? The BR-A2 is for pairs and will allow you to measure plate voltage and cathode current (more on that below)

    The first step is to find the standard rating for a given tube type's maximum plate dissipation. I like the good old RCA Tube Manual, in pdf format here. There are times this site doesn't have what you're looking for, Automatica is a good place to look.

    So here we see the maximum plate dissipation for an EL34 is 25w.

    Carefully remove the chassis from your head. Make sure it is unplugged and off! Once out, locate the bias trim pot on the 'board (for reference), and remove the power tubes. EDIT - Keep the amp's speaker output plugged into your cabinet/speakers. You need them to serve as a load to protect the power section of the amp while it is on.

    Then, you plug the Bias Rite sockets into your tube sockets and then your tubes goes into the top of the Bias Rite probe sockets. Then you turn on the amp, wait 60 seconds, and switch off the standby switch.

    Switch the Bias Rite to the Vp setting (plate voltage). You will see a reading in the likelihood of somewhere between 300-500v. Whatever you read here, this is your plate voltage. Write it down. For sheer example, let's say yours reads 400v.

    Next, you take the maximum plate dissipation (25w for an EL34) divided by plate voltage. Here, we'd get:
    25w divided by 400v = 0.0625amps, or 62.5ma.

    Next you take the amperage you just calculated, and multiply it by 65% (+/- 5%) so you're getting the tubes in the appropriate crossover range that Steve Fryette recommends.

    62.5ma multiplied by .65 = 40.6ma

    Slowly adjust the internal bias trimpot until your Bias Rite (now set to B+ for cathode current) says 40.6ma. Be careful not to touch anything (especially caps) on the circuit board - there are lethal electrical currents present.

    Turn the amp off, let the tubes cool down, remove them, remove the Bias Rite, place the chassis back in the head, put the tubes in, and rock out!

    Long story short, the calculation method is:
    [(Maximum plate dissipation / Plate voltage) x 1000] x .65 = ___ma cathode current. Bias to this value.

    NOTE - this is at your own risk. If you're not comfortable around electrical currents, measurements, etc., I'd leave this to a tech. If you take your time, are careful and mindful...I really think you'd be able to do it. Just wrap you rmind around it before you start. Don't try to figure it out as you work on it .
    Last edited by kleinm; 05-20-2006 at 11:59 AM.

  7. #7
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    Wow, thanks for that post!! I've been searching the net for this info with no luck, and you've just told me everything I've been wondering about!

    Ok, I'm definately ordering the Bias Rite and doing this myself. I'm no electronics expert, but this seems like something I can handle.

    Thanks again for taking the time to type all that, I really appreciate it!

  8. #8

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    They should sticky a thread like this, with bias notes. I know I'll be looking up this thread when I get a bias rite, I honestly do not trust the shady tech I have used, he seems too much like a dipshit. Plus, it's a ripoff for what sounds to be a pretty simple thing.
    VHT CLX w/graphic
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  9. #9

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    No problem guys, I'm glad to help.

    Yeah, its really not too complicated especially with the advent of the Bias Rite probe. It is really slick because it allows you to measure both cathode current and plate voltage without moving your DMM points to the different pinouts and whatnot.

    Like I said however, I'd recommend that you have a clear grasp of what you're doing before you start. That will help you stay focused and not screw up.

    Its like changing the strings on a guitar, once you've done it right its cake doing it again. However, adjusting bias has a little more inherent risk than changing strings . If you're careful though, you'll be golden.

    Good luck guys. Don't hesitate to post questions. I'm sure myself and others will be willing to try to help.

  10. #10

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    EDIT - I just noticed that I didn't clarify that your amp has to be plugged into a cabinet for loading purposes when you turn your amp on and adjust bias. It is possible to damage the power section of your amplifier if you turn it on and do not connect it to a load.

    I edited the above post to include this step.

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