VHT CLX Biasing issues

Discussion in 'Amps and Cabs' started by drjenkins, Jun 12, 2008.

  1. drjenkins

    drjenkins Supporting Member

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    Posted this over on the vhtusers board as well:

    I've emailed support about related issues but haven't heard back yet. I figure I'll post up here and see what you guys think.

    So I got a new matched set of Ruby EL34's from Doug's Tubes to replace the SED's that were crapping out on me. I bought a Biasrite to make sure I get it all biased up and working right. I threw a Tung-Sol 12AX7 in the V1 to see what that'd do as well.

    Well...I do the whole setup with the Biasrite, followed the instructions to the letter, I get a ma rating of 67!!!!!! Holy ****, no wonder I was having a red plating issue with it. So I back that down to 32ma and check everything out. I'm getting a varied reading between the four tubes, but IIRC from past threads, that's normal within a couple ma. The weird thing was that I had to turn the bias control opposite of what was labeled. Is that normal?

    Then, when I popped off the bias rite and loaded the tubes, the amp was harsh as hell sounding. To even stay in the room with it, I had to turn all the treble all the way off, along with the presence control. So I know that's not normal. Tonight I'm going to throw the old tubes in and see what happens there.

    Does anybody have any other insights or suggestions as to what might be the problem? I'm getting frustrated with it really quickly, and I'm coming to the realization that I may have to try and find a tech to go over it for me. There's just no one local that does this kind of work, and I really don't want to ship this thing off if I can avoid it. Thanks in advance for any tips!

    dave
     
  2. drjenkins

    drjenkins Supporting Member

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  3. 6AM

    6AM Supporting Member

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    32 seems kinda low. What's the plate voltage of the amp?
     
  4. drjenkins

    drjenkins Supporting Member

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    470 was what it read out on the Biasrite. The bias calculator said to hit 37ma for 70% dissipation. The tubes were reading within 6-8ma of each other +/-
     
  5. 6AM

    6AM Supporting Member

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    If the tubes are 6 to 8 mA off of each other, I wouldn't call those matched. Doug's Tubes is pretty reputable. I can't believe he sold tubes that mismatched. I'd send those back.
     
  6. drjenkins

    drjenkins Supporting Member

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    Really? I was under the impression that it was pretty normal for them to be a little off, as in within 10ma of each other. Plus I don't know if there's something up with this Biasrite or not. So it may be reading the tubes wrong.:confused:
     
  7. 6AM

    6AM Supporting Member

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    Anything more than 3mA is too off to be considered "matched" in my opinion.
     
  8. drjenkins

    drjenkins Supporting Member

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    Gotcha.

    Hopefully I'll get a response from VHT soon, and see what they say about all of this. Thanks for the help.:AOK
     

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