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Post subject: Vibro King Bias
Posted: Wed Mar 03, 2010 9:18 am
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My Vibro King schematic (Rev. D) shows the bias pot voltage as -45vdc. The posted information on Fender's web-site shows a schematic (Rev. A) with the pot bias at -55vdc, and other web information shows the bias at Pin 5 of either power tube (6V6GC) as -46vdc. My Vibro King was built in January 1995 (Date Designation: FA) and has a EL-84 reverb. While I understand that the Vibro King was designed to be biased "by ear," can someone please tell me what for my amp might be the "correct" bias voltage measurements at the bias pot point, and at Pin 5 of the power tubes. (It seems to me the bias voltage should be different because of the resistors in the circuit. Kindly feel free to correct me if I'm wrong.)
Thanks!


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Posted: Thu Mar 04, 2010 2:58 am
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Hi Vegas, welcome to the forum. The only schem I found was for the 6L6 amp(rev A). So I see your dilemma. The bias point of a tube in general is usually set by measuring it's current draw in conjunction with the plate voltage. And that point is just within the tubes operating range. It's not a set number, just as you mentioned about biasing by ear, it's a preference of tone within the tubes design parameters. Unfortunately, you can't set the bias by just measuring the voltage at the control pot or the the tube pins, but that measurement would surely ensure that the tube won't be in a runaway state. To answer your question about - voltage, those numbers look fine, for a starting point. Without knowing your expertise with tube amps, I would suggest getting yourself a bias probe, they are worth their price. And using your ear, the probe, and the calculator tables you'll be set. Actually, I bought one from Weber almost a year ago because my eyes and hands seem to have grown old, and a slip while measuring current is no good. Art

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Posted: Sun Mar 07, 2010 2:40 pm
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Thanks for taking the time to answer my post. Perhaps you might have some ideas on my main problem with my Vibro King. After several years of beautiful sound, the the amp started sounding "crummy." I put in two new matched JJ 6L6GC, a new Sovtek EL-84, and left in the (5) 12AX7s (Electro-Harmonix). Set the Bias "by ear," and it still sounded crummy. Set the bias to -46vdc at Pin 5 of the 6L6's, still crummy. I then took it to a reputable technician, who checked all the tubes, and circuit bias, and all of the other circuitry. He didn't find anything wrong, and said the amp was O.K. But I've had the amp for years and know that it doesn't sound like it use to.
Perhaps you might have some ideas or thoughts on why after years of a nice full tone (think Wes Montgomery, Joe Pass), the amp all of a sudden sounds "tinny" and "metallic."
Thanks again!
P.S. The power tubes are 6L6, not 6V6 as I wrote in my first post.


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Posted: Sun Mar 07, 2010 11:48 pm
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Hey there Vegas, a crummy sounding Vibro King is unusual, for sure. When you describe a"metallic" sound , the bad speaker flag pops up. Can you test the amp with another cab?(a 2 or 4ohm load will work for the test) Otherwise, I'm going with a wornout tube. The first tube I'd replace is the phase inverter. A matched JAN spec'd 12AT7, 5751, or 12AX7 would be my choice, in that order because you seem to lean towards Jazz. I hope you'll find the problem with one of theses suggestions. If not, I'm afraid you'll have to go through the pre amp tubes. Since you have a grasp of the schematic, you'll be able to prioritize the selection of quality tested tubes. Just curious, did that amp get the mod necessary to run the EL84 at the appropriate levels? Art

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