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Thread: Biasing Peavey JSX

  1. #1
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    Biasing Peavey JSX

    Anyone have experience setting the bias on a Peavey JSX (EL34)? There are bias terminals out the back but I'm not sure what to measure. My guess was that I was measuring mA across 1ohm resistor but I'm not sure of plate voltage and can't find a schematic. I saw somewhere online plates were at 350v so I'd be shooting for somewhere around 45mA. Any help appreciated.
    Thanks
    Doug

  2. #2
    Senior Member e_c_mccollum's Avatar
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    Peavey sends me FREE schematics all the time. I'm sure they will be happy to assist you in this manner. The peavey website does have a pretty good forum on the repair and mods for there amps.

    If you are right, and there is a 1ohm resistor, you would be measuring the voltage drop (mV) across it, not current. Example, 60mV reading = 60ma bias current. Using a 1ohm resistor makes this possibe. Internally should be a bias pot to raise or lower the negative bias voltage which in turn affects plate current.

    The back of your amp may state how much voltage to read. If not call peavey to find out.
    Last edited by e_c_mccollum; 02-05-2008 at 01:44 AM.

  3. #3
    Senior Member Enzo's Avatar
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    NO NO NO NO NO

    Whereas we like to measure current, and the typical 1 ohm resistor is convenient, on this amp those terminals measure bias VOLTAGE. yeah, I know...

    Factory spec is to set it to -55VDC for 6L6 and something like -42 for EL34. Without me looking, does that amp have the 6L6/EL34 switch? ANyway, definitely -55 for 6L6.

    What current that results in I couldn't tell you. It will be a cool setting. AS in not hot.

    The range of that control is limited, so you can pretty much set it by ear.

    Your plate voltage is a lot more than that, I'd expect 465 to 480v, by the way.

  4. #4
    rf7
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    This is from the Eurotubes site:
    "An update 10/09/07 We just measured the bias on a XXX in the EL34 mode and with the bias set at 42 volts using the factory test points (Peavey recommends 42.5) the bias was a whopping 57mA which is over 100% of dissipation! You will burn up your tubes with all but the coldest grades of EL34's or KT77's. With the bias set to 45 volts at the factory test points the bias went way down to a very acceptable 37mA. So in this particular amp a variation of only 3 volts at the factory test points made an incredible 20mA difference in bias!!

    Do NOT bias your XXX or JSX for any EL34 type tube including the JJ KT77's using the test points! The 55 volts that Peavey recommends at the test points for 6L6GC's is on the other hand, on the cool side but using the test points is so completely inaccurate we will not warrantee tubes to players who use the test points to bias these amps!"

    Here's the link: http://www.eurotubes.com/euro-k.htm

    You might want to get a bias probe, to properly bias your EL-34's.

  5. #5
    rf7
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    I've got one on the bench now and I ended up biasing it with a bias probe for 70% dissipation and came up with 41V on the test points. So, Enzo is correct, the Peavey recommended 42.5 is a little cold, and maybe Eurotubes had some tubes that really pulled a lot of current in the amp that they checked out. Plate voltage was 481V.

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    Senior Member Enzo's Avatar
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    I like Bob, and I think he provides good service to his customers, but I also have to take some of his claims with a little salt. he has a sort of running argument with some folks at Peavey. I think a lot of it started when certain PV amps did not like the E34L, while the plain old EL34 worked fine - screen resistor issues. Bob used to be a regular at Peavey forums, and that drove him away. That and a gentle warning from the moderator that Bib's presence was getting too close to advertising for their liking. I have to say in his defense that it wasn't so much him per se as it was lots of the other guys constantly plugging his service. It kind of got uncomfortable.

    I would say that while Bob may have some tunes that don't work out, in general the PV folks are pretty conservative on their bias so no one gets in trouble and burns up his tubes, and in the process their PV amp.

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