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  • JCA50H Biasing

    What do these bias at with a bias probe. Just got this and swapped in JJ 6L6GC and want to set bias.
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  • #2
    they bias at 40mA like most other 6L6 equipped amps. Not sure what is the question here?
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    • #3
      Thanks that what I needed.
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      • #4
        I use my bias probe by setting the dmm to millamps 0-200 range,others use set the dmm too millivolts 0-200 and millivolts really indicates how many milliamps you have going thru the tube!
        Read up on your probe instructions,they are not all the same.
        What probe do u have,you need information from the seller of the probe,u cant ask us how to use something and give zero information.
        You may have a probe that reads plate voltage,careful there !

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        • #5
          You need to know the plate voltage to bias it properly. Taking the screen draw into account wont hurt either.
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          <img src="images/misc/quote_icon.png" alt="Quote" /> Originally Posted by <strong>-Assy-</strong>
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          <div class="message">Okay I summed up this entire thread in three posts. Let's go home and beat our children.</div>

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          • #6
            Probe purchased through Eurotubes. Plugged into multimeter to measure ma. How do you measure plate voltage?
            <div class="signaturecontainer">Jet Setters (Jet City Lounge)</div>

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            • #7
              If the amp info says 40ma i guess thats close enough.
              I also did the same ma reading,but i finnally ordered the probe for plate voltage and found 465volts on my bugera 6260,so webers bias calculator says 32-36ma ,38 is hot ,and i chose 33ma ,im running tungsol el34b tubes,no change in tone within 32-38ma ,they should last awhile now !
              Without the plate voltage i wasnt sure,plus the tube data specs vary alot with some tubes.

              My pot was barely in range.

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              • #8
                I bought my tubes & probe from Eurotubes, and they were a great help to me. 40ma is hotter than what I came up with, but I think most amps will vary. I checked pin 3 (plate) voltage on my power tubes and found it to range between 480 & 503 volts (not sure why, but that's what I got at diff times). Factoring a 25% dissipation & using their recommended 70% bias, I calculated between 34 - 36ma (dissipation / plate voltage * bias %).

                ...and, 'cause nobody seems to have said it yet. Watch yourself in the chassis of that thing. It can blow you up (READ: KILL YOU) if you touch the wrong thing - even powered off, unplugged, prayed over, etc. without discharging caps first.
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                • #9
                  I will second that I used a AMP-HEAD probe that does both plate voltage and current. My plate voltage was right on the mark with Grimes and I used weber bias calculator online and got a 36ma bias setting. From the factory my bias setting was ice cold at 10ma and sounded like crap, new JJ's in ever slot and the right bias then the depth mod made it really open up.
                  I bought my tubes & probe from Eurotubes, and they were a great help to me. 40ma is hotter than what I came up with, but I think most amps will vary. I checked pin 3 (plate) voltage on my power tubes and found it to range between 480 & 503 volts (not sure why, but that's what I got at diff times). Factoring a 25% dissipation & using their recommended 70% bias, I calculated between 34 - 36ma (dissipation / plate voltage * bias %).

                  ...and, 'cause nobody seems to have said it yet. Watch yourself in the chassis of that thing. It can blow you up (READ: KILL YOU) if you touch the wrong thing - even powered off, unplugged, prayed over, etc. without discharging caps first.
                  <div class="signaturecontainer">Jet Setters (Jet City Lounge)</div>

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