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Thread: Fender Deville Bias Issue

  1. #1
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    Fender Deville Bias Issue

    Hey Guys,

    I have a Fender Deville that I bought new 6l6's for. When I install them and measure bias
    at the test point I can only get the pair up to 23 ma. Is there a feed resistor or something
    that I can change in the circuit that will allow me to bring the current up to around 70 ma.

    Thanks,
    Mike

  2. #2
    Old Timer J M Fahey's Avatar
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    WHY would you want to do that?
    If Fender, who should know "something" about tube amps and 6L6 decided "up to 23 mA" is fine, maybe there´s a reason for that , don´t you think?
    Juan Manuel Fahey

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    Senior Member Enzo's Avatar
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    Juan, factory spec is 60ma, 30ma per tube.
    And I see no reason to run it hotter than 60ma.

    Mike - looks to me like one of your tubes is not running, that would be why you are getting roughly half the current expected. Oh the heater may be glowing, but no current flowing.

    First inspect VERY closely the solder on the two power tube sockets. I often find them cracked. In fact, when I get a amp of this series in I just automatically resolder all 16 pins.

    Since you replaced tubes, it is possible there had been a bad tube. Bad tubes often take out a screen resistor. SO check voltages all around on those tubes. Is there good B+ at both pins 3 AND 4 on each tube socket? I will assume there is good bias on pins 5 because without it they should run away. BUT ALWAYS check.
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    Mike, are the old tubes still operational? Can you put them back in and check the bias current and verify that the amp is working with them?

  5. #5
    Supporting Member Jazz P Bass's Avatar
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    I would do the test one tube at a time.

    It sounds like either one of the tubes is bad or both are extremely low in current draw.
    Do you measure -54Vdc at the 6L6 bias pin?

    Which Deville do you have?
    There is a Deville, a II and now a III.

  6. #6
    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Enzo View Post
    Juan, factory spec is 60ma, 30ma per tube.
    And I see no reason to run it hotter than 60ma.
    Thanks
    I thought the goal was 70 mA per tube, which means >30W dissipation each, crazy hot for no good reason (unless it´s a Matchless ).
    Now if it´s for both, it´s more reasonable and within specs.
    Juan Manuel Fahey

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    Supporting Member The Dude's Avatar
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    Quote Originally Posted by Enzo View Post
    Juan, factory spec is 60ma, 30ma per tube.
    And I see no reason to run it hotter than 60ma.

    Mike - looks to me like one of your tubes is not running, that would be why you are getting roughly half the current expected. Oh the heater may be glowing, but no current flowing.

    First inspect VERY closely the solder on the two power tube sockets. I often find them cracked. In fact, when I get a amp of this series in I just automatically resolder all 16 pins.

    Since you replaced tubes, it is possible there had been a bad tube. Bad tubes often take out a screen resistor. SO check voltages all around on those tubes. Is there good B+ at both pins 3 AND 4 on each tube socket? I will assume there is good bias on pins 5 because without it they should run away. BUT ALWAYS check.
    ^^^^^What Enzo said. I just highlighted something in his post that you might miss. Be sure to check those screen resistors! BTW, if you find one bad, you don't need to remove the board. Just unsolder it, shake it out of the chassis, and solder the replacement on the topside.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Hey Guys,

    Thanks for the quick responses. I took some quick measurements and here is what I found.
    pin 3 - 464v
    pin 4 - 463v
    pin 5 - (-38v)
    bias point - 23ma

    After I took these measurements I put the original set of tubes back in and basically got
    the same measurements. Am I correct in thinking that I just cant "dial up" enough current
    through the existing bias pot or can this be indicative of another problem?

  9. #9
    Supporting Member The Dude's Avatar
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    Something seems funky there. For the amount of bias voltage you have measured, you should have lots more current. Did you measure both tubes and they are the same voltages, or is that only one tube? Is your battery good in your meter? Those numbers just don't make sense.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  10. #10
    g1
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    And are you measuring mV there or mA ?
    If you are measuring mA, you may not get an accurate reading depending on your meter.
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    Supporting Member The Dude's Avatar
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    Good catch g1! I didn't notice that.

    Edit: In fact, if you have your meter set for mA instead of mV, you would get roughly half the intended reading because you are providing a secondary current path for the cathodes- one through the existing cathode resistor and one through the meter (assuming there might be some minimal meter path resistance).
    Last edited by The Dude; 10-21-2017 at 04:29 AM.
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  12. #12
    Supporting Member Jazz P Bass's Avatar
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    You may want to check the 1 ohm cathode resistor on the output tube cathodes (TP30).
    It may not be 1 ohm.
    Either that or the diode is bad.
    Hot Rod DeVille TP30.pdf

    I will have to agree that a -38Vdc voltage at pin 5 should make the tubes very toasty indeed.

    I am surprised it goes that low.
    That's not a typical Fender range when you consider the raw C+ voltage is around -54Vdc. (or should be.)
    Last edited by Jazz P Bass; 10-21-2017 at 02:38 AM.
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  13. #13
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    I get plenty of Devilles and every now and then get a set of tubes that won't quite bias to the factory spec. I always set these amps up using mV measurement as per the schematic. There are bias resistor tweaks to bring the pot in range, but in this case I advise against those because something is more drastically wrong, either with your measurement, both sets of tubes, the meter or any combination of these. One thing to look for is to check that none of the components in the bias circuit have already been changed or removed (maybe before you got the amp).

    If only one tube is operational you'll get severe loss of volume and excessive distortion when you turn the amp up a little.

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    Hey Guys,

    Thank you for all the replies. g1 gets the gold star for figuring out what I messed up.
    I did indeed have the meter set to mA. Set it to mV and presto....got a good bias reading.
    I reset the bias to 60mV and cranked it.
    Sounds great!

    Thanks again for all your replies,
    Mike
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