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CompuBias readings on 1974 Vibrolux... does it seems "cold"?

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  • #1

    CompuBias readings on 1974 Vibrolux... does it seems "cold"?

    Hi,
    I have a new for me 1974 Fender Vibrolux Reverb that I added to my collection and I intend to give it some maintenance over the next few months.

    I inquired last week about a squeal that sometimes occurs but all in all the amp is very quiet and seems to work well... so I haven't done any maintenance work or troubleshooting yet.


    I had thought it seemed a bit underpowered compared to my other amps. I have several Champs, a Princeton, a Princeton Reverb, and Deluxe Reverb here in my studio as well.

    Today I hooked up my CompuBias meter to the two 6L6 tubes and noticed that the readings seemed to confirm my idea that the amp wasn't running at full potential.


    The readings are:

    Vp 441vDC Ik 21 W 9.2
    Vp 441vDC Ik 23 W 10.1


    Does anyone have any comments about the fact that the power output seems low?

    I finally got around to buying a new Fluke DMM today (That this forum recommended to me a year or so ago) and plan on pulling the chassis out for a check up.

    Thanks.

    best regards,
    mike
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  • #2
    Within reason, bias level won't affect power output to any significant degree, ie plate dissipation is very different to power output. But yes, they are on the cold side. Pete.
    My band:- http://www.youtube.com/user/RedwingBand

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    • #3
      Thanks,
      One of the things that made me think the readings were low is that the CompuBias docs suggest that I should expect readings of approximately W 15 for a 6l6GC tube.

      I've been playing it through the stock speakers... and I use hi sensitivity speakers like JBL K series in most of my amps so I may just be hearing a bigger difference in the actual speakers.


      best regards,
      mike

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      • #4
        Most tubes are different and IMO after they wear for awhile the numbers go down. That's what I like about the Compu bias is it show wattage and current but I bet brand new tubes or NOS would have a higher reading. What is the negative grid bias reading ?
        KB

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        • #5
          Thanks,
          I'll report back when I pull the chassis and take the measurement. It may be few days.

          best regards,
          mike

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          • #6
            Depending on what I'm trying to accomplish, I run my 45-50 watt amps with 6L6s at around 16-20 watts each....
            The Vibrolux amp uses a smaller, 30-32 watt OT. It is only about 1/3 bigger then a 20 watt Deluxe Reverb.
            I'd probably not run those 6L6s in idle at more then the 16-18 watt level in any case with that OT and PT combo.
            Tonally you will hear a difference with the tubes running hotter at idle but I don't think you'll hear much of a difference it volume.
            Bruce

            Mission Amps
            Denver, CO. 80022
            www.missionamps.com
            303-955-2412

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            • #7
              Hi,

              I have finally pulled the chassis and measured the bias to the power tubes.

              It seems balanced with one 6L6GC tube at -44.54vDC and the other at -44.94vDC.


              On another note, I'm beginning to think that the oscillation is related to the Vibrato circuit... and hope to investigate further.


              Thank you.


              best regards,
              mike

              Comment

              • #8
                Originally posted by mike_mccue View Post
                ...I have finally pulled the chassis and measured the bias to the power tubes. It seems balanced with one 6L6GC tube at -44.54vDC and the other at -44.94vDC....
                Mike,
                The bias voltage at the power tube grid is just part of the story. Remember that you set the bias voltage to achieve a given plate current at idle and then calculate the resulting idle plate dissipation. Maybe you just didn't finish yet but I wanted to clarify for the readers that he wattage figures discussed above are the resulting power tube dissipation in Watts. To balance the power amp may require "unbalanced" bias voltages due to differences in the individual power tubes. Likewise, setting the bias voltage to the same valuse for each tube won't necessarily result in balanced plate current.
                Regards,
                Tom

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                • #9
                  Hi Tom thanks for the comments.

                  The post today about the bias voltage was a follow up to my original post where I had shown the following readings from my CompuBias meter:

                  The readings are:

                  Vp 441vDC Ik 21 W 9.2
                  Vp 441vDC Ik 23 W 10.1


                  I work on tubes amps only a few times a year so I am perennially rusty when it comes to speaking about them, but I thought the Ik is the Cathode current and so I was thinking the tubes were idling at 21ma and 23ma respectively.

                  Thanks.

                  best regards,
                  mike

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                  • #10
                    Hi Mike,
                    OK. I understand now in the context of the thread. I agree that the tubes are biased on the cold side.
                    Tom

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