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Cathode biased 6L6 amp (modelled after twin 5c8) very quiet after playing a while

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  • #1

    Cathode biased 6L6 amp (modelled after twin 5c8) very quiet after playing a while

    I have an older very uncommon amp which was in need of some replaced components which I rebuilt following the twin 5c8 schematic which seemed to have similar tranny's and tubes.
    Here's basically how I have the amp wired up, except with a different tone stack, And I'm just using 6sl7's and a 6sn7 tube in the preamp.
    Fender Twin Amp 5C8 Schematic - KBapps.com

    Anyways, If I crank it way up and play on it for about 2 minutes, it starts to get quieter and quieter, until it's sort of a very low fizzly sounding volume. I thought maybe it was the old tubes I had in the preamp (since I initially did have it set up using these old metal case 6sc7's). So I picked up a new sovtek 6sl7 and rewired the sockets for that.
    And I still get the same thing. All of the resistors and caps are new, but the tranny's are original (hammond), the rectifier tube and 6L6's also are original, and the 6sn7 I'm using as a PI is original too.

    What else could be causing this? Do I need to try all new tubes? Could it be the rectifier or the OT?
    I'm hoping this particular behavior points to something in particular.. If I let it sit for a while it'll start out really strong, and it sounds really really good, and then after about 2 minutes of playing with it cranked right up it's a very weak sounding fizzly output. If I keep the volume low then it seems to stay alright at that low volume without going into the quieter state.

    Any thoughts?
    Tags: None
  • #2
    If you don't have an oscilloscope, the next best troubleshooting tool would be to monitor DC voltages with a DVM and look for a shift when the amp is malfunctioning. Start with pin 8 of the 6L6s. Does the voltage shift when the amp gets quiet? A bad rectifier would show a voltage shift at just about every point in the amp.

    It could be tubes, always a good idea to try known good ones. Is the speaker good? Have you tried it with another amp? It could be the OT, but you should check voltages and tubes first unless you have a spare OT laying around.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    • #3
      I actually did get ahold of a friends o'scope recently, I'm just not very familiar with using it. I don't have any sort of signal generator though that I could measure a waveform on the amp.

      I'll check the voltages when it starts malfunctioning. I know the speaker is good, I use it with another amp fine for extended periods of time.

      If I confirm that the voltages stay steady, would it just be a matter of getting a sine wave through on the input and then checking that with the o'scope at the output of each stage of the amp?

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      • #4
        Originally posted by loudthud View Post
        If you don't have an oscilloscope, the next best troubleshooting tool would be to monitor DC voltages with a DVM and look for a shift when the amp is malfunctioning. Start with pin 8 of the 6L6s. Does the voltage shift when the amp gets quiet? A bad rectifier would show a voltage shift at just about every point in the amp.

        It could be tubes, always a good idea to try known good ones. Is the speaker good? Have you tried it with another amp? It could be the OT, but you should check voltages and tubes first unless you have a spare OT laying around.
        Thinking about what you said, he is nearly describing the way an amp behaves when you hit standby and the voltage drains down.

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        • #5
          Originally posted by mikeboone View Post
          Thinking about what you said, he is nearly describing the way an amp behaves when you hit standby and the voltage drains down.
          That is totally what it sounds like!
          I think what you're saying is if say you're playing the guitar and you continue to play as you switch to standby the output quickly dies, but before it does the sound gets quiter and sort of fizzles out to nothing.
          What I'm seeing is it getting to this quiet fizzly type state and staying like that until I let it sit for a bit and then try again.

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          • #6
            It sounds like something is causing your high voltage to fail when pushed hard. To LoudThuds point, check and record the voltages when you turn it on, then play it until it fails and check and record in the same place.

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            • #7
              Okay, so I just did a quick test, the voltages definitely stay solid when the fading starts to happen.. So I think the rectifier is fine.
              Even though I didn't want to rewire another tube socket, I decided to try rewiring the PI for one of the spare 6sc7's I had, just to see if maybe the 6sn7 I have in there is bad.
              Besides being WAAAY too gainy and poppy and microphonic the 6sc7 (which has seen its day) seemed to be alright, so I think the PI is the culpruit here. I banged away on the guitar for a good 5 minutes with no fading at all.
              I guess I'll be ordering myself a new 6sn7.

              I appreciate the help, I should have just tried this in the first place.

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