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Plexi Marshall blowing HT fuse.

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  • #1

    Plexi Marshall blowing HT fuse.

    Hi,
    I've been working on this old Marshall for quite some time. It keeps blowing it's 1 amp H.T. when the amp is turned on cold. I've replaced the tubes and all the filter caps with no luck. The output transformer seems fine. I've put it up on an oscilliscope and the waveform doesn't show anything odd. If I bring the amp up slowly on a variac it works fine. I can play it for an hour, turn it off, turn it on again and it still works. The next day when it's cold I turn on the power, wait a minute, hit the standby and the fuse blows. The only thing I noticed is when I was checking the pin voltages with no tubes in it the plates were at 560 Volts. I'm wondering if I have a shorted choke. I've never had a bad one before but that might explain the high voltages and why there is a surge at turn on. Any help would be appreciated.
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  • #2
    No, your high voltages are explained by the fact the tubes are not in it. Put the power tubes in it and the voltage will drop below 500. VOltage readings without tubes in an amp are generally meaningless, unless you specifically need that condition for some troubleshooting purpose.

    Are you using a 1 amp SLOW BLOW fuse? You should be. That is what the T in T1A means bu the fuseholder - T for "time" delay.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Marshall Blowing HT Fuses

      I realize that the voltages go down with the tubes in there but that's when it blows the HT fuse. Like I said I tried a brand new set of Groove tubes in there with the bias set so high the tubes were almost off. It still blew a fuse. I don't see any sign of shorting or arching around the tube sockets. I also have been using slow blow 1 Amps. Any ideas? Thanks.

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      • #4
        You are saying then that with tubes it blows the fuse upon cold power up. Are you also then saying that without tubes the fuse holds? Or does a cold boot take the fuse either way?

        A shorted choke seems unlikely, but if you want to be sure, disconnect the thing and tack a resistor in its place. Value doesn't matter much, 500 ohms maybe.

        Here is a specific reason to test without tubes: with no tubes, there will be almost no current draw on the B+ supply. SO there would be no voltage drops across the resistors in the B+ string. So without tubes, go to the end of the string and see if the voltage is pretty much what it is at the start of the string. SInce you replaced all the caps, it seems unlikely a leaky one is in there though. But if there is a drop somewhere, find out why. A small few volts would be normal.

        Don't know what I am looking for exactly, bt you could disconect one side of the OT at a time - pulling B+ from half the power tubes - to see if one side reacts different from the other.
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          Marshall Blowing HT fuses.

          Hi Enzo,
          Thanks for the reply. The amp only blows the HT fuse with tubes in it after a cold start up. Without tubes it seems fine. With tubes bringing it up slowly on a variac it's okay. I can play the amp for an hour at 120V. Plugging my Gerald Webber current-draw devise in with the light bulb inline with the AC. No problems. Plug the amp in strait to the wall. Turn the power on, wait a minute, and hit the standby and the HT fuse blows. It doesn't always blow the fuse if the amp is still warm or even an hour later but it will always do it the next day. Just to say it again I have tried a new set of tubes in there and have changed all the filter caps. I have some more time now to investigate this so I'll post more information as I find it.

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          • #6
            Re: Marshall blowing HT fuses

            Did you find out the cause? I have one doing the same.

            thanks,

            dave

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            • #7
              Check to see if the bias supply powers up in the standby position. This was a factory error in the design of the Plexi, and was corrected with a service bulleton. The cure is to move the bias feed wire from the cold side of the standby switch to the hot side. Strangely enough - I see newer production Marshall amps that have been produced with the exact same fault. Without the bias supply present on turn on, the power tubes will draw massive current until the bias voltage stabilizes.

              RE

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              • #8
                Tube Amp Debugging Page - http://www.geofex.com
                Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

                Oh, wait! That sounds familiar, somehow.

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                • #9
                  BLowing HT fuse

                  Narrowed it down to just the design flaw. Everything else on the amp is in perfect working order. It doesn't blow all the time, just every so often.

                  Thanks,

                  Dave

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