HT fuse problem

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by LESTERWILLIAM, Jun 4, 2005.


  1. LESTERWILLIAM

    LESTERWILLIAM Member

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    posted June 03, 2005 09:31 PM
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    My 68 50 watt plexi is blowing the HT fuse when I turn the standby switch on but Not every time. We thought it was the output tube so we put new ones in and it still blows. then we replaced the filter caps and still the fuse blows when you turn the standy on. This is starting to cost $$$
     
  2. John Phillips

    John Phillips Member

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    One problem with some older Marshalls - with the dual standby switching on the PT secondary, rather than in the B+ chain after the first filter - is that the bias supply is derived from the wrong side of the standby switch, so as you flip the switch to on, you're running the tubes absolutely wide-open for a few seconds until the bias voltage rises. This is a design mistake that absolutely needs to be corrected - even if it doesn't blow fuses, it seriously stresses the power tubes everytime you use the standby switch, which is the exact opposite of what it's supposed to do.

    Luckily it's easy to fix - move the white wire that goes to the standby switch to the other end of the switch, so it's permanently connected to the wire from the PT.

    Secondly, check that the amp has screen-grid resistors fitted - Marshall didn't always around this time. Many modern-production tubes don't like being run without them - although old Mullards will take it, it isn't good for them either.

    If it's not either of those things, it sounds like you may have an intermittent arc or short somewhere nasty - hopefully nothing more serious than something like a tube socket, but I've come across a choke that did this too, and obviously OTs...
     
  3. John Phillips

    John Phillips Member

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    Hmmm... in the chassis pic, it definitely does have both the better bias supply arrangement and screen-grid resistors fitted, so it isn't that!

    It's hard to tell from the appearance of the tube sockets - they don't look obviously fried, but then again they don't always...

    Best advice: it sounds like it might be something not too simple - I would give Hogy at Komet amps a call. He's in your part of the world (Louisiana, anyway) and is a world-class expert on old Marshalls. He may be able to help more easily than I can from over here :).
     
  4. LESTERWILLIAM

    LESTERWILLIAM Member

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    Mike did a cap job and replaced the output tube and its still doing it. Are you sure it has the better bias supply arrangement?
     
  5. John Phillips

    John Phillips Member

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    Yes, definitely - you can see a white wire (the most visually prominent in the right-hand side of the chassis in your pic) running from under the end of the board to one of the terminals on the PT, where there are two diodes soldered. This means the bias feed is constantly on, which is the correct method. The standby switch has only two red wires going to it, not two red and two blue, so it's the earlier B+ chain type.

    This is a very early JMP by the way, it still has the extra tags on the PT for the filament winding (top left of the PT as you look at it in the pic), which means that it's delivering very high secondary voltages - it was designed for a tube rectifier, which has a built-in voltage drop. This means that it may be running at over the voltage rating of the filter caps, unless you've fitted 500V ones in place of the 450s. It doesn't necessarily indicate this as a possible cause (many originals run fine at over the cap voltage), but it's something else to check. These over-voltage models sound really great BTW... when they're working right!

    (For reference, it also appears to be a PA model - 1985 - since it has a common cathode network on the first tube, no bright cap on the Channel 1 mixer resistor, and a 56K resistor in the tone stack. These are actually very nice-sounding for guitar... warmer and fuller than the Lead model, although IMO they benefit from fitting the bright cap, which should be a 500pF on the 470K mixer resistor - the right-hand one of the two just above and to the right of the Channel 1 jacks in your pic - so you can blend the two channels for more tonal range.)

    This might be a tricky one to diagnose... one thing you might do is to try flipping the standby on while watching the inside of the amp very closely in a dark room - if there's an arc somewhere (like a tube socket), you may even see it - fuses aren't too expensive to keep trying for a bit. FWIW, I found the arc in that choke I mentioned by hearing it, almost by accident. I might not have found it otherwise, it didn't show up on a meter at all, as it was only doing it under the start-up surge.
     
  6. LESTERWILLIAM

    LESTERWILLIAM Member

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    Wow. I'm learning. Hogy thinks it is a bass model. I will try the dark room method but you know its want blow a fuse for days and it seems to blow it when I flip the standby on to soon but it did blow one time when I first got it while I was playing before we changed the tubes and caps. Paul at two tribes music in the UK where I purchased it says it never happened to him and he thinks it's because of the 120 volt operation in the US. Thanks for your reply.
     
  7. LESTERWILLIAM

    LESTERWILLIAM Member

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    I tried the dark room and I see no sparks. I will send more pics. Thanks for your time.
     
  8. LESTERWILLIAM

    LESTERWILLIAM Member

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  9. John Phillips

    John Phillips Member

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    Have a closer look at the power tube socket nearest the filter caps - I can't be sure from the pic (even with the better lighting in this one), but it looks slightly carbonized. If it is, that would cause this problem for sure.

    It's highly unlikely that it's anything to do with running it on 120V, that only affects the primary winding of the PT, which is on all the time when the amp is on, no matter how the standby is set. The secondary voltages are (or should be) exactly the same.


    (BTW, if it was a bass model it would already have the 500pf cap on the mixer resistor, which is the only difference between that and a PA one.)
     
  10. LESTERWILLIAM

    LESTERWILLIAM Member

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    This is what Paul at two tribes were I purshased the amp is saying:

    Hi Mark
    I have just checked the circuit again and that HT fuse should be antisurge
    and not fast blow ! it might be the simple answer.
    Those reservoir caps may be struggling to cope with the surge to the
    eletrolytic capacitors every time.
    Let me know how you get on.
     
  11. John Phillips

    John Phillips Member

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    That is possible... although generally a fast-blow is OK in the HT position (though not the mains).

    If it's not doing it every time, it is something to check. Try using an anti-surge fuse instead. Don't increase the fuse rating though - 500mA is more than enough unless there's a fault with the amp.

    Hopefully it's that simple.
     
  12. LESTERWILLIAM

    LESTERWILLIAM Member

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    What is a anti surge, is it the slow blow fuse
     

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