6AQ5 tube up in smoke. Help me diagnose.

Discussion in 'Amp Tech Center' started by Whatizitman, Oct 17, 2019.

  1. Whatizitman

    Whatizitman Friend of Leo's

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    I'm fairly new to working on amps, so this is foreign territory for me. Is it common for tubes to pop and burn up like this when they die?

    I just acquired a Musicmaster Bass Amp. One of the very early ones, probably all original tubes and caps. I added an inline fuse, made sure plug was properly grounded, took out death cap, removed and jumpered input cap (typical mod for guitar use).

    I have not changed caps or tubes. I just wanted to make sure everything was working before I did any other work on it. It worked gloriously for about 2 minutes, then started cutting out, distorting in a nasty way, and eventually died. Fuse didn't blow. Tubes and light were still glowing. I changed the output tubes around and no change. I then swapped the preamp tube for a known good one. No sound for a minute or so. Then a pop and smoke.

    When I opened it up at first I couldn't see any noticeable physical damage. So I went about sniffing around the chassis to see if I could locate the area that burned with my nose. I took out the tubes - one of the output tube smelled, and has a clear burned area. I checked the socket, and sure enough Pin 5 on the socket is black. That's the plate for 6A5Q tubes. The other tube and socket have no burn.

    There was no noticeable redplating before this. But the plates are actually hard to see in these tubes. I will go about getting some numbers on components, and will change caps and tubes before I do anything else. But right off the bat, what are some causes of tubes blowing up like this? Just getting old? Too much current? Poor bias? Could it be a power issue, or one of the transformers failing? Is the plate the typical point of failure? Can bad filter caps be culprits? Any insight will be helpful. Thanks!
     
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  2. muchxs

    muchxs Doctor of Teleocity

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    The pin spacing is so close on 6AQ5s and the wafer sockets are so cheap in Musicmaster Bass amps, they tend to arc across the socket. Once they establish an arc trail, that socket tends to arc.
     
  3. muchxs

    muchxs Doctor of Teleocity

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    Causes: It might have a large AC component (hum) in the signal due to spent filter caps. That's old caps for you. Sometimes they'll re- form if you bring 'em up slow on a Variac. Most of the time it's a good idea to start fresh and replace 'em.

    Show the people pics of your new toy. Little bird told me it's a rare early MMB. No tag board, it's all on a couple terminal strips.

    Let's see a close- up for the burned socket, views from inside and outside the amp.
     
  4. Whatizitman

    Whatizitman Friend of Leo's

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    I put some pics in the mafia thread. I'll post them here for all to see. Not a good pic of the socket, but pin 5 is clearly toast.

    Just my luck that these sockets are impossible to find. Are they repairable in any way? Solutions? Convert to noval/EL84?. Not sure I wanna go down the 6V6 route, but if I can't keep the amp stock anyway..... Hmmmm. I guess that would make sense since the later ones didn't change PT/OT, right?

    Definitely gonna change the caps. But now I'm gonna have to deal with the tube socket issue before anything else... :oops:

    EDIT: Yup. Very early model - first year production. I kinda wanted to keep it stock because of that. Oh well....

    [​IMG][​IMG][​IMG]
     
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  5. Whatizitman

    Whatizitman Friend of Leo's

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    Wait. I kept typing 6A5Q. No wonder I couldn't find anything. Edited title. Correct googling brought up Triode USA. I'm sure there are others. Thanks, @corliss1. I will check those out, too.
     
  6. muchxs

    muchxs Doctor of Teleocity

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    That's because I bought them all.

    Seriously, I think Fender got them when Emerson Radio closed up shop in Hoboken, NJ.

    Tube junk from New Jersey's defunct consumer electronics manufacturing was available on Canal Street in NYC through the 1980s.
    Reason I wanted to see the other side of the chassis was to see the hole size. How big is the hole? What are the mounting centers?

    7 pin sockets typically fit a 5/8" hole with 7/8" mounting centers. 9 pin sockets fit 3/4" with 1 1/8" mounting centers. Knowing what I know about the MMB Fender probably used something weird.

    Every other Fender amp of the era retains its sockets with sheet metal screws. This MMB has rivets.
     
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  7. Whatizitman

    Whatizitman Friend of Leo's

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    Right. Rivets. Same as the multivox. Cheap and quick way of mounting, I guess. But I have no tools to punch those out. I thought I recall the D-Lab guy just drilling through the rivet in some video? Not sure of the hole size. I will measure. Apparently ceramic 7pin sockets I'm seeing online are 5/8". Not sure about the mounting holes, though. I don't think those match up. Would have to drill anyway.
     
  8. corliss1

    corliss1 Friend of Leo's

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    Yup, there's not a tool for it. Just drill it out.
     
  9. Whatizitman

    Whatizitman Friend of Leo's

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    OK so I'm just gonna add on ceramic sockets to my to-do and shopping list. I'm hoping to not have to put in too much dough to get this running regularly. But I did buy it as a project amp. So I'm not too bumbed. I will return with pics and war story after I attempt to change out the sockets.

    In the meantime, anything else I should look into? What type of extra damage can an arcing tube do?
     
  10. corliss1

    corliss1 Friend of Leo's

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    There's literally a handful of parts in these, so I doubt it'll take much to get it working.

    Besides the tube and the socket, just check the resistor values and make sure they're in spec. If this had hit my bench it would have been power cord and caps before any power was applied, and then see what else it needs, if anything. Not to say you still wouldn't/couldn't have had the tube/socket issue.
     
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  11. Whatizitman

    Whatizitman Friend of Leo's

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    Yeah. My plan was to change out caps first. But I got a little impatient. Home Depot had a nice inline fuse holder in stock, and I had some fuses.... Power cord was grounded fine - just needed a new plug. Fuse didn't blow, but I'm not sure if that's good or bad in this situation. I did find that the preamp tube is bad, as well.

    I will be ordering new tubes, complete set of filter, bypass, and coupling caps. No hurry on that. I can check the resistors and replace anything that looks too far out of spec.

    I just got...excited. Live and learn, right?
     
    muscmp and corliss1 like this.
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