1958 Bell Sound Model 5630 Head scratcher

Discussion in 'DIY' started by Sean Colvin, Feb 12, 2018.

  1. Sean Colvin

    Sean Colvin New Member

    Messages:
    5
    Hello everyone, this is my first post, and it might be a real head scratcher.
    I recently purchased my first broken tube amp with the intent of fixing it, and now I've reached the point where the knowledge I seek is not readily available online. I see many experts on here and would love some help.

    What I know (or don't): The amp is original except it's had the filter caps done. The large older caps were left sticking up off the top of the chassis for looks. Below you can see what the original caps were on the schematic. After the cap job the amp supposedly ran for a few hours then started blowing fuses. Then it sat for a few years until I bought it. I took all the tubes out, turned it on and it's still blowing the 250v 2a slo-blo. One funny thing I noticed is that one of the large old caps above the chassis was actually not to spec. The cap in the manual known as C2 was supposed to be 20uF, 450v, but rather, you can see it's actually 16uF 500v. But none of this should matter since the filter caps were replaced, right? Well, I wonder if any damage could have been done previously to the amp because of this part. The power transformer looks like it's gotten real hot at some point in the past and has some tar on its surface.

    Another alarming thing is that the diode of the old Sprague (C2) Cap is being used to connect all sorts of wires together still (also pictured). As far as I can tell, it's still fully connected to the circuit and running in parallel to it's replacement. Is that okay?

    Speaking of its replacement, I noticed that the new, smaller caps also have a different capacitance than the schematic calls for. C1 and C2 are supposed to be 20uF, 450v capacitors, but the ones installed under the chassis (pictured below) are in fact 22uF 450v. Does this matter?

    Next, I noticed that on the schematic, pin 8 of the 5U4GB rectifier tube is supposed to be connected to the 20uF(C1) cap, then on to ground. Instead, the 8th pin is not at all connected to C1, rather the 8th pin is connected the first pin by a very short wire, then pin 1 is connected to a (newly installed) 47uF 500v capacitor(???), before it's connected to a 22uF(c2) cap (like it's supposed to) and so forth on its journey. My question is, what is this 47uF capacitor doing here? The schematic does not (as far as I can tell) call for one of these anywhere in the amp.

    I realize that I may just be grossly misunderstanding something, but that's why I'm here. IMG_0070.PNG IMG_0068.JPG I will be very grateful for any help. Cheers. IMG_0062.JPG IMG_0069.JPG IMG_0063.JPG
     

     

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  2. UncleBingo

    UncleBingo AK Subscriber Subscriber

    Messages:
    777
    Need to see the rest of the power supply to help. C1A and C1B (I think- the pic is cut off) look to be 20uf connected in parallel. If so that makes 40uf. OR, C1C or C2C are spec'd at 50Uf and whoever worked on it had a 47uf (close enough to 50) handy at higher voltage, and put it in there to serve as one of them. Hard to say without more schematic.

    Also, it looks (in the pic) like C2 is still in the circuit, and one of the 22uf/450 caps is paralleled across it...

    As far as the cap values- the standard value in UF as manufactured have changed. 22uf for 20uf, 47uf probably okay for the 50uf. Voltage rating the same or greater! Todays line voltage is higher and the extra voltage rating wont hurt anything (maybe the wallet, but not much).

    Pin 8 jumper to the unused pin 1 on that tube is just acting as a tie-point. More room to attach wires instead of trying to jam them all onto the pin 8 lug.
     
    Last edited: Feb 13, 2018
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  3. Sean Colvin

    Sean Colvin New Member

    Messages:
    5
    Bingo,
    I suspected that about pin 1. Thanks so much. So, is the fact that C2 is still wired in a potential problem, being that it's 60 years old? Here's the rest of the schematic of the power section. IMG_0072.PNG

    One other burning question I have is, are C1A and C1B two different capacitors? Why are they listed as being the same part but have different capacitance ratings? What do the triangle and the square next to their name on the parts list signify? This stuff is sort of like hieroglyphics to me still. It looks like they are wired in parallel to ground. Is that correct? Bad filter caps would just make the amp noisy right, not cause fuses to blow?

    Thanks for entertaining these entry-level questions!
    -Sean
     
  4. UncleBingo

    UncleBingo AK Subscriber Subscriber

    Messages:
    777
    C1A, C1B and C1C comprise a multi-section cap in a single can. Not unusual back in the day. They share a common terminal (the metal can) and the triangle and square denote which lug belongs to the + lead of each capacitor section; the outer lugs are the "common" or ground lugs (Though in the case of a -voltage bias cap, the metal can will NOT be connected to the chassis, but will be "hot" in relation to ground). I learned this the hard way...Zing! So be careful. That's why bias or "Negative can" caps will have a cardboard sleeve.
     
    Last edited: Feb 13, 2018
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  5. UncleBingo

    UncleBingo AK Subscriber Subscriber

    Messages:
    777
    According to the schematic, the 47uf at 500V cap is acting as C1C (or it could be C2C, perhaps) and goes from ground to pin 8 of V7 which is a 6L6 according to the schematic. If it's connected to pin 1/8 of the 5U4, something is fishy or we need a better gut-shot of the interior of the amp to see what goes where in more detail..
     
    Last edited: Feb 13, 2018
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  6. Sean Colvin

    Sean Colvin New Member

    Messages:
    5
    UncleBingo,
    Thanks again. I've got a lot to work through with this amp but I may follow up with this later. I'm going to give the last tech who worked on it the benefit of the doubt and hope that he knew what he was doing. For now I'm going to check out the power transformer and work my way through the circuit.

    One question that I also can't find any information on is, why are there three 'dummy' tubes next to the 6v6's? You can see them in the foreground of the second photo in my original post. They seem to be empty inside. I can attach another photo if needed later. Cheers.
     

     

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  7. gadget73

    gadget73 junk junkie Subscriber

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    35,201
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    If the tubes are out of it, particularly the rectifier tube, the caps simply do not matter. They are not connected to the fuse in any way. If you're blowing the 2.5 amp fuse with no tubes in it, one of the transformer circuits has a short outside the transformer, or the transformer is internally shorted. If the transformer shows signs of overheating, you may have a bum trafo on your hands. To confirm you can unhook all of the secondary wires and try it again. if it blows the fuse with the transformer connected to absolutely nothing, the transformer is toast.
     
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  8. Sean Colvin

    Sean Colvin New Member

    Messages:
    5
    Gadget, that will be the next thing that I try, along with making sure there are no shorts in the AC power circuit before the transformer (very unlikely).

    One thing I'm still wondering is, what does this section of the schematic represent? There seems to be a 'dummy' tube installed into this spot with no apparent function than to complete the circuit. It's a short, like 1 1/4" hollow aluminum cap with 8 pins. What is it's function? There are three of them, one in each preamp section (it's a 3-channel PA).
     

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  9. UncleBingo

    UncleBingo AK Subscriber Subscriber

    Messages:
    777
    They may be sockets for microphone input transformers.
     
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  10. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    35,201
    Location:
    Southern NJ
    yep, mic impedance matching transformer. Common on PA gear. Some things had a switch that either routed through the trafo or bypassed it. Looks like yours requires either a transformer or a bypass/dummy plug to make it work, depending on what impedance is in use.

    Also possible its for balanced / unbalanced mic use. The tube itself is an unbalanced input, but the XLR connector implies a balanced line. A 1:1 balun would get used to make that work.
     
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  11. Sean Colvin

    Sean Colvin New Member

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    5
    Ya'll are the best, thank you!
     

     

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