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Thread: cathode cap question

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    Supporting Member Randall's Avatar
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    cathode cap question

    Working on a 50's juke box amp. One of the cap cans is a 30/20/20/25 where the 30/20/20 are 500/400/400v and the 25uF is 50v and is the 6L6 cathode bypass cap. I have a 40/20/20/20 @ 525v that I'd like to use. I know it is OK to go over voltage rating, but will 10x over make a difference in this case? And would going under capacitance a little make a lot of difference in a cathode? This section measures 21.7 uF .

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    In the fifites, cap value tolerances may have been +100 to -50%, so 21.7uF is fine.
    As voltage ratings increase, parasitic ESR and dc leakage do too (all else being equal) but the cathode bypass application isn't very demanding, so neither should be a problem.
    But if you're in any way concerned, for an extra $0.5 leave the can cap section unused, and fit an external cathode bypass cap

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    Lifetime Member Enzo's Avatar
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    You know how I like to say "it's just a guitar amp"? Well, I worked on juke boxes for many years and really...it's just a jukebox amp. There is nothing critical. Go ahead and use the high voltage section if it is convenient. Though like pdf said, a 25uf/50v is tiny and cheap, and easily tucked into the circuit on its own.

    25uf is not a standard value like it was back then, 22uf is the current standard value.

    What make and model jukebox amp?

    I don't know if you have gotten there yet, but I can almost guarantee you will need to replace most of the coupling caps. They were often waxed paper-types.I rebuilt a zillion Seebergs, but also saw a lot of Wurlys, and some AMIs. Fewer Rowes, and for some reason, not many RockOlas. I saw my Rockolas mainly from the solid state era.

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    Supporting Member Randall's Avatar
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    "What make and model jukebox amp?"

    Wurly 530. Someone already did most of the coupling caps, but I'm going to check them all. I'm under time pressure on this, the customer wants it for his granddaughter's birthday party in a couple weeks.

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    Lifetime Member Enzo's Avatar
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    I just go down the circuit path looking for unwanted DC, usually doesn't take long. On the Seeburgs, If the first old paper cap was leaky I just change them all right there.

    I still have a couple CObra styli if you wind up needing a needle. I think I can get at them.

    You may not want to get into it this far:
    I don't know the Wurly 530, but certainly a lot of juke amps have an AVC - automatic volume control. No, not the remote volume control. Some records are recorded hotter than others. So they come out louder. IN a restaurant or bar, they set the volume to their desired level, and don't want to have to turn it up and down as louder and softer records play. SO towards the end of the preamp, there is a branch and the audio is simply processed to assess its level, then a control signal is fed back to earlier stages to turn it up or down, so overall the volume is more consistent record to record.

    But just as a tone stack is subtractive and loses signal level, this AVC never lets things get to full output. SO for home units, I disable the AVC. On some models you can just yank the AVC tube. On others, there is a control for how strong the effect is, and I alter that. The net improvement is you can get all the volume the box has to offer.

    I won't detail the circuit unless you are interested.

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    Lifetime Member Enzo's Avatar
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    BY the way, if you are interested, on the expanded schematic you posted, see the 12AX7 right above the muting plug? That is your AVC. First stage amplifies the signal. VA for voltage amp. The second section is wired as a diode to make a rectifier. The audio signal is rectified to make DC. That DC controls the grid of the 12AU7, which acts as a variable resistor. That acts to control the level going into the 6AN8 cathode follower stage.

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    Quote Originally Posted by Randall View Post
    Working on a 50's juke box amp. One of the cap cans is a 30/20/20/25 where the 30/20/20 are 500/400/400v and the 25uF is 50v and is the 6L6 cathode bypass cap. I have a 40/20/20/20 @ 525v that I'd like to use. I know it is OK to go over voltage rating, but will 10x over make a difference in this case? And would going under capacitance a little make a lot of difference in a cathode? This section measures 21.7 uF .

    Only issue you may face is a bit lower actual capacitance than nominal capacitance on the 25uF, but as others have pointed out wherever electrolytics are used in guitar amps (or jukebox) you usually don't expect (or require) precision.

    In guitar amps, electrolytics are like nuclear bombs just drop it in there anywhere you hit, however far from the target, gets the job done.

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