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Thread: Have Juke Tube Amp can't repair correctly thinkin abt swappin Mosfet Amp pros/cons?

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    Have Juke Tube Amp can't repair correctly thinkin abt swappin Mosfet Amp pros/cons?

    Hi all,
    I have a 1947 seeburg trashcan jukebox that has a tube amp. I got the whole jukebox working except for the amp. Changed all the cap, out of tolerance resistors, etc. Still doesn't work. I was just wondering the pros and cons of using a mosfet amp instead of the original tube amp.
    Sending it out for repair would be very expensive due to the heavy shipping weight.


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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Thomas57 View Post
    I have a 1947 seeburg trashI was just wondering the pros and cons of using a mosfet amp instead of the original tube amp.
    A friend did just that with his jukebox. He bought a reasonably priced Behringer bass amp and is very pleased with the result. Cost for the amp was about $250 if I recall, it delivers 150 watts, has 5 eq controls. Juke man added a separate speaker externally for mid/hi clarity, with an L pad on that speaker to balance it against the juke's built-in 15" woofer. For all that he has more volume than he could ever want plus a great deal of tone control. If he ever gets a buyer for the jukebox they can elect to take the bass amp or not, and if not he has a bass amp he can repurpose or sell. Drop in a quarter and rock on everybody!

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    Senior Member Enzo's Avatar
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    I used to repair and rebuild jukeboxes, mainly Seeburg in this part of the country.

    I don't recall on the trashcan specifically, but on MANY Seeburg amps the 6v heaters ran all the time, but the HV rectifier didn't come on until the mechanism went into action to play a record. SO if it won't make sound, are all the tube heaters lit? Including the rectifier? Is there high voltage in the circuits?

    Test the speaker connections: find the wires connected to the chassis for speakers, Pull them off and touch them to a 9v battery briefly. Do you hear a pop or thump from the speaker? If not, the speakers or their wiring are not right.

    My Seeburg manuals are all in storage, do you have the schematic for this amp, you could post?

    The Juke itself has a model number, but the amp chassis itself should also have a model number. Is there a serial number plate on the amp that includes a model number?
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    Speaker passes battery test. Amp is model MA1-L6. Schematicma1.jpg attached

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    Senior Member Enzo's Avatar
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    OK, what does "doesn't work" mean? Tubes don't light? Tubes light but no sound? etc?

    Are B+ voltages present? Is there a volume control assembly plugged into the correct jack? YOu said the speaker works, is it the original field coil speaker or is it some permanent magnet speaker someone has installed. If the latter, did they put something in place of the field coil in the circuit? Note the center tap on the B+ winding goes to ground via a plug, so if ther is no jumper in that socket, there will be no high voltage.
    Education is what you're left with after you have forgotten what you have learned.

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    What does "doesn't work" mean? Tubes don't light? Tubes light but no sound? etc?
    Tubes light but no sound.

    Are B+ voltages present?
    Is there a volume control assembly plugged into the correct jack? Yes.

    Is it the original field coil speaker or is it some permanent magnet speaker? PM Speaker with appropriate resistor to replace field coil. Seeburg did that also in mid production.

    If there is no jumper in that socket, there will be no high voltage.[/QUOTE]
    Jumper in place

    Amp is coupled with another unit SDU (Solinoid Drum Unit) which picks the records and supplies 115v to the amp and 6.3v to the tubes. Problem is I can't bench test it as it uses a jones plug / socket. Also tested with by using a guiter amp, and it works fine. So problem in Jukebox is isolated to amp.

    Thx for your help

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    Senior Member Enzo's Avatar
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    I have a cord I made up with an AC power cord wired to a Jones socket for 120vAC plus a couple wires for the 6v which I plugged into a bench supply.

    The schematic you post shows one power transformer including 6v. Is your actual unit receiving both 120v and 6v on separate pins of your Jones plug?

    I also have around the shop a couple "death cords", which are AC power cords with alligator clips on the end of the wires. They are used spcifically when something like a male Jones lacks a mate.

    In any case, it should be possible to run this on the bench with some sort of temporary power connection.

    In later models, the 6v was always on, so the tubes stayed hot. But the 120v was only switched on by selections being made. The 120v powered the 5v rectifier heater and the B+. So those started cold until selection was made. Point being if yours is a later type with separate 6v and 120v, make sure the 120v is coming on when selections are made. If you sub another amp in place of this one, it won't be thus controlled, so you might be overlooking a bent/dirty contact in the 120v system in the controller.

    So we still need to know if B+ is present or not when it should be there.
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    Both the two 6L6's, and the rest of the amp is powered on only when record selections are made. The amp is supposed to receive the 6.3 and 120v on separate pins. I guess I will obtain a filament transformer and make up a cord like you mentioned.
    Last edited by Thomas57; 07-05-2017 at 04:05 AM. Reason: Added Infor

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    Senior Member Enzo's Avatar
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    You have a working guitar amp? You could pull the tubes and make up a cheater cord. Clips on the jukebox end and a old octal base to plug into a socket wired to the 6v pins. Or if you are into conectors, I bet a pair of Molex 09 series pins would fit an octal socket. Just thinking of what is at hand. And a transformer need not be dedicated, if you have a spare guitar amp power transformer around, tape the high voltage wires and clip to it.

    So we know your unit differs from the posted schematic at least as far as AC power connections.

    And L2 top center appears to be connected to a muting circuit on the 6-pin Jones? MAke sure that mute switch is not stuck closed.

    Even before we get it to the bench, is the underside accessible in juke? So we can take readings on the tube socket pins while it runs?

    I used to do this for a living, so I made up all manner of test cords and fixtures. Would like to save you the trouble if possible.
    Education is what you're left with after you have forgotten what you have learned.

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    Working guitar amp? Pull the tubes and make up a cheater cord. Clips on the jukebox end and a old octal base to plug into a socket wired to the 6v pins. I bet a pair of Molex 09 series pins would fit an octal socket. A transformer need not be dedicated, if you have a spare guitar amp power transformer, tape the high voltage wires and clip to it. The unit differs from the posted schematic at least as far as AC power connections. And L2 top center appears to be connected to a muting circuit on the 6-pin Jones? Make sure that mute switch is not stuck closed.
    Even before we get it to the bench, is the underside accessible in juke? So we can take readings on the tube socket pins while it runs?

    I used to do this for a living, so I made up all manner of test cords and fixtures. Would like to save you the trouble if possible.


    The guitar amp is transistor one unfortunately. I do have an oldddddddddddd variac, so I could get close to the 6.3v (could meter the output) but what about ampere? Underside is not accessible.

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    Senior Member Enzo's Avatar
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    No, please do not use a variac for the heaters, too dangerous.

    On many later models, the amp and control center plug into each other and are bolted to the rear panel A sub panel could be removed to uncover the underside of the chassis, I was hoping the trashcan had similar. Too many years for me to recall.


    Not much we can do without getting it powered up while we have access to the innards.

    Well, one way: pull one or the 6L6 tubes, and probe in the socket pins. Is there several hundred volts on pins 3 and 4?
    Education is what you're left with after you have forgotten what you have learned.

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    Cool

    Quote Originally Posted by Enzo View Post
    No, please do not use a variac for the heaters, too dangerous.

    On many later models, the amp and control center plug into each other and are bolted to the rear panel A sub panel could be removed to uncover the underside of the chassis, I was hoping the trashcan had similar. Too many years for me to recall.


    Not much we can do without getting it powered up while we have access to the innards.

    Well, one way: pull one or the 6L6 tubes, and probe in the socket pins. Is there several hundred volts on pins 3 and 4?

    I will give that a try tmr or during the week, it's close to 2:30am in the morning here. Time to get some sleep.

    Thanks for all of your advice. I will pick your brains further, later

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    You were right, 6.3v is on all the time, I took readings off the SDU1 which feeds pow

    On MANY Seeburg amps the 6v heaters ran all the time, but the HV rectifier didn't come on until the mechanism went into action to play a record.

    You were right, 6.3v is on all the time, I took readings off the SDU1 which feeds power to the amp. HV doesn't engage until a record is selected.

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    Senior Member Enzo's Avatar
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    So was that part of the problem? The amp wasn't working because it wasn't getting turned on by a selection?
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    Quote Originally Posted by Enzo View Post
    So was that part of the problem? The amp wasn't working because it wasn't getting turned on by a selection?
    No, if I missed that I would kick myself in the head!
    There is a Solenoid unit (SDU1) that feeds the proper voltages to the amp. What I did was measure the voltages from the SDU, so as you stated the 6v is always present, but the HV is not until a record is made. With the amp in place I get no sound, the speaker is good, and to recap if i used an external amp, all is good. I ordered a jones plug and socket, and a tube transformer, so I can make up a cable to test the tube voltages.

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    Senior Member Enzo's Avatar
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    OK, one other simple thing, the mute. The 120v only comes on when selections are made. So teh high voltage is only present then. But they also mute the box between selections, this so that while it is moving records around, the amp doesn't amplify mechanical noises into the speakers. Look on the left at the 6-pin jones, the two right pins are wired up and over to an inductor and resistor tied to the signal path. When a selection is made, but record not playing, it should mute the amp there. So check that, see if that resistor is being grounded, or lift the wire to it from the jones plug.

    Your external amp tests the speakers and the tone arm pickup, but it bypasses both the power thing and the muting. Worth a check.

    But if it is none of that, I'd start by making sure I have B+ throughout, and I'd isolate the problem. You could inject a signal at the volume control jack, does it come through? At this point I;d be looking for a go/no-go test, so even just hum is a good test. You can also see if the input signal is reaching teh volume control. The control is halfway through the amp and accessible, so a good place to start.
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    Senior Member Enzo's Avatar
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    By the way, when you changed ALL the caps, did you mean ALL of them or just the electrolytics? It is my experience that ALL the caps will be leaky in the old Seeburgs. The paper caps I replace with film.
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    Quote Originally Posted by Enzo View Post
    By the way, when you changed ALL the caps, did you mean ALL of them or just the electrolytics? It is my experience that ALL the caps will be leaky in the old Seeburgs. The paper caps I replace with film.

    All lytics and caps, and a few out of tolerance resistors.

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    Senior Member Enzo's Avatar
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    On more recent models than yours, there is also an AVC circuit - Automatic Volume COntrol - or something like that. Some records are louder than others, so this AGC variant controls the overall volume to bring louder ones down and quieter ones up, so all are roughly the same loudness for any given volume control setting. But it is subtractive, so for home jukes, I often disable that feature and a simple rewire to the switch, and it gets me a few free extra decibels of loudness for them.
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