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Thread: ampeg reverberocket r12rb hum

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    ampeg reverberocket r12rb hum

    Hi,

    I am trying to repair an old reverberocket R12RB with a hum problem.

    It has 7591 7591 6SL7 6SL7 6SN7 6SL7 tubes.
    It has the bridge rectifier.
    It does not match the Joe Piazza schematic exactly.
    I don't have the original schematic because the back of the amp is missing.

    So.....

    For starters the OT was open on one side.
    No voltage at all on the plate of one of the output tubes.
    Replace with new OT from Fliptops.
    That is all good now.

    I also replaced the original power cord with a 3-prong grounded one.
    I did not see any death capacitor to be removed.

    Replaced the 6 main electrolytic caps.
    Got the new can from antique electronics.
    60 uf @ 525 V
    40 uf @ 525 V
    10 uf @ 450 V
    25 uf @ 50

    Also replaced the other two electrolytics 20Uf @ 500 V

    Amp sounds great except for the faint hum.
    When I plug it into my ballast lamp (200W) the voltages are reduced slightly and there is no hum at all.
    At full line voltage it hums faintly with no input and all controls turned down to zero.
    The hum is constant regardless of how the controls are set.
    I have also unplugged the reverb tank altogether and it still hums.
    Reverb does work fine, however.
    Tremolo works too


    I am not sure what kind of hum I am hearing in the speaker.
    I think it is 120Hz.
    I also see some ratty looking oscillation on the high voltage rail marked "C" on the schematic.
    This is for the phase inverter and the first input stage.
    The ratty stuff is there on the scope irregardless of the hum.
    It is present with no tubes plugged in.
    Shouldn't the new 10uf electrolytic cap take this stuff to ground.
    Is it possible for a brand new electrolytic to be bad?

    I am a little puzzled.
    Any help would be greatly appreciated.
    Thanks again for your time.

    Bob Savino
    Bergenfield, NJ

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  2. #2
    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by bobs426 View Post
    Shouldn't the new 10uf electrolytic cap take this stuff to ground.
    Is it possible for a brand new electrolytic to be bad?
    Yes. Pain in the neck but sometimes new parts aren't good as they should be. Try clipping another electrolytic filter cap in parallel with the suspect one. If the noise goes away you know what the problem is.

    Other hum/buzz source is the typical 7199 output inverter/drive tube. Some hum, some don't. Now they're very expensive & almost unobtainable, what a PIA to compare.

    There's a built in ground loop in many of those 60's Ampegs too, that will result in a low level hum/buzz that won't go away. You'll see a series of black ground wires looping from one eyelet to another, from the power supply and across the circuit board. In self-biased Ampegs I find removing one that links from the ground end of the output tube cathode resistor to the preamp section stops the hum. To check just remove one end of the link wire. Hum gone? remove the wire.

    Piazza schematics are (in)famous for inaccuracy.

    Welcome aboard MEF and good luck on that Reverberocket from an ex NJ boy, West Orange. Those old Ampegs can sound fantastic when they're working right.

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    Supporting Member Jazz P Bass's Avatar
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    I have had bad sections of new cans already.

    The only way to prove it out is to parallel another cap across the one in question.

    If the separate paralleled cap helps at all, I would suspect the internal section & disconnect it.
    Try the separate cap by itself.

    It would help if you would measure the idle Vac ripple at the speaker terminals & post the value.

    4mvs max is really good.

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    Supporting Member loudthud's Avatar
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    Did you replace the 7591 output tubes? An open OT on one side will tend to burn up the screen grid of the tube on that side. The imbalance in output tubes will cause the type of hum you describe. Check the screen resistors if the amp has them.

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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
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    Lifetime Member Enzo's Avatar
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    Seems you have a scope. Set up the scope to display one complete cycle of the signal you get when touching a finger to the scope probe. That is 60Hz. Now do not readjust the scope sweep, leave it alone. Scope the hum. If it also has one complete cycle on the screen, then the hum is 60Hz. if you see two complete cycles, it is 120Hz. it doesn't matter if the "sine wave" is real distorted on the screen, the frequency is the frequency.

    LT makes a good point, I don't trust tube testers, but if one tube is damaged, that might show up in one. One thing you can do is pick one socket, the side with good OT, and install ONE power tube. Listen to the hum and check current. Now put the other power tube in that same socket. Does it have the same level of hum or different? Are currents similar? Unbalanced push pull stages do hum. Balanced ones do much less.

    Isolate the problem. it hums with the controls down. SO pull the phase inverter tube. Hum stay or go away? That will tell you power stage or not. With the PI returned to the amp, pull V1, any hum reduction? Pull V2, same question.

    And I agree with Leo, that some of these old Ampegs were built with a hum. I recall a GU-12 or something like that came in with a hum complaint. I went nuts trying to find what had failed. Ultimately I discovered a ground wire between the filter cap ground and the transformer was the cause. I moved the grounding point to the other end of a four inch hunk of black wire, and hum solved. This amp had hummed from the day it left the factory, and it took 40 years for someone to fix it... me.

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    Don't forget the joker g1's Avatar
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    Does this schematic match?
    Attached Files Attached Files

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    No, it's close.

    This amp has a different tremolo circuit.
    There's a metal tube with what looks like a auto lamp inside and some sort of photocell?
    The wire coming off this photocell? goes to the grid of V2 (6SN7 ) and then to a 1M resistor to a ground lug bolted to the chassis.
    It's hard to make any sense out of this.
    I am wondering of a poor ground here could introduce hum into the grid here.
    I see hum on the plate of this particular tube.

    Here is a picture of Click image for larger version. 

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    Hi guys,

    Thank you all for your help.

    I was able to do some more troubleshooting.
    I think I can rule out the new filter caps.
    When I grounded the scope to the ground lug at the input jacks, the noise I was seeing is gone on the voltage rail "c".
    So, lesson learned, clipping the ground to the metal chassis is not necessarily the best place to ground the scope.

    I replaced the 7591 output tubes with new ones, no change.
    I believe the originals are OK, so we can rule them out.

    If I put in the original 6SN7 tube, hum goes away.
    The old tube tested weak in my tube tester, which is why I replaced it.
    With the new 6SN7 I hear the hum in the speaker, once the amp really warms up.

    This is what I see on the speaker, again with no input and all controls at zero.

    Click image for larger version. 

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    scope at .1 V/cm , 5 msec/cm


    I have traced the source of the hum back to the plate (pin 2) of V2 (6SN7)
    This is what I see.

    Click image for larger version. 

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    Same scope settings as above, 60 Hz if my calculations are correct.

    Here is the weird part.
    This amp does not match the schematic at this point.
    The tremolo circuit is completely different.
    When I go to the grid (pin 1) of this 6SN7 the input comes off the tubular device shown in the photo below.
    I think there is a light bulb and a photocell? similar to what fender did. Not sure.
    It's hard to make sense out of this.

    Here is a picture of the tremolo "thing".

    Click image for larger version. 

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    The photocell goes to pin 1 (grid) of V2 and then a 1M resistor to ground. But the ground is under a screw that supports the tubular device.
    Could there be a bad ground here?

    I am also going to try to find the ground loop mentioned earlier.

    Thanks again.

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    Last edited by bobs426; 12-16-2015 at 08:58 PM. Reason: missing picture

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    Lifetime Member Enzo's Avatar
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    Um... one 6SN7 makes the amp hum, and the other 6SN7 has no hum? Then I'd have to say the hum question is answered, the hummy tube is bad. Doesn't matter how new it might be. The old tube may test "weak" in the tester, but how does it sound in the amp? That is the real question. Tube testers are very limited.

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    Enzo,
    You are absolutely right.
    Thanks.

    Problem is solved...

    Brand new 6SN7 tube from EH was causing the hum, but only when it really got warmed up.

    Put the old one back in and problem is solved.
    Not sure now how or why it showed weak in the tube tester, maybe I had the settings wrong.
    Tested it again today and there is nothing wrong with it.
    I can't believe this drove me nuts for so long.

    So this amp sounds really good.
    Has all its original tubes, a new OT, and new electrolytic caps, plus 3 prong AC plug.

    Reverb sounds great, too.

    I have to return this amp to the gentleman who asked me to fix it.
    Anyone know where I can find another one?

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  11. #11
    Lifetime Member Enzo's Avatar
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    Just keep your eyes open. Watch online sources, but also make regular visits to area music stores, and don't forget pawn shops. Pawn shops usually don't service anything, so if you can find anything wrong with an amp, you can often point it out and get a reduced price. We can fix most anything if it makes sound.

    I can't believe this drove me nuts for so long.
    One thing experience teaches is never to assume things, and never think of reasons not to check something. For example, endless numbers of people have asked for assistance and claimed that such and such part CAN'T be the problem because it is new. Nope, new parts can be bad too. Not only that, new parts that were good, can be destroyed by the rest of the circuit the moment it is powered up. Any time a fuse blows during the repair process, we have to go back and check everything. Never say to yourself, "Oh, it COULDN'T be THAT." Because it COULD be.

    But frustrating or not, you saw it through, and now you have a good sounding amp.

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    Bobs426, you should be able to find quite a few old Ampegs locally, since they were built in New Jersey. Search Craigslist.

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    Member Usable Thought's Avatar
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    Quote Originally Posted by Enzo View Post
    Set up the scope to display one complete cycle of the signal you get when touching a finger to the scope probe. That is 60Hz. Now do not readjust the scope sweep, leave it alone. Scope the hum. If it also has one complete cycle on the screen, then the hum is 60Hz. if you see two complete cycles, it is 120Hz.
    This is a good trick that I discovered on my own when I was first learning to use a scope a couple of months back - I am pleased to see it is a real trick & not just something I found helpful because I don't have a clue!

    I also read that "LINE" input is useful when checking out hum or ripple, so that was another thing I did (since I was in fact chasing down hum). But that is just to ensure a good trigger, I guess - not related to the period.

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  14. #14
    Lifetime Member Enzo's Avatar
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    LINE sync? Yes, that syncs to the 60Hz mains, but 120Hz is just an octave higher, so it syncs as well. But even if it doesn't sync and the waveform crawls along, you can still usually see whether it has one lump or two, so to speak. I started doing that a long time ago because I was too lazy to flip on the signal generator and set it to 60Hz. 60Hz is in the air all around us for free.

    Hey, if you came up with something like that on your own, it is a real trick whether anyone else does it or not. What works works. When I train technicians, I show them my way, but I also tell them there are many other ways that also work. it isn't about right or wrong, it is about effectiveness. I have been doing this for many years, I have been soldering for over 60 years, for gods sake. But I still learn something every day, and I learn tricks of the trade all the time. I find a bit of joy in it when someone says "You know you can do THIS to find out THAT", and I was completely unaware of the technique.

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