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Thread: 135 watt Twin Reverb loud hum

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    135 watt Twin Reverb loud hum

    SO,,i had a late 70's Twin Reverb come in with a loud hum.

    It had been taken to another shop and had the following replaced: all filter caps,all bias caps and resistors,the cathode bypass caps,power tube screen and grid resistors(470 and 1.5k),the big 10 watt resistors in the power supply,bridge rectifier diodes,hum balance pot tested and working, tube matching pot tested and working,bias in acceptable range. New matched quartet power tubes,all preamps pulled and hum still exists. Speakers test well(JBL orange frame) Plate load resistors test well

    At this point im thinking an issue with a transformer. Not much else left?

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by lo-watt View Post
    SO,,i had a late 70's Twin Reverb come in with a loud hum.

    It had been taken to another shop and had the following replaced: all filter caps,all bias caps and resistors,the cathode bypass caps,power tube screen and grid resistors(470 and 1.5k),the big 10 watt resistors in the power supply,bridge rectifier diodes,hum balance pot tested and working, tube matching pot tested and working,bias in acceptable range. New matched quartet power tubes,all preamps pulled and hum still exists. Speakers test well(JBL orange frame) Plate load resistors test well

    At this point im thinking an issue with a transformer. Not much else left?
    Depends on the hum, a carelessly soldered connection will make all the hum in the world. Divide an conquer, is it in the preamp or the power amp?

    nosaj

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    Since the hum exists with all the preamp tubes out,id say power supply section.

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    Lifetime Member Enzo's Avatar
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    Absent preamp tubes - and I assume you include the phase inverter in that - that leaves the power amp.

    I don't know how it would be a transformer.

    Does it hum with power tubes removed?


    I don't care what they replaced, don't assume anything is OK. Is there ripple on the bias supply? Scope it.

    Are any of your power tube screen resistors open?

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    Yes, I pulled all six preamp tubes. It does not hum without the power tubes. I alligator clipped in new F and T caps under the dog house one by one,to make sure it was not a bad filter or bias cap. No change there. I verified all the screen and grid resistors were within spec and voltages on the power tubes were within spec.

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    l also swapped the new power tubes with another known good quad with no change in hum. Thepower cord was also changed and the outlet and ground switch bypassed and new power and standby switches installed as well.

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    Supporting Member The Dude's Avatar
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    100 ohm resistors for the filament artificial center tap good?

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    Lifetime Member Enzo's Avatar
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    Bias caps not in the dog house.

    But that is why we scope the supplies. Ripple is ripple, whether it makes sense to be there or not.

    Is it 60Hz or 120Hz? Be sure.

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    Quote Originally Posted by The Dude View Post
    100 ohm resistors for the filament artificial center tap good?
    This amp has the hum balance pot tied into the heaters at the V1. No resistors

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    Supporting Member The Dude's Avatar
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    Ok. So does the pot test good?

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    Quote Originally Posted by The Dude View Post
    Ok. So does the pot test good?
    At this point the scope will be your best friend. Once you find the answer you'll wonder how you looked over it so much. If you have had the amp for a while put it away go watch a game quit thinking about it then go back fresh eyes.

    nosaj

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    Quote Originally Posted by Enzo View Post
    Bias caps not in the dog house.

    But that is why we scope the supplies. Ripple is ripple, whether it makes sense to be there or not.

    Is it 60Hz or 120Hz? Be sure.
    On this one they are in the doghouse. This is the one with the rectifier diodes to the right of the powertransformer

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    Quote Originally Posted by Enzo View Post
    Bias caps not in the dog house.
    Wow.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	Doghouse Original.jpg 
Views:	27 
Size:	285.0 KB 
ID:	53361  

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    How loud a hum are we talking about? With all the preamp tubes still out report the ACV across the speaker. Check the frequency while you are at it.

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    Last edited by nickb; 04-19-2019 at 03:59 PM.
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    Quote Originally Posted by lo-watt View Post
    This amp has the hum balance pot tied into the heaters at the V1. No resistors
    I am a novice, but is the hum balance pot tied to an elevated ground ? If that connection is bad, you won't get the same degree of cancellation.

    But take what I said with a grain of salt if it was never tied to ground in the first place. Unlike others here, I am still taking shots in the dark...

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by HaroldBrooks View Post
    I am a novice, but is the hum balance pot tied to an elevated ground ? If that connection is bad, you won't get the same degree of cancellation.

    But take what I said with a grain of salt if it was never tied to ground in the first place. Unlike others here, I am still taking shots in the dark...
    Don't kid your self. You've shooting straight thus far in your posts.
    A bad solder joint can make a bad ground.
    nosaj

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    Does it still have the original 'output matching' pot wired up like stock? Does adjusting it affect the hum?
    Obviously, the previous shop used the 'shotgun' approach and it didn't work. Lack of troubleshooting skills have already cost the owner hundreds, I would highly recommend against continuing down the 'shotgun' road.

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    Master Destroyer nosaj's Avatar
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    How about some hi-res pics?

    nosaj

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    I suggest to find and post the appropriate schematic.

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Helmholtz View Post
    I suggest to find and post the appropriate schematic.
    http://ampwares.com/schematics/twin_reverb_sf_135.pdf
    I'd suggest taking the hum balance pot out of circuit and connecting it to a meter and doing a full sweep to check the ohm range and see if there are any descrepancies. I'd also use an analog meter.
    nosaj

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    Quote Originally Posted by HaroldBrooks View Post
    I am a novice, but is the hum balance pot tied to an elevated ground ?
    It is tied to the chassis right at the pot itself.

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by 66 Kicks View Post
    It is tied to the chassis right at the pot itself.
    Maybe in a stock one by the schematic, but at this point and already shotgunned amp we really do not know.
    We can only speculate.
    nosaj

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    Quote Originally Posted by nosaj View Post
    Maybe in a stock one by the schematic
    More than just by the schematic. Every one I have ever gotten my hands on had the wiper connected to a solder lug held to the chassis by the pot mount.

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    Lifetime Member Enzo's Avatar
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    Does it still have the original 'output matching' pot wired up like stock? Does adjusting it affect the hum?
    I don't recall an answer to that.


    Hum from tube balance will be 120Hz, hum from the bias supply will be 60Hz.

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    Quote Originally Posted by 66 Kicks View Post
    More than just by the schematic. Every one I have ever gotten my hands on had the wiper connected to a solder lug held to the chassis by the pot mount.
    Point being someone has already tried working on this one for the same problem, we do not know what has been attempted without pics.

    nosaj

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    Quote Originally Posted by nosaj View Post
    Point being someone has already tried working on this one for the same problem, we do not know what has been attempted without pics.

    nosaj
    In post #15, he asked if the Hum Balance was tied to an elevated ground. You can interpret this as tied to an elevated ground originally or tied to an elevated ground presently. Given the level of ignorance displayed here about the original condition, I went with the former.

    So you came back with the statement, "Maybe in a stock one by the schematic,". The comma makes that a statement by itself. Well here you are questioning what I said. Maybe a stock one is like that or maybe not. And even then only if they happened to wire it like the schematic. It also strongly implies that I am making my assertion based on the schematic.

    You have a valid point after that, but you should have started with something like, "Yes in an original one, but...". Do you see the difference?

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    Lifetime Member Enzo's Avatar
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    I apologize for having the wrong interior layout in mind.

    We can see the schematic, but we need to know how the OP's amp is wired. Did someone elevate it or is it grounded? And I still want to know about the bias balance control.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Enzo View Post
    I apologize for having the wrong interior layout in mind.

    We can see the schematic, but we need to know how the OP's amp is wired. Did someone elevate it or is it grounded? And I still want to know about the bias balance control.
    Well certainly something is wrong, regardless of everything being fixed and there being nothing wrong , because I'm betting that amp and the thousands like it that left the factory didn't all hum.

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    Thanks everyone for the responses,sorry I didn’t check these earlier. The hum balance and output matching pots are stock and unmodified. As far as I can tell what has been changed so far was changed correctly and properly grounded. Both of those pots test within tolerance and are properly grounded tested with a Fluke 87/3. No modification from stock have taken place,just replaced parts as deemed to fix the hum. Once all of the normal parts that could cause a serious hum were chucked,not much else was left. In the Power Supply anyway.

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    I actually used a massive soldering iron to make sure all wires from the PT were freshly soldered directly to chassis instead of via mechanical connection like a transformer screw. All of the grounds from fiberboard to brass strip were checked too.

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    Quote Originally Posted by lo-watt View Post
    I actually used a massive soldering iron to make sure all wires from the PT were freshly soldered directly to chassis instead of via mechanical connection like a transformer screw. All of the grounds from fiberboard to brass strip were checked too.
    I'd wirebrush the brass strip and star washers and the pots it all other efforts fail and make sure the brass strip is well grounded.
    If you have a scope you can build this an check your grounds.
    http://tone-lizard.com/high-resistance-ground/
    nosaj

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    So how does the hum respond to adjusting the hum balance and output matching pots?
    That might help to at least determine if we are dealing with 60hz or 120hz hum. Does one of those pots introduce more hum of a different frequency, etc.

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    The hum balance pot decreases and increases hum mildly,as expected on AC heaters. Tube balance pot does adjust as well,I set the tubes at 35mv. Both are factory and stock

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by lo-watt View Post
    The hum balance pot decreases and increases hum mildly,as expected on AC heaters. Tube balance pot does adjust as well,I set the tubes at 35mv. Both are factory and stock
    Does Tube balance pot affect hum at all?

    nosaj

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    Quote Originally Posted by nosaj View Post
    Does Tube balance pot affect hum at all?

    nosaj
    If I recall,no. But it does change the “bias”.

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