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Tech question regarding anomalous and intermittent 60 cycle hum

Discussion in 'Amp Central Station' started by gmm52, Sep 9, 2018.

  1. gmm52

    gmm52 Tele-Meister

    339
    Sep 23, 2016
    Southern Ontario
    I was playing my Blackstar HT40 a short time ago, at very modest level, and out of the blue there was a pop noise, and the 60 cycle hum kicked up quite a bit louder. I shut the amp off for about 15 minutes and turned it back on. This time the hum was lower than usual, then faded to almost nothing for a few seconds, and then it popped again and the hum came back quite loud. The actual amp tone remained unchanged, and the amp otherwise sounds healthy.

    When I bought this amp it was dead quiet. Then after some time I noticed 60 cycle hum, but there were a few weeks where I didn't play it, and I didn’t entirely trust my memory that it might have been dead quiet initially like I thought, so I just lived with the hum, but this latest event tells me there is indeed something amiss, and that it is even intermittent. When on standby (before and after this latest incident) there is virtually no audible transformer hum. I’ve seen older amps where even in standby there is a good deal of hum to be heard directly from the transformer. This transformer seems to be relatively quiet in and of itself.

    Any ideas as to what could cause transformer hum that varies?
     
  2. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    I am going to doubt that you have a transformer humming. Ime, a power tranny that hums does so due to loose laminates, and one can feel the hum in the transformer.
    The hum which your amp exhibits makes me suspect that there is a bad connection or a weak filter cap. Tech time, imho.
     
    Andy B likes this.
  3. gmm52

    gmm52 Tele-Meister

    339
    Sep 23, 2016
    Southern Ontario
    Thanks Wally. Will take the advice. I just learned the HT40 uses a switching power supply, so the caps are pretty small. Not sure if or how that might affect the fix, but thought it was worth noting. The amp did the same thing last night, but I happened to be playing at the moment the hum kicked in (zero hum for about 1 minute after turning on, and then it kicked in) and the volume dropped substantially at the same time. The tone seemed less affected, but there was some trimming of the high end. I'm glad to learn I my memory wasn't faulty and that I had indeed heard this amp without any hum, and that I should be able to get it back to a hum free state with some kind of fix.
     
  4. beninma

    beninma Tele-Afflicted Ad Free Member

    Age:
    41
    Mar 17, 2017
    Massachusetts
    I'd try plugging it in somewhere else in your house or taking it somewhere else completely and seeing if it has any effect.

    AFAICT there can be things turning on and off in your house that can change how much noise you're amplifying.

    You didn't mention if you've got it on a good surge protector/filter/whatever/etc.. that can affect this kind of stuff.

    I have a THR I often carry around to practice away from my house. I've noticed there are some locations where the power is WAY cleaner than my house.
     
    studio1087 likes this.
  5. JMac52

    JMac52 TDPRI Member

    Age:
    57
    57
    Apr 26, 2018
    Mesa, AZ
    ^^^

    Try to find something in the house/building that’s turning on and off. I have this problem when the dishwasher runs. The hum comes and goes as it switches wash cycles.
     
  6. AndrewG

    AndrewG Friend of Leo's

    Age:
    65
    May 15, 2007
    Exeter, England
    My Cornford Hurricane developed a very nasty hum. The culprit was a cable whose retaining clip had failed, leaving the cable flapping around too close to a transformer. Simple fix involving a blob of silicon sealant. Interference can come from some strange places; my Tivoli radio, for example, picks up radar pulses on FM from my local airport, 5 miles away.
    Years ago I gigged regularly in a bar close to a railway line, and my amps would pick up track-circuit signalling pulses as a series of rapid bleeps.
     
    Last edited: Sep 10, 2018
  7. gmm52

    gmm52 Tele-Meister

    339
    Sep 23, 2016
    Southern Ontario
    I appreciate the ideas, but I think Wally's got it. I didn't mention it but my AC seems pretty clean, and I've also tried the amp with an Oneac power conditioner, with no difference.
     
  8. gmm52

    gmm52 Tele-Meister

    339
    Sep 23, 2016
    Southern Ontario
    Strangely, the problem turned out to be a bad tube or tubes. I was very surprised as they were NOS Mullard's, and they worked terrifically for a while. The first thing I tried was measuring bias after turning the amp on. It crept up to 66mA and then the pop, followed by a fall to 28mA and instant hum. I then installed two TAD EL34's (used, but not heavily) and the hum was nowhere to be found. I balanced and biased the amp and have put about an hour on it without any problem. I think I was biased (pun intended) in approaching the problem because I could not understand how one or two NOS Mullard's with such low mileage could go south. Lesson learned. Mind you, I would still be curious to know what kind of defect a tube might have that would present that kind of symptom.
     
    Last edited: Sep 13, 2018
    RoscoeElegante likes this.
  9. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Kudos on curing the problem. It would be of interest to know which Millard was going south. The hum was due to the imbalance of current draw when the problem exhibited itself....so I would think only one of those Millard’s is defective. Bad control grid??
    I take it that the bias reading procedure reads both tubes at once...and 66ma indicates a current draw of 33ma per tube. Fwiw, this is one situation where measuring current draw via the transformer shunt method would reveal which tube has the problem.
    Knowing which tube had the problem would lead one to making sure that that tube was making good contact in all of the pin sockets. Taking measurements on the plate, screen grid and the control grid during the hum situation might have been revealing. A tube tester might provide some insight...maybe not due to the intermittent nature of the proble.
     
    Last edited: Sep 14, 2018
  10. Silverface

    Silverface Poster Extraordinaire Ad Free Member

    Age:
    66
    Mar 2, 2003
    Lawndale CA
    A day late and dollar short since the problem was unrelated.

    But I recommend every electric player - especially those that gig or move amps from room to room - own a $7 outlet tester and check every power outlet before plugging anything in. Especially in older houses, 3-prong outlets have NO ground connection - 3 holes are meaningless; you have to check.

    And a huge number of outlets in both old and new houses have some outlets where the polarity is fouled (polarity is a "term of convenience" in this context) - the "hot" and "neutral" supply wires are connected backwards on the outlet. There is a reason for one plug side to be wider than the other, and that's to ensure neutral and hot are supplied to the equipment correctly. "Neutral" is NOT an extra ground wire, but having the outlet reversed can cause some dangerous issues, especially in some vintage amps.

    An outlet test has 3 simple colored lights that tell you if the outlet is grounded and wired correctly. I check the power source every single place I play; if I can't access a correctly wired source I use an isolation transformer or don't play.

    Glad it was easy. Good lesson - check tubes first when hum is present. But while it's unusual for a newer amp to have a filter cap fail, it DOES happen - that would have been my next item based on the description of the hum.

    Because it was intermittent and varied it was very unlikely to be due to some piece of nearby equipment, especially anything run continuously.
     
  11. gmm52

    gmm52 Tele-Meister

    339
    Sep 23, 2016
    Southern Ontario
    As fate would have it, I had a third NOS Mullard, so I tried finding the bad tube by switching them out one at a time. All three permutations hummed (1-2, 1-3, 2-3) so either both tubes were bad, one of them and the third one I introduced was bad, or perhaps even all three. I only checked bias reading on the first permutation and didn't bother checking the others because they were humming. By the time I got to this point I was frustrated enough that I opted to forget the Mullard's. I started with the TAD's, and when they didn't hum, I decided to balance and bias and call it a day, happy that I had my amp back in nice quiet condition. Although, ironically, the reverb pot decided to stop working after putting the amp back together :-/ I tried Deoxit, but the pot still didn't work, so I touched the three lugs with an iron and that seemed to do the trick.

    If I get ambitious in the future, I'll try again to see if it was a bad contact with the Mullard's. I'll have to learn how to read screen grid and control grid, but as a non-tech, I felt I did o.k. this time around with getting plate voltage and bias, a bit more comfortably each time one does it. I've blown things up before, so I really like to proceed with extra caution these days.
     
    Last edited: Sep 14, 2018
  12. gmm52

    gmm52 Tele-Meister

    339
    Sep 23, 2016
    Southern Ontario
    Indeed, I have one of these: http://images11.palcdn.com/hlr-system/Thumbnails/80/800/8003/8003373.jpg and use it. Thanks.
     
    Silverface likes this.
  13. alnicopu

    alnicopu Friend of Leo's

    Oct 3, 2009
    georgia
    This^^^. I have seen “pops” caused by connection issues. Get the elecrolytics replaced in the power supply and if it has push on connections in there, fully disconnect and reconnect ALL of them. First thing I do servicing pcb amps (and older printing systems I work on at my personal accounts.) You have no idea how breaking a poor connection and reconnecting has corrected intermittent issues.
     
  14. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    “I have seen pops caused by connection issues”...
    I overhauled a ‘67 Bandmaster last week that had a pop issue. There was a loose connection on the standby switch....the wire was sitting in a hole in the solder at that connection post....maybe for all of these 51 years.
     
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