HUM, HUM ISSUES (resolved !)

One of my M-125 started to hum and making crackling noises. I swapped power tube, rectifiers, coupling caps, front and back tubes and the noise still there. I plugged the amp to a different outlet with and without the Variac, etc., etc. Any thoughts please help? I have own these amps for 7 years and never had any issues until now. Thank you. Also, when I tap the chassis you can hear the noise through the speakers.

One of your tubes may have become microphonic, tap each tube with a pen or something similar and if one is microphonic you will hear it through the speakers.

That's one of the issues that everything I tap on will transfer noise to the speakers. It seems like the amp is a microphone.

The only thing I can think of that would make the whole amp microphonic is a faulty tube. Faulty tubes can also hum and crackle like the symptoms you describe. When you say you swapped the tubes around did you mean from the good M-25 to the faulty one?

mijohn wrote:The only thing I can think of that would make the whole amp microphonic is a faulty tube. Faulty tubes can also hum and crackle like the symptoms you describe. When you say you swapped the tubes around did you mean from the good M-25 to the faulty one?

Yes. I swapped all the power tubes and the small tubes from the good amp.

Another thing to consider is a cold solder joint, particularly around the input and output tube bases. Sometimes these can show up after years of on-off recycling. Reflow all solder connections.

Email Roy Mottram for further suggestions:
info@tubes4hifi.com

To me, hum means a ground loop.  Having eliminated tubes as a source, I'm thinking that this issue emerging after 7 years is some sort of solder joint failure.  These are a pain to track down, and reality is M125 grounding scheme has multiple contact points with the chassis.

So maybe below as some shots in the dark?

Maybe look at joints on input and speaker jacks?
Signal wire connection to PCB?
All the tube sockets?


I had ongoing hum on my M125's that was helped by replacing alloy PCB standoffs with non-conductive nylon type and isolating the board from chassis.  The solder mask on mounting eyelets had abraded off the driver board exposing the PCB ground plane.  Seems unlikely, but maybe years of tube swapping abraded yours?

Well the amp is also making a static noise.

I agree that most hum issues are ground related.

I had similar issue with a pair of MKIIIs. it was a pair of Bad rectifier tubes. Not sure about the Copper Caps by Weber.  Swap from one mono to the other.

A bad power transformer will also cause your described symptoms. They can and do go bad.   I know you don't want to hear that.

If you've eliminated every other possibility, and again by swapping all tubes, etc.  to see if the symptom follows, you can with a bit of work, swap the power trannys to see if it follows.  Swapping the circuit boards, if thats the problem, would at least get you closer to locating the issue, but only after ohming all resistors first and anythign else you can check such as diodes.  I haven't quite figured out a way to accurately check capacitors in the cuircuit.

gener8r wrote:I agree that most hum issues are ground related.

I had  similar issue with a pair of MKIIIs.  it was a pair of Bad rectifier tubes.   Not sure about the Copper Caps by Weber.  Swap from one mono to the other.

A bad power transformer will also cause your described symptoms.  They can and do go bad.   I know you don't want to hear that.

If you've eliminated every other possibility, and again by swapping all tubes, etc.  to see if the symptom follows, you can with a bit of work, swap the power trannys to see if it follows.  Swapping the circuit boards, if thats the problem, would at least get you closer to locating the issue, but only after ohming all resistors first and anythign else you can check such as diodes.  I haven't quite figured out a way to accurately check capacitors in the cuircuit.

Thank you. When I turn on the amp the hum gets louder and louder. I think swapping the circuit boards are a good suggestions if it comes to that.

First disconnect the amps from the source and insert shorting plugs to the inputs.

Power one amp at a time.
If the hum disappears, it's a source issue.

Disconnect the speakers.
Measure the output with a voltmeter. Should be below 5mVAC.
With a WOODEN chopstick, tap every component and its connection.
If you get a jump on the voltmeter, determine the issue with that component, fix and restart.

Could be the Quad cap if the hum gets worse when the amp crackles.

If you do a lot of tube rolling, it's quite possible that there is a cracked solder joint or a socket with a spread pin clamp.

With the amps off and B+ reading 0vDC, remove all tubes.
With a drill bit of the correct size, check all the socket jacks for size and reasonable freedom of movement.
Slightly close any loose jacks.

Seamus wrote:First disconnect the amps from the source and insert shorting plugs to the inputs.

Power one amp at a time.
If the hum disappears, it's a source issue.

Disconnect the speakers.
Measure the output with a voltmeter. Should be below 5mVAC.
With a WOODEN chopstick, tap every component and its connection.
If you get a jump on the voltmeter, determine the issue with that component, fix and restart.

Could be the Quad cap if the hum gets worse when the amp crackles.

If you do a lot of tube rolling, it's quite possible that there is a cracked solder joint or a socket with a spread pin clamp.

With the amps off and B+ reading 0vDC, remove all tubes.
With a drill bit of the correct size, check all the socket jacks for size and reasonable freedom of movement.
Slightly close any loose jacks.

I was about to do that but then it blew up the fuse.

Well, what caused the fuse to blow was that the 100 0hm 1 watt resistor was touching pin 8 on one of the power tubes. I am going to replace it and see if that was the problem.

Issue has been resolved after replacing the resistor and one of the driver tubes