crackling sound - Marshall Major

[fperron_kt88]

Hello All,

Got a crackling sound on a 1970' Major when hitting hard. All four inputs, anything past about two on the input volumes. No real effect of the other knobs. Sound is a "Krrrrricklyckliclkiclki-ick-ick-ick" that dies off with the normal bloom of the amp. Very annoying, very akin to P.O.

Amp is stock lead with a PPIMV added (nice layout, with shield grounded at the dual 250K pot with 2.2Meg safety res.)

Flaky impedance selector is now bypassed (can I have put a lot of stress on the OT ???)

Problem remains identical with a different set of power tubes, preamp tubes. Power supply caps are new. Chopsticks can excite the sound, but not on any particular part. Tapping the chassis while playing is more likely to excite the sound.

Extracting the signal before the PI coupling caps and sending to a 50W plexi clone fixes the problem. Unfortunately for me, the Major NFB is tapped very early in the preamp, so this is not really 100% isolation of the problem... and that opens the loop of the 50W as well...

Sockets? Output transfo dying (already replaced 2 years ago...)? Phases of the moon?

Any tests you would perform to isolate more (next step is to buy a scope...)

BTW, how do you guys go about re-tentionning tube sockets???

Thanks

[Phil_S]

Have you tried swapping known good tubes for the PI and power tubes?

Does this problem resolve when you pull the PI? So far, you have only eliminated the preamp section(s).

Look for all the easy stuff first. I don't think you've done that.

Carbon comp plate load resistors? You might change for metal film. (Usually a cure for hiss, not crackle.) There is no tone on the plate load resistors, so I don't view this as any sort of compromise. Actually, I see it as an improvement. Some purists will disagree and that's OK by me.

Re-tension of sockets isn't always possible if the metal has lost its flex. The typical tools of choice are a dental pick or a small jeweler's screw driver. Gently push the tool into the outer edge of the socket and coax the contacts to be a bit narrower. Typical safety considerations apply, including discharge of caps.

[CaseyJones]

Got a crackling sound on a 1970' Major when hitting hard. All four inputs, anything past about two on the input volumes. No real effect of the other knobs. Sound is a "Krrrrricklyckliclkiclki-ick-ick-ick" that dies off with the normal bloom of the amp. Very annoying, very akin to P.O.

Oh no, the Dreaded Crackle of Doom. There's only one sure way to get rid of the problem and that's to sell the amp to me. I'll give you fifty bucks for it, I can fly in there this evening.

Seriously... look for a cold solder joint and re-tension your tube sockets. Look for anything that isn't quite right, for instance is everything kosher and correct with your PPIMV? Is there a solder whisker or a stray ball of solder stuck somewhere it shouldn't be? Have any of the solder lugs on the tube sockets been pushed together resulting in an "alternative" signal path?

While you have the chassis out... plug in, turn out the lights, bang a chord and watch the inside of the chassis. Is anything arcing?

Eliminate the easy stuff, eliminate the obvious and move forwards.

Extracting the signal before the PI coupling caps and sending to a 50W plexi clone fixes the problem. Unfortunately for me, the Major NFB is tapped very early in the preamp, so this is not really 100% isolation of the problem... and that opens the loop of the 50W as well....

That gives you a very accurate idea of where the problem is. Your preamp is o.k. up to that point. I don't care how pretty your PPIMV is, I'd scrutinize that area.

Do your output tubes arc internally? Do they arc at the sockets? An internal arc indicates either a bias supply fault or bum tubes. A really dirty amp and I don't mean dirty tone, I mean full o' dust and roach wings... can carbon trail between tube socket solder lugs. There's enough juice in a Major for it to arc nicely.

[Firestorm]

Concur with CaseyJones. Sounds a lot like arcing. Some resistors can look fine, but have bad lead connections that arc almost invisibly. If a metal or carbon film resistor is stressed it will sometimes burn through one of the sprial traces in the film and arc there.

[fperron_kt88]

Thanks All,

Good idea to play it in the dark, will try that tonight! Arcing is definitely something that I would suspect to happen... No obvious trace of carbon anywhere I looked...

I re-tensioned the outer half of the sockets and tried with both brands of KT88. Same sound for any combination I tried of the 8 power tubes I have on hand... I will re-tension the inner-ones tonight as well.

Any idea on how to remove oxidation on the sockets. Any magic lube or compound? WD-40, contact cleaner??? I can surely swap things and cycle the sockets, but this might not be the best if that is the case that they are already worn-out...

I just saw the previous post about arcing resistors. Would this somehow glow in the dark enough for me to see it?

[fperron_kt88]

BTW, as for the Dreaded Crackle of Doom, I'd throw it away against any decent Twreck or Dumble with 50$ (Canadian, though) for good measure... Still interested CaseyJones ???

[Firestorm]

Any idea on how to remove oxidation on the sockets. Any magic lube or compound? WD-40, contact cleaner???

Any kind of tech spray, contact cleaner or just plain alcohol. BUT NOT WD-40.

[CaseyJones]

BTW, as for the Dreaded Crackle of Doom, I'd throw it away against any decent Twreck or Dumble with 50$ (Canadian, though) for good measure... Still interested CaseyJones ???

Sounds like a deal! Pay me fifty bucks Canadian plus your Major. I'll only accept payment in Doubloonies, I'll spend them on Rue Wellington in Sherbrooke on the way back.

Any idea on how to remove oxidation on the sockets. Any magic lube or compound? WD-40, contact cleaner???

Any kind of tech spray, contact cleaner or just plain alcohol. BUT NOT WD-40.

+1 on that. WD40 is an insulator. Fill a small pan with WD40 and check continuity through it with your multimeter. Pretty good insulator, no?

Use Deoxit. Although... unless your Major is a New Orleans refugee or has recently been submerged in Des Moines chances are the sockets aren't terribly corroded. They may be loose if you swap tubes every other day but they're probably not corroded.

My top picks would be something screwed up in the bias supply resulting in incorrect bias or...

Some new KT88s just arc.

Turn out the lights and watch those babies arc!

[Beerman]

Cracking noises with Marshall Major on two...... could be the Earth's crust breaking....

[electric]

i once had the same issue with a Marshall Super Lead Clone with PPIMV added

it was the master volume pot which was brand new.
first i thought it was a bad solder joint at the pot. Remelt everything but it kept going
So changed the pot and voila.

could also be one of your preamp volume pots or a capacitor near them that gets leaky

[Andy Le Blanc]

did work on a super lead last year.... its issue was noisey resistors..... but
didnt crackle..... use napa CRC QD electronic cleaner for the socket and pins
one small drop on the tube pins is enough to clean the contacts in the socket
a bad coupling cap or filter cap can make a similar sound if you cant find
evidence for arc....

[breakfastbuddy]

check your power caps , they are getting old , look for bubbels on the end of the caps

[selloutrr]

If the major can currently be played. and the bias is set, and holding,
try this...
Take the amp out of the housing and prop the chassis up in an amp cradle or on books so you can get to the guts.
hook it up the a tone generator 1Khz and a matching ohm load speaker cab.
now this is the best part (order asian food and get an extra set of chop sticks) now take a chop stick and push on all the contacts in the amp one by one and see if you can recreate the pops on your terms.

THE AMP IS POWERED ON FOR THIS TEST AND HAS DEADLY VOLTAGE!!!!
ONE HAND IN YOUR POCKET! DISCHARGE THE CAPS BEFORE YOU FIX ANYTHING!!! IF YOUR NOT COMFORTABLE DON'T DO THIS!!!
IT'S A GREAT AMP BUT IT'S NOT WORTH A STROKE OR DEATH

If you have a layout diagram available as you go threw circle any places that might be of interest so you can go back and test more in depth... after you unplug the amplifier and properly discharge the stored voltage left in the caps

most pops i've encounted have been,
loose pins on tube sockets ( dental tool does the trick )
Bad tubes (I know you just got them but even new ones can be bad)
Cold Solder Points (But just cause you found it and remelted it doesn't mean you fixed it why is it going or did it go cold?)

and the 100K Carbon Resistors i call the V's in preamp section of the blackface fenders

Sometimes a CAP job will do the trick but usually you will also be experiencing a signal to noise ratio that leans toward the noise. as a 50/60/120hz ground hum even while you play quietly.

BEST OF LUCK!

[selloutrr]

If the major can currently be played. and the bias is set, and holding,
try this...
Take the amp out of the housing and prop the chassis up in an amp cradle or on books so you can get to the guts.
hook it up the a tone generator 1Khz and a matching ohm load speaker cab.
now this is the best part (order asian food and get an extra set of chop sticks) now take a chop stick and push on all the contacts in the amp one by one and see if you can recreate the pops on your terms.

THE AMP IS POWERED ON FOR THIS TEST AND HAS DEADLY VOLTAGE!!!!
ONE HAND IN YOUR POCKET! DISCHARGE THE CAPS BEFORE YOU FIX ANYTHING!!! IF YOUR NOT COMFORTABLE DON'T DO THIS!!!
IT'S A GREAT AMP BUT IT'S NOT WORTH A STROKE OR DEATH

If you have a layout diagram available as you go threw circle any places that might be of interest so you can go back and test more in depth... after you unplug the amplifier and properly discharge the stored voltage left in the caps

most pops i've encounted have been,
loose pins on tube sockets ( dental tool does the trick )
Bad tubes (I know you just got them but even new ones can be bad)
Cold Solder Points (But just cause you found it and remelted it doesn't mean you fixed it why is it going or did it go cold?)

and the 100K Carbon Resistors i call the V's in preamp section of the blackface fenders

Sometimes a CAP job will do the trick but usually you will also be experiencing a signal to noise ratio that leans toward the noise. as a 50/60/120hz ground hum even while you play quietly.

BEST OF LUCK!

[BJF]

Hi,

Check for oscillation. Make sure the probe can take the voltage at the points you measure.

If you take an output from the phase inverter outputs and sound is fine without artifacts, you can have a faulty output transformer.

I'd stress the point of being carefull with those amps as I had once just one like that in for service and while I could isolate the problem to a faulty OT the amplifier also took the HT of the oscilloscope I was using, hrmpfff.

The point being I could see the disturbance at the primary windings.

To secure oscillation marginal, use one dominant rolloff and place that across the phase inverter outputs.

Check grounding and location of wires and use your intelligence on that.

Be careful when stabilizing the amp as you'd not like to kill the response but just make it stable: if you overcompansate the amp will be stiff and if you undercompensate your amp will be oscillating like a pig.

The formula for trading gain for bandwidth states thatgain will approach infinity- of course only then at one spot frequency- but at nearthis point gain will be much higher than below.

Be careful when tuning this.

On its own the preamp of a Major will have a crunchy sound when pushed allthough this will be extremely loud when amplifier is running with speakers.

I would agree that if you don't absolutely know what you are doing sell the amp because Majors have raised voltages and are loud as kingdom come and are dangerous to work with.

If you however apply the the formula of artificial gain you would have the loudest distortionmachine on planet Earth.


Crackling sounds may arise from oscillation and if you focus you will find the oscillation.

Arching can be a problem due to the voltages involved and you might want to protect amplifier against this by means of backwards diodes to ground from each primary.

I would think though that from what you describe there is an oscillation problem.

There are numerous ways of listening for this like grounding the input and hearing the noise and especially when adjusting the presence control and volumes.

Sometimes an oscillation is lurking and only present at the decay of a transient and you'd view that as a burst on the scope

The best of luck and be most careful

BJ