amp crackles with nothing plugged in help

Carbon comp plate load resistors are famous for that snap crackle pop thing.

 

Time to change the caps - I just had it done on a 68 sf twin and it solved the problem.

Or it could be a bad 12AX7 or 7025 but probably capacitors.

 

If the crackle sounds like twigs catching on fire it is probably the plate load resistors like
alnicopu said.

 

I have a vintage (61) amp with the same intermittent bacon frying problem. So the resistors are probably a similar construction. But could loose/corroded tube sockets also cause the symptom?

 

Old, cracked carbon comp resistors. Yep.

 

Loose tube sockets will do it too. I have had an amp that crackled until I fiddled around with the offending tube.

 

If it sounds like distant lightning over the AM radio, my experience is that that's a bad tube. YMMV

 

There are easily half a dozen different potential sources of crackle. They should be addressed in a logical order. Like anything intermittent it's easy to "fix" something other than the root cause only to have the problem come back later.


Has the amp been worked on or modded recently? Mods or service make the amp "good as new" or better when done properly. Done improperly... well, a cold or broken solder joint is a classic source of crackle. Any poor connection can crackle.


Cap job. 45 year old caps are like 45 year old people. You're lucky if they're still holdin' up. They ain't "good as new" no matter how good they look.


Cap job 2.0. Check the coupling caps. When we say "cap job" we mean the big electolytics in the power supply. Might as well replace the small electrolytics as well. A little gain lost here and there to funky bypass caps adds up. Coupling caps can cause crackle if they're leaking DC. DC on preamp tube grids is like "phantom bias". The coupling caps block DC if they're working correctly.


Dirty pots. Sometimes a spot of corrosion develops inside. It may crackle or do a couple ofhter weird things. I had an old Ephiphone amp that would play annoyingly clean except for that one magic spot low on the volume knob. It would do a glorious fuzztone impersonation between "3" and 4" on the volume knob.


"Dirty" pots / cap job 2.0. A failing coupling cap passing DC to a pot may cause crackle. Sounds like a dirty pot. No amount of cleaning will fix it.


Noisy plate resistors. They tend to hiss rather than crackle. Hissy plate resistors aren't as common as some people say they are.


The dreaded "Bill the Cat" hairball. This is frequently a 12AX7 tube doin' a Bill the Cat impersonation.


The dreaded "Bill the Cat" hairball 2.0. Acts like a tube problem, may be caused by dirty or loose tube sockets. Fender's vendor pardon the illiteration changed the contacts on their octal sockets between the '60s and the '70s. The '70s sockets aren't as good as the earlier ones.



Bottom of the list because that's where they belong:


Dirty speaker voice coils. Maybe a flake of rust developed. Maybe moths ate the felt dust cover (it happens) and their little carcasses are jammed up against the voice coil. Maybe a flake of speaker glue got loose and wedged in there. The OP eliminated this one. I figured I'd mention it.

Crackly transformers. A hot Schumacher power transformer with its guts boiling will crackle. It frequently surprises me how long they can cook before they fail.

Crackly transformers 2.0. An output transformer that's been abused may do some odd things including crackle.


Don't overlook that Hendrix era curly cord (also eliminated by the OP). Those crackle due to the tiny bit of capacitance change between the conductor and the shield. Try this: Hit your amp with as much gain as you can find. Boost it with an overdrive if there isn't "enough" gain in the amp. Turn your guitar down and play jump rope with the cord. Or just whip the cord. It's a good test to see if a premium cord is worth the extra money.


.

Bill the Cat, Official Amp Repair Mascot.

 

The FIRST step should be cleaning all spring-tension contacts, because doing so is free and has a very high probability of fixing the problem. If that doesn't work, there's plenty of time to try the high priced solutions.

 

Yeah, but that takes all of the fun out of it!

 

Now that's a sound description that I can figure out. Not like "White noise".

 

I had a bad crackling/bacon frying in my Fender amp.

By using a chopstick I located the problem at V1 - 12ax7.

Put a new tube there, no crackling whatsoever.

Fender now quite as mongoose in heat.

 

The fact that it takes the heating time before your noise begins makes me suspicious of the carbon comp resistors.

Have a look and see if there are any cracked or peeling ones, if not you can get some freeze spray or use the old cheapo method of the upside down compressed air can with the blower straw to direct a small stream of propellant on the resistors one at a time.

Listen carefully for changes in the sound, because if it is more than one resistor, cooling one will still have the other noise makers doing their thing.

Be very careful to keep any of the propellant from being directed towards the tubes, they may crack or shatter.

 

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