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Does it still have a hum/buzz when NOTHING is plugged into the INPUT? If so, you MAY have a tub going bad. If the noise is only there when you have your guitar/effects, etc. plugged in, then the problem is the guitar grounding or effects faulty...
FIRST: When the amp is "cool", UNPLUG THE AMP POWER CORD FROM THE WALL! ! ! ! Then CAREFULLY remove and reseat the tubes one at a time. Be sure to insert back into the socket correctly. You MAY just have a bit of corrosion on the tube "pins"....
SECOND:Don't plug anything into the amp, turn on and warm up. GENTLY tap one tube at a time with a WOODEN pencil. DO NOT USE anything metallic (like a screwdriver....) and be VERY gentle. If the noise increases or decreases, you probably have a tube going bad...
That's all I have. If this doesn't cure the issue, best take it to a repair shop . . . .
If it's humming when something is plugged into the input, but not when nothing is plugged in, it's most likely the cable. If it hums when nothing is plugged in, the amp is defective and you need to return/exchange it.
check out your tubes....it sounds like one is working its way out of its socket or you have a microphonic tube...may be time to replace them and have the amp serviced while you are at it. ...Does one tube have a whitish look? inspect them/.....are there any cracks? while it is on...tap them with a lead pencil...something NOT metal...does it make a loud noise? These are all indicators there is something going on with your tubes..Good luck
On mine, the guitar input jacks get tarnished and causes all kind of interference and noise. Take a Q-Tip soaked with WD-40, run in down into the jack and brush up and down. Don't overlook the filter caps as they are a common cause of noise. You need a capacitor checker (the older tube type) to thourghly check those out.
Sounds like a bad power supply filter cap...the buzz you're hearing is the 60 cycles hum that filter caps reduce or (hopefully) eliminate altogether. It may also be related to the tubes but finding the bad one will require some work. Any friends have this same amp? If so, try swapping one tube at a time and see if you get lucky. If this amp is new take it back ASAP as it is a manufacturing defect.
If the humming ONLY happens when a cable is plugged into the amp, the problem is in the cable or the guitar. Poor quality cables or wrong cables will result in hum. Defective wiring or connectors in the guitar OR having the guitar near a source of magnetic fields will result in hum. We cannot diagnose these possibilities remotely.
You say the "tubes are fine" I take it that you have either replaced them with new or used a very sophisticated tester on them. A heater-cathode short might cause the symptom. There are a bunch of tests I would want you to run alnd get back to me with the results.
Heat up the amp but DO NOT play anything into it for the usual failure time or more and they start to play i... did hum occur WITHOUT playing? With all volume controls down does the hum occur?
The right tools is an oscilloscpe to fing out if the filter caps are good. These are C31, C33, C35, C36.
With hum present in failed stater, plug in a grounded plug into the power amp in jack... if hum is stiil there, check the +/- 16 volts regulating Zener for ripple. get schematic here:
The key is to keep the guitar away from magnetic fields. Transformers in amps are a common source. You could use a graphic equalizer to "notch" out the power line frequency. Make sure your amp isn't the source of the hum by unplugging guitar cable. Then make sure the guitar cable is good by plugging it in and shorting the open end. Then plug into the guitar to check.
You may find your cable is sub-standard, or you MAY have to set yourself up to use balanced connections if your guitar has XLR output connector.
Sometimes the grounding of the conductive paint in the electronics well of the guitar gets ungrounded... check that using an ohmmeter.
Try disconnecting the speaker from the amp with the amp unplugged from your power. Never work on an amp when its plugged in unless you know how dangerous it is. Check for resistance across the speaker terminals with a volt ohm meter to see if you have a blown speaker. Thats the first thing to look for. You should here a hiss or a low hum when your amp is on and you plug a guitar cord into the input without the guitar. Check those two things first.
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