Tube amp noise

I’m about at my wit’s end trying to figure out noise issues I’ve been having with my AC30.

I replaced power tubes a few months back, noise persists…replaced preamp tubes two nights ago, and noise is to a more normal level…until today when it’s back. Nothing has changed (guitar, power, amp knobs, mics, other stuff on in the room) between the two so I don’t understand what this noise is.

Here is a recording I did two days ago:

And here’s one from today:

Here’s an FFT of the difference (new in white)

Does anyone have any idea what this nasty high frequency hum is/what I need to do to get it to go away. you can make the level go up and down based on the guitar volume pot, as well as preamp and master volume knobs on the amp. but I don’t think it’s the guitar as I’ve tried it with another guitar and my SG shortscale bass, present on all of them.

Really any ideas would be good, I don’t know of any amp repair that I trust in the area, I’m going crazy trying to figure out noise with my guitar setup, it’s like a never ending saga.

That sounds like dirty power to me. Maybe there is a transformer on the circuit with the amp. Fluorescent lights?

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no fluorescent lights in my house. a few weeks ago when I was experiencing this I went and unplugged every other thing/turned lights off in the upstairs of my house and it did not go away.

That’s a bummer. Have you tried plugging the amp into a different circuit in the house? I would try swapping instrument cables too.

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Lemme try moving the amp around that is one thing I haven’t done.

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I was getting a weird random clicking noise on my ac10 and pathfinder 15r one day, and I finally concluded it had something to do with construction going on next door. It went away when the workers went home for the night, so maybe it’s something like that?


Just tried a few different outlets (front room, garage) and they exhibit the exact same sound. It is a fairly newly constructed house, and we did have someone come and inspect the electrical work before we moved in who said it looked clean and solid (of course there could be something that wasn’t caught so not ruling that out). I don’t see any construction or anything out of the norm going on with neighbors, but who knows…I’ll check again tonight.

It does seem to be exclusively the amp having these issues at this point…my synth case is very quiet, other stuff (record player/tape player/reel to reel) are good

EDIT: Forgot to mention, I have tried multiple instrument cables and IEC power cables

Power tube bias issue? Long shot and not an area of my expertise, but with my Ampeg B-15 I always had a tech “adjust the bias” when changing tubes, not that I completely understood what that meant, which the tech very politely understood.

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I believe the AC30 does not need to be biased

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Ahh, long shot indeed! I learned much from that link though, thanks!

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Stupid questions but are you plugging directly from guitar to amp? Also does it make the sound with nothing plugged in?

If it has a reverb tank try moving it around, rotating it etc. I have a custom AB763 DR clone, and moving the spring tank and using shielded Mogami cables to and from the amp made a HUGE positive difference in noise levels. I also notice way more noise when I put my Eventide H9 on top of it, (the closer to the power tranny, the worse it gets), so now I keep it on the floor below.

I have tried going straight into the amp (removing pedals). Noise level is about the same.

This bad metallic kind of noise only occurs when something is plugged in. There’s some low white noise if you turn the amp completely up without anything plugged in but that seems normal levels

I can try removing the spring reverb tank. I dont know if I have any fancy shielded cables but I’ll have a look.

Single coil pickups? Tried a different guitar or amp? Has this combination of amp/guitar ever run noise-free at your current house?

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If you have the reverb turned down it shouldn’t make any sound? Also you’re saying it only happens with an instrument plugged in? I would consider RF (I think that’s it) interference in the room or area maybe?

This sounds like a guitar issue or something funky at the input jack. If the amp is reasonably quiet with nothing plugged in, then the problem is happening before the gain stages otherwise you would hear it being amplified in the preamp or power amp.


Yeah, does the noise level go up and down when you move guitar around and change pickup settings?

If you move around parallel to mains power etc sound like this can change if from pickups.

Also light dimmer switches can cause noise like this, even if “off”.

Honestly shit like this drives me crazy. If in house I’d go to breaker and shut off everything but room you’re using just to see what happens.

I’d borrow a radial JDI and xlr to ts just to see what happens too.

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It was …a cable. I swore I tried a different cable maybe I somehow just swapped them around (with the effects loop there’s 4 cables).

My main guitar is an hss strat. I had it recently set up and they did away with a lot of excess wiring inside which helped improve its noise performance significantly. They said it is still somewhat susceptible to noise and I could follow up with getting the inside covered in shielded tape to make it perfect. Knowing this I did try my other guitars (a strat retrofitted with p-90s and a short scale sg bass which I believe is single coils. I don’t have anything super low noise like a les Paul humbucker

It seems like the amp is working at normal levels (out in the garage, just the guitar plugged straight in) with the other cables besides this one.


With my hss strat i can move around or pivot to find better noise spots, but I think that’s the guitar not the amp.

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This would be ultimate solution

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