#1
Hi.

I have Peavey Classic 30. I bought it second hand a few months back. Since the day I got it is has had a constant hum. It hums when the guitar is plugged in and when nothing is plugged in and is just a constant hum. It is not over powering but I did some recording with it yesterday and the hum was to present on my track.

But what could be causing it?

I read this on a Pro Guitar Shop piece:

Amps
So if you’re finding the noise is originating in your amplifier, this is a tougher problem to tackle. If you’re playing a tube amp, have all your tubes tested for microphonics. A microphonic tube can be a sign that the tube is failing, which can cause all kinds of mischief. Simply take a nonconductive item such a pencil or chopstick and tap your tubes lightly one by one. If you hear any feedback or tones coming from the speaker, you could have a bad tube. Although an improperly biased amp could produce many different side affects, hum can be one of them some amps have a “hum bias” control on the back precisely for this reason.


If I knock my amp on the side by mistakes the knock can be heard through the speakers in a high sound like knocking on glass. Does this mean it is the tubes or is this normal?
#2
When you hit it was the reverb on? It can cause noise to transmit as well.

I would start with tubes and go from there.

When was the last time they were replaced? If you don't know then you should replace them anyway. Keep them as spares

Lastly you can tap the tubes and see if you get any noise through the speaker. No guitar plugged in and turn the volume up a bit.
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#3
As RV said, tap testing. It is normal for v1 (usually the tube nearest the input jack) to have some microphonics so swap it with one from another position and test them all twice
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#4
What frequency is the hum? You can find out by using a tuner that also displays frequency. Gstrings on android will do it if you have an android phone.
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#6
Quote by Cathbard
What frequency is the hum? You can find out by using a tuner that also displays frequency. Gstrings on android will do it if you have an android phone.


I'm actually a tad ahead of you on this. I put a recording of the hum through a spectral analyser. It had some peaks around 100-300hz, and then danced around a lot above 1.5K.

I can EQ the hum out mostly on the recordings. Though I lost a little bit of highs from the actually awesome tone.


-

To above questions.

Yes - The reverb was on when I knocked the amp.

I did the tap test on all the tubes and they all passed. But all I had handy to do it with was a tightly rolled up bit of card. I jabbed them pretty hard.


How do I find out if it is 60 cycle?


I can record the hum for you all if you like.
#7
^The wiki link has everything you need to know. You'll have to Google the rest.

Basically something isn't grounded right in your mains, amp or guitar.
Try an outlet in another room, or another building to see if it makes a difference.
Try another guitar with the amp.
Single coils are more susceptible.
Consider shielding your guitars electronics cavity if it's not already.
Try a gadget..
http://www.sweetwater.com/store/detail/HumX
#8
Quote by 667
^The wiki link has everything you need to know. You'll have to Google the rest.

Basically something isn't grounded right in your mains, amp or guitar.
Try an outlet in another room, or another building to see if it makes a difference.
Try another guitar with the amp.
Single coils are more susceptible.
Consider shielding your guitars electronics cavity if it's not already.
Try a gadget..
http://www.sweetwater.com/store/detail/HumX

ok thanks
#9
Quote by jkielq91
ok thanks


Also, your amp might have a ground lift switch on the back. If so, try another switch position (there may be two or three possible options).
#11
Those peaks at 100Hz will be power supply hum caused by dodgey filtering. ie the filter capacitors.

The higher frequency stuff is likely to be a dodgey tube.

To test that, start by grabbing a new preamp tube and try it in every position taking notice of what effect that has. It could also be a power tube so next try your spare power tubes.

If that fails it could be a dodgey earth so you'd have to go in search of. If it's that then it gets trickier and pretty hard to describe on a forum.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#12
I discovered today that the hum is only really a big issue when the pre amp gain is turned up.

Does that suggest preamp valves?


Another questions. Is it normal for the amp to make a loud pop when I turn it off? Just as I flick the switch.
Last edited by jkielq91 at May 8, 2013,