Weird Hum

Hi all-

My 5:25 Express Plus head has started doing something strange. When in 5 watt mode there is this low "hummmmm" noise. It does it on both channels and with different guitars, but only on the 5 watt setting.

Since 5 watts runs one power tube, a bad power tube was my initial thought. However I put in another set of tubes and still got it. However the tubes I put in, while I believe they are a matched pair, are not Mesa tubes, so maybe that is still it?

Thoughts? Could it be preamp tubes or something else?

Thanks!

mesa works normally with other brand tubes...i use JJs...

get one 12ax7 and swap each preamp tube to find out if there is a bad one ...
every time after you swap the tube in one socket, turn the amp on and see if theres any hiss...
then turn off the amp and repeat until you swap all...

try the phase inverter tube good luck len Im sure its nothing serious

Thanks guys I will try your suggestions.

Preamp tubes might be the issue, but isn't it more likely to be a power tube? I am asking this because I only hear it in the 5 watt setting which only uses one power tube. I don't seem to hear it on the other settings. Of course it might just be getting masked.

Krek13 said:

mesa works normally with other brand tubes...i use JJs...

every time after you swap the tube in one socket, turn the amp on and see if theres any hiss...

Click to expand...

I am aware that you can use other tubes, particularly for the preamp tubes. However, its not hiss, but more like a hum sound.

I went crazy tracking down hum on a plexi and it turned out to be a preamp tube. I do agree with you that you would think the hum is from a power tube fist and especially that its only in the 5 watt mode but you did change power tubes already right? You could call mesa on the phone they're pretty good at trying to help over the phone. i hope its just a tube, most times it is. good luck.

Swapped out the PI tube and that didn't seem to do it. I have new power tubes on order. We will see what that does. Any other ideas!

I sold my egnater to get away from these type problems.

I contacted Mesa Boogie. They said that there is a bit of a hum on some of these amps in the 5 watt mode due basically to how the biasing changes going down to the one power tube. They also suggested some fairly standard things like trying a different electrical plug and whatnot.

They did offer for me to send it back and they would give it the once over.

Put in new power tubes. Still got the sound PLUS... heat and burning smell! SSSOOOOOOOOOO... back it goes!

We'll see what happens.

good for you-piece of mind love this amp

I don't know that they will exchange it. I have a feeling they are going to find nothing or maybe a blown fuse or transformer. Still, they offered to look at it so why not?

Len Rabinowitz said:

I contacted Mesa Boogie. They said that there is a bit of a hum on some of these amps in the 5 watt mode due basically to how the biasing changes going down to the one power tube. They also suggested some fairly standard things like trying a different electrical plug and whatnot.

They did offer for me to send it back and they would give it the once over.

Put in new power tubes. Still got the sound PLUS... heat and burning smell! SSSOOOOOOOOOO... back it goes!

We'll see what happens.

Click to expand...

if you didn't get mesa power tubes, did you get matched power tubes that were tested to fit the mesas' bias?

The new ones are Mesa power tubes.

I think it could be bad screen resistor or plate resistor

Turns out that I am pretty sure there is nothing wrong. It's in a room with some electrical issues- rheostats for lights and whatnot. Changing which plug I used helped. Some of it was getting exacerbated by sitting too close to the amp, even with humbuckers. Mesa said that new tubes can have a burning smell but that it goes away, which this did. They also said that the EL84's do run hotter than the 6l6's in my 5:50.

So, all's well that ends well I guess. I certainly learned some things about the amp, and if the worst that happened is buying a new set of power tubes, what's wrong with having a spare set?

Thanks for the help!

Good to hear you got it worked out. just got a replacement express today(noticed the old box had an RA# on it?)any way- new one sounds great- this is mesa's best amp does everything well

What was wrong with the old one?

never found out- they sent me new preamp tubes and I tried new power tubes -still crackling hiss and hum. Seems they sent me one that was returned before maybe (ra# on box)? I let them hear the noise on the phone and they imeditly said it sounded like more then just bad tubes so they replaced it.

Bit of an update here. Amp worked okay for awhile and then started giving me almost no volume on the 5 watt setting only. Did that on both channels. Mesa said to send it back to them which I did. They have it now and I am waiting to hear from them.

I am wondering why they wanted it rather than take it to the local authorized repair guy. Maybe it's a weird problem and they will just send me a new one!

Len Rabinowitz said:

Bit of an update here. Amp worked okay for awhile and then started giving me almost no volume on the 5 watt setting only. Did that on both channels. Mesa said to send it back to them which I did. They have it now and I am waiting to hear from them.

I am wondering why they wanted it rather than take it to the local authorized repair guy. Maybe it's a weird problem and they will just send me a new one!

Click to expand...

Mine has just developed a hum on the 5 watt setting as well. I don't perform at that setting, but do practice with it there. Let us know if Mesa finds an issue with your.

Al