Help me diagnose a hum issue

Any help appreciated!

I recently bought a (used) Mark V. It was working fine. Seller said tubes were old, so I swapped in some new ones I had (all genuine Mesa):

4 x 6L6
1 x 5U4
1 x 12AX7 (in V1 position, I left the other pre-amp tubes untouched)

...then I played a gig. Went great! Amp sounded amazing, so far so good.

Then I took it to rehearsal a couple of days later, and noticed an intrusive hum. It's like a flat buzz, very constant. Characteristics:

• 
  The hum is only present when amp is fully powered up (i.e., NOT in standby mode)
  The hum is present in all channels, and is gain dependent. It's barely noticeable in low gain channel settings (e.g., Channel 1, Clean, low gain). It's very loud in high gain settings (e.g., Channel 3 pretty much anything!)
  It's there (but pretty quiet) if nothing is connected to the amp.
  It gets worse when I plug a (known good) guitar cord into the amp input.
  It gets even worse when I plug my guitar in.
  The hum basically disappears if I turn the volume on the guitar down.
  The hum varies a lot as I move the guitar around - pickup orientation seems to matter, and pickup proximity to amp matters a lot... the hum gets really bad when I move the guitar close to the amp.

I thought it might be bad AC at the rehearsal room, but it's also happening at home (where I've used lots of gear with no problems like this). I've tried two different guitars and a couple of different cords. It really seems to be the amp.

I'm guessing it's tube related, but I'm wondering if anyone has any tips for diagnosing which tube(s) it's likely to be?

Also, could this be caused by the amp getting jostled in the back of the car, driving too/from gigs and rehearsals?

Thanks in advance for any help!

Hmmmmmm....

Like that!?!?

Last time I had an issue like that, I swapped out the inner
power tubes and it went away. It was a bummer though
'cause the tubes were less than 9 months old from Mesa.
I had a "scratchy" type of sound, like a fizz that I found
to be pre-amp tubes...these I found easily by bypassing
the loop and tapping tubes.

Good luck!!

Is your Effect Loop engaged? I have had some issues with the effect loop and hum previously.

Is Reverb engaged? Try taking that out as well.

Try reseating your power tubes, I do this pretty much every time I move the amp. As well, you can swap the pairs to see if that has any impact.

Try replacing the V1 tube, I have had 12AX7 tubes die within 20 hours of playtime previously (unfortunately, not spectacularly.

Thanks for the responses.

Thought I'd keep a log of progress in case it helps anyone in the future.

• Tried switching loop to HARD BYPASS - that made the hum worse (about twice as loud)!

• Tried re-seating power tubes and rectifier tube - no difference

• Ensured reverb and EQ were both disabled - no difference

• Ensured reverb and EQ were both disabled - no difference

• Switched V1 12AX7 with spare (new from box Mesa) - no difference. Initially I thought it was worse, but it turned out I just had the amp near my computer, which was creating a new high pitched noise. Moving the amp away a bit fixed that. So I think the original V1 was probably fine.

• Switched power tube pairs - no difference

• Tried a different AC cable - no difference

Taking a break - my perception is shot at this point. Especially since I tried plugging my guitar into my Roadster combo... and I'm hearing more or less the same hum there now. So weird. I would swear neither of these amps was making this noise previously. Now with two different guitar cords and two different guitars, I'm hearing it in both of them. Maybe it's normal and I somehow wasn't hearing it before?

Will keep investigating.

I'm wondering if you have a loose ground somewhere in your power circuit.
I've heard florescent lighting could cause hum as well. Hope you find the
issue.

Like JB says, could be your power
- try different outlets, rooms, go to a bud's house, etc
Search electromagnetic interference
My old house was a ***** concerning the lights and DIMMERS

There will be a hum and may not be avoidable. Lighting, yes florescence lamps will cause noise (includes the ones that can be used in place of tungsten bulbs). Power consumption by other devices on the grid of your home or environment (dryers, air conditioners, washing machines, etc..) will place transients on the power supply that may not be fully suppressed and will end up in your amplified signal. Power conditioner may be the only means to remove electrical noise from the supply.

The Mark V uses two types of heater circuits. There are only three tubes that use DC heaters (v1, V2 and V5), all the rest use AC heaters (includes rectifier and 4 power tubes). AC heated tubes can pick up the 60 Hz hum. with all of the settings turned all the way to lowest setting, it may be a power tube causing the hum if present when the amp is not in standby. I have often found that some preamp tubes are noisy in V3 and will only effect CH1 and CH3, CH2 remains quiet. I typically get hiss with JJ or Mesa tubes which is an easy fix. EH or Tung Sol when used in v3, v4, or V6 typically is hit or miss on the hum issue. I have had my Mark V super quiet with some tubes (not changing V7, rectifier or power tubes) Hum may return after swapping out V3, or any tube there after in the chain. Most of the time it is tolerable and does not irritate me that much.

Thanks folks, good tips.

Bandit, yes, I'm expecting some hum... I'm not expecting total silence. My Express 5:25+ Combo has a wicked hiss to it when gain is maxed, and that's fine by me.

This is a kind of grating buzz, though, I don't think it's normal (although this is my first and only Mark V, so...?). I have an email in to Mesa, they should answer Monday/Tuesday. I'm guessing they'll set my expectations regarding how much noise to expect. And of course it's a tough thing to describe in text. That's why I took a break the other night... I just couldn't tell what was normal or not any more!

After some more fiddling today, I am pretty convinced that the noise is coming from the pre-amp stage. Two reasons:

1) The noise goes away completely if I dial down the gain.
2) The noise is shaped by the pre-amp tone controls (treble, mid, bass)

Further, I think it has to be V1 (if it's a tube issue), because it's present in all channels. It seems to be purely a function of gain, so it tends to be more audible in CH3 because that's a gainier channel. But CH1, Brit, with high gain settings has it almost as bad.

But I've tried a coupled of 12AX7s in V1, with no noticeable difference. So I'm starting to think that this is not a tube issue at all. I wonder if a cap has blown or something? Or a wire has come loose? After I've spoken to Mesa, I'm thinking it might be time for a pro service.

I will try switching V3... although the manual says that V3 should only affect CH1 and CH2, not CH3. But still, I'll give it a go. I don't have a better theory!

>Power conditioner may be the only means to remove electrical noise from the supply.

I wondered about this... would it make sense to invest in one for the Mark V just generally speaking? If so, any recommendations? I know Furman are common... is that a good brand to look at? This would be for gigging, so I'd need something small/portable (I don't take a rack to gigs).

If you are using the effects loop active for using the master volume, V6 will influence all channels. But since you mentioned that the tone controls will effect sound, chances are V1 is the culprit. Does the noise go away in the low power mode? That will force the amp to run on the rectifier tube in all channels. try turning down all of the presence controls as that is a feed back signal path from the output. if it goes away, hum may be caused by a tube latter on in the chain than V1.

It is odd that I get noise in CH1 and CH3 but CH2 remains quiet without input signal. I found that V3 (depending on tube type) I will get hum or hiss in CH1 and CH3 but not CH2. Some tubes even if not part of the gain chain may effect what you hear on the output if it is caused by self induced issues and if it is not physically disconnected from the signal path. Tube operating at its idle point with no input signal will make noise if there is something wrong with it.

Looking at your initial post, I see a few key things:

It gets worse when I plug a (known good) guitar cord into the amp input.
It gets even worse when I plug my guitar in.
The hum basically disappears if I turn the volume on the guitar down.
The hum varies a lot as I move the guitar around - pickup orientation seems to matter, and pickup proximity to amp matters a lot... the hum gets really bad when I move the guitar close to the amp.

If the hum is worse when the instrument is plugged in, and changes with the volume on the guitar - this is a sign your instrument and cable are acting as antenna/reciever - fluorescent lighting, computer monitors, fans, dimmer switches, or proximity to other electrical-based influences. The hum varies when you move your guitar around and also when you are close to the amp because of interference being picked up by your instruments pickups.

When troubleshooting hum in the amp, its best to have nothing plugged into the input jack, as that will isolate hum from the amp vs. hum from your guitar and/or cable.

Thanks!

Regardless if the tube chart indicates V3 is used by CH1 and CH2 and not CH3, if V3 was not used by CH3 I would be able to pull it with no effect while the amp is set to CH3? Not the case. I was tube rolling last night and wondered that myself so why not pull it and hear for myself. V3A controls all three channels, signal for CH3 then passes on to V6B for the master volume control, V3B provides signal for the master volume controls of CH1 and CH2. If V3 is creating the noise issue it will affect all three channels. I cheated ( I looked at the signal path in the schematic).

I normally run a noise check when tube rolling, I first switch all three channels and listen to the change in noise levels before I plug in my guitar for the sound check. One thing I do not want to listen to is hum or hiss , a low level is okay but not excessive.

Once in a while I encounter a tube that causes hum. Usually this only happens when I am tube rolling. I get bored with the tone, I need to update preamp tubes, Rectifier tube, or the power tubes. I do have a preference for EH and Tung Sol in my other amps. They do sound good in the Mark V but on occasion I get a tube that causes hum. It does not matter what I do, turn off reverb, hard bypass or with effects loop on. I used to believe it was associated to V3 since that has an AC heater circuit (Along with V4, V6, V7 and of course the power tubes). However, my favorite tubes (vintage 1990 Mesa Chinese 12ax7A) that I had in the amp became unstable and sounded terrible. CH3 was really noisy. Overall tone was dull and the reverb went absent. I decided to scrap the old tubes and begin again with the stock Mesa 12ax7 (JJ tubes). I usually begin by re assigning the preamp tubes that make the least amount of noise (I find the JJ type tubes create abundant white noise or hiss). This time I worked from the back end to the first tube. Considering V6-V3, I usually get a quiet arrangement or it gets noisy. Once I cycle through all channels and modes and listen to the change in tone I will continue with the other tubes. This time though I encountered an issue. Reverb was out. Not present in the signal chain. (I pull the amp chassis out along with the reverb tank as I found the tubes that drive and receive the reverb signal also can effect overall tone of the channels. ).

I was able to repair the reverb circuit but wound up with an abundant reverb presence. Volume controls for the channels did not have much of an effect. I was able to restore the amplitude of the reverb to normal with a 12AT7 in V4 (yes that impacted CH3 gain structure, actually I like it more now). V4 is the noise generator due to the method of how the reverb transformer is driven. My oscilloscopes I have at work are not suitable for the high voltage of the DC supply so I cannot measure the ripple on the 480VDC which is used to drive the reverb transformer. It is this ripple that causes the hum. The 10K used to set the reverb transformer current may be a bit too high. Other Mesa designs, a 22K resistor is used.

If you still have the hum issue, there is an option, 12AT7 in V4. (typically more common for reverb circuits). Mesa has one available and is used on the RA100. The Mesa 12AT7 is Chinese. Actually the old Mesa tubes I liked the most are made by the same source. I do have an NOS RFT 12AT7 (using this in the RA100), seemed a bit noisy compared to the Mesa branded tube. I believe the 3 mica spacer design of the Chinese tube lends to its low noise level. Those I found work great as a driver tube. Unfortunately, since the Mark V uses V4, V5 as part of the ladder gain stages for CH3, what you put into V4 and V5 will also effect your tone of CH3. I found the 12AT7 took out some of the blurred character of the CH3 MKIV mode. Extreme sounds much better as does the MKI. V3 can also contribute to noise but V4 is more prone. Even with the Reverb switch out via the foot switch, or the volume pots turned all the way down, it will not remove V4 from the circuit. Also the noise will be present with or without the reverb tank connected. Since the signal path used to drive V4 is not a one way circuit, any noise caused by the ripple on the DC supply will back track into V3 and V6 which are the master volume for CH1, CH2 and CH3. Reason why the noise may go away when switched to CH2 is the change in voltage divider network for reduction in reverb drive amplitude.

Posting an update to this...

This turned out to be nothing to do with the Mark V, and everything to do with the electrical interference in our rehearsal room. Switching to a different guitar (different pickups) reduced the hum a lot.

Turns out we are right opposite a power station, so the amount of EMF is pretty high!