C+ transformer hum

[mule#1]

I've heard transformer hum is a common problem. Does anyone have an idea on how it can be eliminated or made better? This is a 60 watt non EQ model.

Thanks!

 

[dodger916]

Hey Rob. It's not always the transformer. I've read that power tubs can be the source, especially if they are not matched.

 

[mule#1]

Thanks Frank. I am getting it out of my STR-415 and 5881's though. I thought it may be the tubes so that's why I tried another set. I'll try another set to be sure. It does seem like it's worse than it was. Maybe I should check the pre-amp tubes?

 

[zodiac272]

yeah check the preamp tubes too... maybe the phase inverter?

scott

 

[dgr888]

If it has reverb check out my post for the fix. This is a common problem on the reverb models where the tanks were installed backwards by "Mesa Engineering". Go Figure...

 

[Boogiebabies]

1. Does the hum stop when you physically hold down the transformer ?
2. If yes, get nyloc washers and new mounting bolts
3. If it still hums, chech the two 100 Ohm Resistors below the speaker jacks for 50 Ohm in parallel.
4. If you get 50 Ohm in parallel look at two green wires from the power transformer to next to the phase inverter on the power amp board. Make sure they are not loose. If you don't get 50 Ohm, replace both resistors with 100 Ohm 1/2 watt. These resistors balance the hum from the heater circuit and are elevated reference to ground.
5. Check the twisted wires from the right power tubes pin 2 and 7 to the V1 Pin 4/5 and 9.
Make sure they are not loose.
6. Preamp tubes can hum
7. Imbalanced power tubes can hum.

That's all I can think of.

 

[DaveP]

There is a certain level of hum Mesa will deem passable. I had a Mark IV that was beyond the normal and Mike B replaced the transformer to correct it.
Not saying yours is bad but some will hum to a certain extent.

 

[mule#1]

1. Does the hum stop when you physically hold down the transformer ?
2. If yes, get nyloc washers and new mounting bolts
3. If it still hums, chech the two 100 Ohm Resistors below the speaker jacks for 50 Ohm in parallel.
4. If you get 50 Ohm in parallel look at two green wires from the power transformer to next to the phase inverter on the power amp board. Make sure they are not loose. If you don't get 50 Ohm, replace both resistors with 100 Ohm 1/2 watt. These resistors balance the hum from the heater circuit and are elevated reference to ground.
5. Check the twisted wires from the right power tubes pin 2 and 7 to the V1 Pin 4/5 and 9.
Make sure they are not loose.
6. Preamp tubes can hum
7. Imbalanced power tubes can hum.

That's all I can think of.

The hum doesn't stop when I touch the xfmr but I'll push it as well as check out all the suggestions you all mentioned and report back. I'm pretty sure it's not the tubes as I've had 2 matched sets in there. It's not horrible but noticeable especially through a PA system.

 

[dodger916]

Rob: Did this just happen or an old nagging problem? I take it you did not hear hum when you played my C+.

 

[Boogiebabies]

Rob: Did this just happen or an old nagging problem? I take it you did not hear hum when you played my C+.

That amp is dead quiet, plus it has fresh filter caps and had every component verified with and without load to check for drift. It's a terrible example to use as a reference because it only comparable to the best of the C+'s and it has the export. I highly doubt anyone would ever get the opportunity to test enough C+'s to get even close to yours. 1 in 1900 shot.

If the hum is not tracked down to any of the common issues it may be AC sneaking through the caps.

 

[mule#1]

Rob: Did this just happen or an old nagging problem? I take it you did not hear hum when you played my C+.

It seems to be a new development! I don't recall hum in your C+. It can't be heard when playing only when idle.

 

[mule#1]

Rob: Did this just happen or an old nagging problem? I take it you did not hear hum when you played my C+.

That amp is dead quiet, plus it has fresh filter caps and had every component verified with and without load to check for drift. It's a terrible example to use as a reference because it only comparable to the best of the C+'s and it has the export. I highly doubt anyone would ever get the opportunity to test enough C+'s to get even close to yours. 1 in 1900 shot.

If the hum is not tracked down to any of the common issues it may be AC sneaking through the caps.

I will pull the chassis out this weekend as I never looked to see if it ever had the caps replaced! I will check all suggestions out at that point. It sounds awesome though lol! Just more hum than I would like. I will check the reverb idea out as well. However my reverb is at 0 most of the time.

 

[dodger916]

Rob: Did this just happen or an old nagging problem? I take it you did not hear hum when you played my C+.

That amp is dead quiet, plus it has fresh filter caps and had every component verified with and without load to check for drift. It's a terrible example to use as a reference because it only comparable to the best of the C+'s and it has the export. I highly doubt anyone would ever get the opportunity to test enough C+'s to get even close to yours. 1 in 1900 shot.

If the hum is not tracked down to any of the common issues it may be AC sneaking through the caps.

I will pull the chassis out this weekend as I never looked to see if it ever had the caps replaced! I will check all suggestions out at that point. It sounds awesome though lol! Just more hum than I would like. I will check the reverb idea out as well. However my reverb is at 0 most of the time.

Send it to Ed for blueprinting...

 

[dgr888]

Check the reverb out as even with it at zero hum still is apparent. A quick check is to pull the reverb out away from the cab to see if it gets quiet and if it does turn the tank 180 degrees around and put it back in. Let us know what you find.

 

[mule#1]

Check the reverb out as even with it at zero hum still is apparent. A quick check is to pull the reverb out away from the cab to see if it gets quiet and if it does turn the tank 180 degrees around and put it back in. Let us know what you find.

I pulled the chassis out to check these things out. Unplugged the reverb and that is not it! Also checked phase inverter and that is not it either. I'll dig into this more now that Thanksgiving is over and report back.

 

[gts]

I could be totally off the mark here (no pun intended) but wasn't there something awhile back about the wiring in someones house causing a similar problem?

 

[mule#1]

I could be totally off the mark here (no pun intended) but wasn't there something awhile back about the wiring in someones house causing a similar problem?

That would be bad as I built my house 7 years ago and wired it myself. I am an electrician by trade :lol: I even have separate neutrals on the outlets in my studio.

This is the first I've had the problem but I will say I have made improvements. I found a couple of 12AX7 tubes that were slightly micro-phonic and amplifying the hum. I didn't notice the issue when I first got the amp so I think was a new development.