Mark III hum problem??

[rgx612a]

Just got this head in today (simul-class, red stripe) and it sounds great, but i'm getting some hum out of the lead channel, which gets worse with more gain and volume.

I tried some different pre-amp tubes in all positions, but they made no real difference...?? tried the ground switch, same...

Power tubes maybe?? :? I'm out of ideas.

It's not really loud or anything, but it's enough to be annoying when not playing at higher volumes. My .50 caliber + is alot quieter in comparison.

 

[AustinK]

Are you using single coils?

Also the filter caps may need to be changed.

The mark III is very sensitive and it really makes these things obvious.

 

[rgx612a]

Are you using single coils?

Also the filter caps may need to be changed.

The mark III is very sensitive and it really makes these things obvious.

No, i'm using humbuckers.

the sound is there with the volume knob on my guitar at zero, so i know it's not my guitar or cables...

 

[disassembled]

If power tubes are bad or mismatched they can produce hum. Maybe you should try swapping out the power tubes with good ones and see where that takes you.

Greg

 

[Boogiebabies]

IF a power and preamp tube swap does not help, look at the two 39ohm and two 100 Ohm resistors directly to the left of the first 30uf/500V blue filter cap at the bottom right hand of the circuit board. This is looking from the back of the chassis. The 100 Ohm are for hum balance and reference to ground for the preamp tube heaters. The 39 Ohm resistors work with the DC rectifier bridge for the V1 tube heater voltage. If either blow or drift you will get a hum that you will not get rid of until they are changed.
A quick check is to read the two 100 ohms at the same time in parallel. They should read 50 Ohms. The two 39 ohm resistors may read 19 ohms in parallel, but they are in the circuit with the two 100 ohm resistors and four 1N4007 diodes, so your reading will more than likely be off. This is a seriously common issue with MK III's. If you send it to a tech, make sure they check the 33K resistor on the far left of the board that connects to the Red lead of the reverb transformer. These are also known to go bad.
A quick look may tell you that the 33K and 39 Ohm resistors are bad if they are turning from blue to brown. In the MK III Mesa used predominantly metal film resistors. When they turn brown this is a sign that they have been overheated and can kill or imbalance the circuit.

 

[rgx612a]

IF a power and preamp tube swap does not help, look at the two 39ohm and two 100 Ohm resistors directly to the left of the first 30uf/500V blue filter cap at the bottom right hand of the circuit board. This is looking from the back of the chassis. The 100 Ohm are for hum balance and reference to ground for the preamp tube heaters. The 39 Ohm resistors work with the DC rectifier bridge for the V1 tube heater voltage. If either blow or drift you will get a hum that you will not get rid of until they are changed.
A quick check is to read the two 100 ohms at the same time in parallel. They should read 50 Ohms. The two 39 ohm resistors may read 19 ohms in parallel, but they are in the circuit with the two 100 ohm resistors and four 1N4007 diodes, so your reading will more than likely be off. This is a seriously common issue with MK III's. If you send it to a tech, make sure they check the 33K resistor on the far left of the board that connects to the Red lead of the reverb transformer. These are also known to go bad.
A quick look may tell you that the 33K and 39 Ohm resistors are bad if they are turning from blue to brown. In the MK III Mesa used predominantly metal film resistors. When they turn brown this is a sign that they have been overheated and can kill or imbalance the circuit.

Thanks very much, i will have a look at those resistors soon.

 

[rgx612a]

Different power tubes didn't do anything.

I cleaned out the pre-amp tube sockets with some contact cleaner and that actually seemed to tame the hum a fair bit... there was some dust/dirt build up all in and around the tube sockets, i'm guessing from the fan constantly blowing air in that area??

Still gonna check out those resistors, but it's relatively quiet now. Not dead silent, but reasonable. It's just the higher gain settings that are any problem.

 

[Boogiebabies]

Dirty or loose preamp sockets will spit and crackle when you wiggle the tubes.

 

[rgx612a]

Dirty or loose preamp sockets will spit and crackle when you wiggle the tubes.

Just to be sure, the first 500v Blue capacitor (of the three) you're talking about is the very bottom one right? (when looking from the back of the chassis). So you're talking about the four resistors in a row right beside it there?

Sorry, I don't know too much about this type of stuff...

 

[MrMarkIII]

"...Sorry, I don't know too much about this type of stuff..."

Uh, there are lethal voltages present...
Perhaps a trip to a tech is in order?

 

[Boogiebabies]

Dirty or loose preamp sockets will spit and crackle when you wiggle the tubes.

Just to be sure, the first 500v Blue capacitor (of the three) you're talking about is the very bottom one right? (when looking from the back of the chassis). So you're talking about the four resistors in a row right beside it there?

Sorry, I don't know too much about this type of stuff...

Yes. It would be 39/100/100/39 and have four diodes above them.

 

[Boogiebabies]

"...Sorry, I don't know too much about this type of stuff..."

Uh, there are lethal voltages present...
Perhaps a trip to a tech is in order?

What ? It's not like I told him to stick his left nut on a fully charged cap.
He just needs to look at it. It's a visual inspection. So, take your UH and cram it in your cram hole LaFleur !!! :roll:

Now, back to a visual inspection and Dodgeball.

 

[rgx612a]

"...Sorry, I don't know too much about this type of stuff..."

Uh, there are lethal voltages present...
Perhaps a trip to a tech is in order?

What ? It's not like I told him to stick his left nut on a fully charged cap.
He just needs to look at it. It's a visual inspection. So, take your UH and cram it in your cram hole LaFleur !!! :roll:

Now, back to a visual inspection and Dodgeball.

Haha!

I may not be that knowlegable about the inner workings of an amp, but i do know about capacitors and that they hold a potentially lethal charge. I'm not getting anywhere near those things.

Anyways, i checked those resistors with an ohm-meter and they read pretty much what you said they would... the 39 ohm ones read around 19 - 20, and the 100 ohm resistors kinda bounced between 48 - 52. So i guess they're alright?

Actually, i think this amp is all good now, those sockets just needed a bit of a cleaning.... and i just swapped out a bunch of 12ax7's in the V3 position and found one that helped it even more. So, the hum is alot quieter now.

Thanks. 8)

 

[Boogiebabies]

Excellent news !!!