Mesa .50 Caliber Hum

[mrtoobz]

Hi, everyone. Nice to be a new user...

My son just bought a used .50 Caliber, ~1986 issue, 4 EL84's, 5 12AX7's (none from Mesa) and a lot of hum. Replacing the preamp tubes with new, low noise ones does not apprecialbly reduce the hum. Same for the EL84's. The hum occurs as soon as I plug in to the input jack and defeat the input shorting. Before then, no appreciable hum is evident.

Any thoughts ? I have not pulled the board yet and flipped it to check the components. Looks like a bit of work to do.

 

[laptop44]

Hello I have an 88 Mesa 50 Cab with EL84s. I would check the grounding wire on the reverb tank. It is a very thin wire that is grounded to the rear
send/return pots ground. Being very thin it had seperated from the ground and gave a consistent hum on mine. I replaced the small toy tank with a larger 3 spring accutronic reverb tank and mounted it outside the head. Much smoother reverb now- no hum at all. Also check the two wires soldered to the board. Could be oxidized/cold solder creating the hum. Nice clean amp. Gain channel ok on low volume. Use JJs 12ax7 and Mesa el84s. Tried the Groove tubes el84 but wanted to play safe and stayed with the Mesa.


Good luck

 

[Monsta-Tone]

Sounds like you might need some filter caps too.

Most of the hum that I've seen in amplifiers comes from bad grounding, old filter capacitors, or bad tubes.

 

[bartchamdoh]

High laptop, i,ve got an 87 long head .50 caliber(also with hum lol) and was interested in your comment gain good at low volume.You seem to know your stuff do you know of any fixes for this problem?

 

[jpdennis]

Sounds like you might need some filter caps too.

Most of the hum that I've seen in amplifiers comes from bad grounding, old filter capacitors, or bad tubes.

So here I sit in a pickle with the hum from my .50 Caliber (4 x EL84 style). Tonight I am going to take out V5 to see if the hum is either in the pre v5 stage or post v5 stage (preamp vs power). I am going to check the little white wire in the reverb tank itself as well. Kind of a bummer since I do have a new Mercury Output transformer and new mercury reverb transformer which reduced the hum prior to that change to zero after the normal complete re-cap job new tubes/valves as well. This was about 3 weeks ago. Now when I set my volume above 3.5 with my master at or above 4 it starts the hmmmmmmmmmmmmmmm. I also noticed that the reverb will give the hum some gain as I increase its dial. I have been using a 9 band Boss eq in the effects loop with the two prong boss power adapter. I throw that out there in case I should stop that and just use the tone pots of the amp.

I did read about another Mesa amp (not a 50 cal) where the person said, and I quote

"...I have had this problem & have just completed the repair, the cause is an earth loop effecting the phase invertor, unsolder the earth wire from the pot, (the other end of the resistor goes to the 100 ohm resistor in the neg feedback loop), solder a solder tag, to the wire, then attach the solder tag to 1 of the power transformer mounting bolts, take off the other nuts on the power transformer & clean the aluminium chassis, ALL the hum had gone after this had been done..."

Not sure about the resistor in the 50 cal which would equate to this, but ...

So I thought I would throw this out there in case Boogiebaby, or Monsta-Tone or anyone might have an idea before I move forward or if they find fault with my method of testing for the hum.

Thanks for any help since it may apply to the slight increase in hum in my Mark III. Also the Fender and the Acoustic do not exhibit this problem and they all go into the same building for power (even same recepticle as a check). And of course these high gain amps may just very well go through tubes and I am O.K.

Dennis

 

[jpdennis]

Just want to say thanks to Monsta-Tone and the wealth of info in this forum. Monsta gave me some direction and a lot of food for thought. My hum has all but vanished. I notice all three of my Mesas have an inherent hum which I think is the nature of being a high gain amp and I also notice that the reverb in all three adds to the noise. If I turn the reverb to zero regardless of whether it is truely out of the signal or not the noise is very minimal. The next step is too run a subpanel and dedicated power to my computer systems room and the music room.

Anyway, my Heartbreaker arrived yesterday and after putting the 50 Cal back together I am going to eat then start up the HB and start getting used to it.

Thanks again to Monsta & all of you for the warm welcome I am feeling in this forum.

Dennis

 

[Monsta-Tone]

I'm not Hawaiian, but I believe in Aloha!
My pleasure!

:lol: Next time though, you have to break it so bad that it needs to come see me! :lol:


Sorry to have hijacked the thread!
MrToobz, please feel free to PM me!

 

[mrtoobz]

Thanks to all for the replies. Yes, as I and most of you thought, it was an excessive 60 cycle ground hum. Turned out while working on the amp, someone touched the input jack and the chassis similtaneously, and the hum sudsided. A much closer look revealed that, during a previous repair, a couple of ground straps to the chassis had been cut to pull the pcb but had been re-soldered poorly or not at all. After some work, the hum is now reduced to acceptable levels.

This amp does run way hot though and probably accelerates normal contact oxidation, especially with humidity present in addition to obviously browning & embrittling the pcb around the EL84 sockets. As next steps, I will replace the solid tube cover with a perforated cage and add a low speed brushless fan or two.

Thanks again,

mrtoobz

 

[Monsta-Tone]

Right on!

I usually use a Mark IV fan. They are big enough to move a lot of air.

Also, the Maverick 1x12 has a tiny fan, I think it's the same one as the LSS. It is very quiet and small enough to fit anywhere.

If your amp is a head, you could simply find a Mark IV widebody head cab. It already has a place for the fan.

 

[jpdennis]

Thanks to all for the replies. Yes, as I and most of you thought, it was an excessive 60 cycle ground hum. Turned out while working on the amp, someone touched the input jack and the chassis similtaneously, and the hum sudsided. A much closer look revealed that, during a previous repair, a couple of ground straps to the chassis had been cut to pull the pcb but had been re-soldered poorly or not at all. After some work, the hum is now reduced to acceptable levels.

This amp does run way hot though and probably accelerates normal contact oxidation, especially with humidity present in addition to obviously browning & embrittling the pcb around the EL84 sockets. As next steps, I will replace the solid tube cover with a perforated cage and add a low speed brushless fan or two.

Thanks again,

mrtoobz

I am interested in a picture showing your ground strap work. Is that possible?


Dennis