Mark III hum

Lorenzo_8 · Sep 27, 2011

My Mark III blue stripe started making a low frequency hum even with the volume set at zero on the clean channel.

Other than that it works perfectly.

Could this mean that I have to change the caps?

If this is the case. Where do I find a schematic with the values?

Boogiebabies · Sep 27, 2011

On the far right back of the circuit board there are two 50 Ohm resistors and four 1N4007
diodes for the preamp heaters and DC foe the relays. These two 50 Ohm resistors are easily damages by a power tube spike. If they go, or are damaged to lower than 50 Ohms, or 100 Ohms in parallel the heaters in the preamp will hum as these are for ground reference for the circuit.

SonicProvocateur · Sep 27, 2011

Boogiebabies said:
On the far right back of the circuit board there are two 50 Ohm resistors and four 1N4007
diodes for the preamp heaters and DC foe the relays. These two 50 Ohm resistors are easily damages by a power tube spike. If they go, or are damaged to lower than 50 Ohms, or 100 Ohms in parallel the heaters in the preamp will hum as these are for ground reference for the circuit.

This.

I ran some Mesa branded 5881's I had been shelving that had only seen about 30 mins of playtime for a few days to break them in, then plugged in my trusty old Sylvania's back and there was some weird hum I couldn't pin down. After a day or so of frustrated tube changing, I noticed the hum went away when I turned my EQ (in my loop) off. Turned out it was a bad cable in the loop. Always work from the little problems to the big ones.

daveg62 · Sep 27, 2011

Same thing with my amp. Two of those resistors were bad and my amp hummed for months until I finally pinned it down. I remember 4 resistors there though for some reason, not just two.

dgr888 · Sep 28, 2011

Check my post on Reverb Hum fix

Lorenzo_8 · Oct 4, 2011

Thanks for the replys.

I'm not really skilled to do any of those things.

Just wanted to know what the problem could be before I bring the head in for service.

Now that I think of it the Reverb is not even working (never use it). When I try raise the Reverb knob it just distorts in a bad way.

So what I will point out to my tech is to:
-check the Reverb and try to turn it around
-check the resistors mentioned

Since I'm here I'll just throw all my questions and maybe someone will have the patience to answer.

I've had this head since 2008 and never changed the tubes...

Should I just change them all?

I'm thinking of having a head shell built since my head is in the rackmount kit.

WIll I need additional screws to mount the amplifier on the head shell?

Is anyone willing to exchange a rackmount kit for a short head shell? :mrgreen:

dgr888 · Oct 4, 2011

The hum can be a power tube or one or more of the filter caps going bad. It also can be a resistor that feeds voltage to the reverb tank that can cause hum/ distorted signal. Your best bet is to take it to a tech to find the problem. After you get it running if there is still a low level annoying hum/buzz try my fix with turning the tank. The tank is much closer to the power transformer in the head versions so extra shielding of the tank might also be required to get rid of it. Good Luck