Hi All,
One thing makes me worry : the closeness of the power transformer and the rever unit... I suspect magnetic induction, which would make impossible to get rid of the hum, unless you do not use reverb, or make a taller head, or shift to a combo !
To check that possible problem, I have made some "closeness simulations" : using my MKIIA Tolex cab, speaker removed (listening via my Thiele cab), I placed several reverb units in different positions more or less close to the power Xfo, including of course the position used by Mesa in the stock MK series Rev option heads. Same amp settings for all, indeed. Here are some pictures of the trails :
The normal reverb unit in its normal combo position on my MKIIA. The tank is acoustically insulated, and the open side is down, fixed on a carton shielded with glued alimentary aluminium foil : no hum and no larsen at any setting, tone is deep and excellent.
Another reverb unit in the combo position. The tank is closed by a srcewed aluminum plate (1.5mm), Open "closed" side down, no acoustic insulation except 5mm felt glued externally on the aluminium plate, output (recovery) of the reverb is the furthest from the power Xfo : no hum but sensivity to external noise and larsen, tone is more metallic.
Boogie head position. The reverb unit will remain the same now. open "closed" side down. Reverb tank pushed near the front of the cab. Distance from power Xfo is 10mm between metallic parts, but reverb output is now the closest from the power Xfo : unacceptable hum, non valid position. Then reverb is returned to its original "furthest output" position : much less hum, but more hum than in the combo position.
Boogie head reverb position. The reverb is now placed open "closed" side up and remains "furthest output" position, same other conditions. Slightly less hum than the open "closed" side down position, so it is the best boogie head position, but always more hum than the combo position, and the metallic tone, the sensivity to external noises and larsen remains.
VERDICT : an acceptable compromise... if :
- the position chosen by Mesa for the MK heads is the best one, given the room available : open "closed" side up, and "output furthest", pushed near the front face of the headshell.
- the tank must closed by a shielding plate.
- the tank may need to be directly grounded via a separate wire to the chassis.
- the tank would be very profitably insulated acoustically in order to avoid metallic tone, sensivity to external noise and larsen.
But the combo position remains obviously better in terms of hum because the tank is as far as possible from the power Xfo.
A+!
Alors le verdict ! le A ou le B ?
