Mesa Boogie mini rectifier problem

[stratgate47]

Hi, I've got a problem with my mini rectifier.

There s always a hum on the 10 watts of the gain channel....
No hum when I switch to 25 watts or if I go on the clean channel (10 or 25 watts).

Even if I put the guitar volum at 0, I still have the hum...

I tried to change the tubes witout sucess !

Anyone knows why ?

Thanks

[soundguruman]

Possible design defect...
but could probably be modified.

Although they are always telling people to "change all the tubes"...
it really only addresses 50% of the real problems.
(and sells a LOT of tubes, hahahahahahahahahahahahahhaha!)

Hum and noise is an ongoing problem in many of these designs.

MOST of the time, in many of these designs, it's AC filament voltage which is entering the audio path.
That AC voltage is then amplified.
This can be addressed by converting to DC heaters for the preamp tubes.

Other times, it's
a bad grounding plan.
This can be addressed by splitting the grounds.
The preamp ground needs to be separated from the power supply grounds.

They have tried to compensate, by "doubling" the filter capacitors,
however this is a lame solution.
The real problem is that the power supply hum / ripple is combined into the preamp ground.
OR that the filament AC voltage is being amplified in the audio path.

Like I said, it "can" be modified.

[Mick Bailey]

You say you tried to change the tubes. What tubes did you use and did you just change the preamp, power tubes or both?

Additional hum is quite common in amps that switch the output pentodes into triode mode to reduce power and the mini-rectifer is known for it. But, it usually affects both clean and drive channels when in 10W mode. You say yours is OK on 10w clean - are you certain? I thought the preamp remained as-is and only the output tubes were switched on these amps to reduce the power. Can anyone verify this?

[Chuck H]

Mesa has been known to employ channel switching that causes buzz on the clean channels of their amps because the stages that are switched out aren't isolated well enough. This seems to be the opposite problem. But no one can tell about that without personal experience or a schematic.

[Believersblues]

Ah yes, a schematic. I'd like to have one for this amp to fix this very problem for a co-worker. Unfortunately, i could have BUILT one of these amps in the time i have spent VAINLY searching for one!!!
Sez JMB

[Pedro Vecino]

You say you tried to change the tubes. What tubes did you use and did you just change the preamp, power tubes or both?

Additional hum is quite common in amps that switch the output pentodes into triode mode to reduce power and the mini-rectifer is known for it. But, it usually affects both clean and drive channels when in 10W mode. You say yours is OK on 10w clean - are you certain? I thought the preamp remained as-is and only the output tubes were switched on these amps to reduce the power. Can anyone verify this?

Yes. The only automation for the 25w / 10w change is the assignment of the EL84īs in triode mode but has a relay added to assign a different bias setting depending on the mode selected.
I agree that the noise coming from the lack of symmetry (current at idle) in the tubes in triode mode should be common for the whole amplifier. I had one and I did not have that symptom.