Mesa Mpulse 600

[Wilder Amplification]

I've got a Mesa Mpulse 600 here for repair. The main problem has been fixed (shorted bridge rectifier on the main supply) and I've load tested it at about 65W output @ 1kHz and all appears well.

In checking the idle bias across the .2R resistors on the power block I measured roughly about 9-10mV. I've brought this down to 4mV and I'm seeing a DC offset of about 100mV at the output. On the scope there appears to be no sign of crossover distortion at this bias setting.

Before I give it a clean bill of health, does anyone happen to know what the idle bias should be for the power block on the Mpulse 600? Also, does anyone happen to know what the acceptable output DC offset should be as well? I appreciate any information you all can give me regarding this.

[soundguruman]

The DC offset on the output should be zero, obviously.
There's something else wrong.
But once again, dealing with a company that prevents customers from obtaining information, prevents customers from servicing their own equipment.
Sometimes they have updates, that only the service center can know about. They won't tell YOU.

[Steve Conner]

Yeah, it's all a conspiracy, blah blah.

100mV DC offset is fine for a musical instrument amp, IMO. There may be a trim pot for adjusting it. But also bear in mind that it could settle down as the amp warms up.

And about the idle bias, in musical instrument service the less the better. Nobody will notice the crossover distortion after 5 beers anyway, so we set it to keep the heatsink as cool as possible. 4mV seems fine.

[Enzo]

Is that 100mv with or without a load? A lot of amps show a tiny offset like that when not loaded, but don't push the issue when a speaker is connected. And besides, I wouldn;t worry about 100mv anyway.



I don;t use those currents anyway. I adjust up to crossover notch disappearance, then back off a little to the cool side.

[soundguruman]

Is that 100mv with or without a load? A lot of amps show a tiny offset like that when not loaded, but don't push the issue when a speaker is connected. And besides, I wouldn;t worry about 100mv anyway.



I don;t use those currents anyway. I adjust up to crossover notch disappearance, then back off a little to the cool side.

Run it hot, a guitar player knows it sounds better that way.
Conspiracy? Hardly, more like stupidity.
If you go to all the trouble to build an amp with a G10 fiberglass medical grade circuit board, and then locate the circuit tracks too close together, so they arc and burn, that's just plain stupidity.
Add to this the arrogance (supreme) and you have a mesa amplifier.
I would NEVER buy one.
There are too many other manufacturers that support their product without the arrogance.
Even PV has much better support.

[Jazz P Bass]

If the offset is worrying you, sometimes you can get it closer to zero if you change out the LTP input transistors.
Matched is ideal but same batch is generally close enough.

[Wilder Amplification]

@soundguruman...in a perfect world yes the offset should perfectly zero out. That assumes everything 100% matched and that there's a way to adjust out any offset to correct for any mismatches. This amp however has neither.

@Enzo...100mV is the loaded offset voltage with a 4R resistive load.

I went ahead and set it just to the point of zero crossover and didn't worry about the offset. Thanks for the replies guys.

[soundguruman]

You are enough of a tech to recognize that there is DC on the output, but not enough to find out WHY.
There should be NO DC on the output.
DC on the output will cause excessive heating of the speaker, or damage it.
The first step would be to disconnect the feedback loop, and see if the output transformer windings are actually leaking. Very dangerous condition if true.
There are plenty of techs who make amps "work." Now let's see if you can make it work RIGHT.

[g1]

Soundguruman: You understand this is a solid state power amp? All this talk about biasing hot for guitar players and leaky output transformer windings makes no sense.
Tube amps are not biased at around 4 millivolts across .2 ohm emitter resistors, but solid state amps are.

[JoeM]

Well, fixing a SS amp RIGHT, you've got to check the OT windings!

[mozwell]

not sure if this has ben posted here, found it in my travels, might help someone else.....
SS power amp, a valve on the input circuit & also to drive the power amp mosfet gates.

no DC offset adjustment, just a bias adjustment.
Mosfets directly driven from a 12AX7A through a DC blocking caps.
4mV & 0.22 ohm is 18mA, 1.6W dissipation on each mosfet at idle, should be fine. Even then 10mV is approx 45mA & 4W dissipation on each mosfet and this bias setting shoudl be fine also.

[soundguruman]

Maybe I should have said that you have a bad MOSFET, which explains the DC on the output.
If you leave the old ones, it will get worse. You should change the MOSFETS and make sure there is 0 volts on the output.
Even a little DC is a bad idea, and I would not leave it that way.

[xtian]

Hello, zombie thread. I have a client's Mesa M-Pulse 600 on the bench. He said he saw sparks shooting out. I've never seen something so thoroughly obliterated--half the FETs and bias resistors were blown to bits. I'll call Mesa in the morning, see if I can get an entire replacement PCB.

[The Dude]

Let us know how you come out. I'm curious about what Mesa says. Those BUZ's are discontinued. If you can find them at all, it'll cost 3 arms and 4 legs. Otherwise, the board itself doesn't look too bad.

[J M Fahey]

Agree the board is repairable and add: Profusion PLC and Exicon, 2 branches of the same company, still make those transistors for about one arm and one leg, quite acceptable .
In fact they supply the dies which are encapsulated by others as BUZ9** , 2SK/J*** , etc. and sell ready to use transistors as EXCN*** .

Search them, they are in the UK (Scotland?) and sell direct to public.
Of course, replace all with fresh ones from the same batch .