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Old 30th December 2012, 04:50 PM   #21
llwhtt is offline llwhtt  United States
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Q2, AP1, R10 and R11 form an adjustable zener diode with AP1 controlling the conduction of Q2 and therefore the voltage between the gates of the output MOSFETs. The closer together the gates are, voltage wise, the colder the bias and vice versa. Since Q2 is an NPN the more positive the base the more it conducts and vice versa. With the wiper of AP1 in the full CW position, as you look at it on the schematic, basically the wiper is connected to the base of Q2 the conduction is low and the further apart the gates are, and the hotter the bias. With the wiper in the fully CCW position the base is more positive , the more it conducts, the closer the gates are together and the colder the bias. So you need to start with AP1 in the highest resistance position.

As far as the proper setting Ampeg is usually pretty good about putting all of that info on the schematic somewhere as there were no service manuals that I know of. I was a warranty station for SLM long ago. If you need factory help/parts contact LOUD Technologies these days.

Craig
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Old 30th December 2012, 06:48 PM   #22
JMFahey is offline JMFahey  Argentina
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Correct.
And just to make it 200% certain: START WITH THE BIAS PRESET AT MAXIMUM RESISTANCE which means MINIMUM bias. Funny, huh?
Check it with the amp unplugged and your multimeter.
Even safer: start with the amp without speaker and plugged into a lamp bulb limiter with a 100W lamp.
If it blinks and dims, fine; if it stays somewhat bright , check bias.

Unfortunately, the Schematic page posted does no explain the bias setup.
If somebody posts it, fine.
If not, you'll need 3 or 4 hands, or clip your multimeter with red to the top of either R32/34/36/38 0.47 ohm ballast resistors, and black to the bottom of either R33/35/37 or R39
Set it on the 200mV DC scale, it should show 0 mV or a *very* low value.
Then start moving the trimmer, slowly because it "jumps", until you read around 10mV .
That means that each MosFet is passing 10mA , which in absence of other info, is a reasonable value.
Good luck
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Old 30th December 2012, 10:55 PM   #23
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Hi.

Thank you for clearing this one up.
Max value on the trim is where I start.

I have the initial schematic for ampeg SVTIII poweramp. and it has some voltage values. I will upload it when I am at my computer. It turned out that I could not get thouse 0.47 resistors today so I ordered them and they will probably arrive next year (like waiting the whole year) So I will be back with results.

Though on question.
Once I start to look for that sweet spot. What exactly am I after? Bias is needed to remove crossover distortion.
But what indicates the sweetspot?
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Old 30th December 2012, 11:26 PM   #24
JMFahey is offline JMFahey  Argentina
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Your ears are not acurate enough (mine neither ) for a precise adjustment without help.
*Best* would be to drive some sinewave (say 1000 or 400 Hz ) into a 4 ohm load, connect an oscilloscope, and you will clearly see crossover distortion as a "kink" or bend at the base of the sinewave.
Quite unmistakeable.
Then slowly raise the bias pot setting (in this case lower the resistance value until you *start* to see the distortion "melt".
A little more and it disappears.
That would be the "sweet spot", high enough to dissapear; still not *that* high that it overheats the transistors.
But setting them to a measurable yet low bias current (10 or 20 mA per transistor) is a very reasonable value.
Previously you must have set it to, who knows? , maybe 200 to 500 mA, way too much.
Oh well, now it's repaired, congratulations :thumbsup:
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Old 30th December 2012, 11:38 PM   #25
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sounds like what I have seen way too many times with these amps. you had stressed gates on some of the mosfets and adjusting the bias changed it just enough to make it go away, then when cold too much bias and it broke into oscillation and poof.

These amps are very unstable. which they shouldn't be. when I first power one of these up i do it a bit differently then most amps. i turn the bias pot to the middle instead of all the way down. then watch the scope and the line current drawn and bring the variac up slow. once above 90v quickly adjust the bias down if needed keeping an eye on the scope at all times.

Once running, I will let the amp warm up for an hour and recheck the bias. then shut the amp off and let it cool at least 2 hours and power it up and watch the bias and the output for oscillations. while still cold I will lower the variac down to about 100v and watch for oscillation. then run it at 1/3rd power for 5-10 minutes and watch the heatsink temp. again watching the scope all the time. then when at 1/3rd power I will lower the variac again to 100v and watch the scope. it's overkill but that's the only way to be sure.

Unlike a home audio amp, PA and bass guitar amps get the crap beat out of em! especially small or underpowered amps. they get pushed past their limits and fail. bass guitar amps really need a lot of power to play onstage. and these little 200 watt @8 ohm amps just take a pounding! the weak little VI limiters just don't stand a chance. inevitably a peak or 10 makes it past the limiter and the gates get stressed then fail.

Zc
P.S I need a power transformer for a SVT3 if anyone has one.
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Old 4th January 2013, 12:36 PM   #26
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Hi.

Are you sure that the bias pot should be max resistance?
I think it should not as if it was maxed out I got a reading of 8V on the MOSFET but it should have been 65V instead. So MOSFETS were wide open.
It could be that there is something else still broken.
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Old 4th January 2013, 03:35 PM   #27
JMFahey is offline JMFahey  Argentina
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The way it's wired, it works "backwards" meaning max BE resistance in the bias transistor means minimum voltage across it.
Measure CE voltage across it with pot on max and post it here.
And yes, something else may still be bad , but let's go step by step.
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Old 4th January 2013, 05:10 PM   #28
llwhtt is offline llwhtt  United States
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Why do you say it should be 65V on the gates? With the back to back 13V zeners the most it can be is 14V or so (13V zener plus a PN junction), Source to Gate. The source is or should be at 0VDC. Are the zeners good?

Craig
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Old 4th January 2013, 05:57 PM   #29
JMFahey is offline JMFahey  Argentina
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Not sure he means "at the gates", I undesrstood it as "across the Mosfet" meaning +/-65V rails.
Of course he can never ever have 65V gate to source.
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Old 6th January 2013, 02:54 PM   #30
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Hi.

I got myself a 100W bulb. So I will build the light bulb tester.
Will be back if I have something.
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