Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: phsyconoodler on April 19, 2010, 06:46:18 pm
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I'm having difficulty wit a mesa studio 22 combo.The bias supply is wonky.I have replaced all the components in the bias supply and it's still wonky.What it does is this:I fire up the amp to check the bias voltage and the voltage starts off at -16v and then steadily climbs to -65v and stops there.
What the heck is going on here?
It looks like a blue wire goes to the footswitch supply and I'm thinking something there is causing the issue but what?
Any insight would be greatly appreciated.
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> The bias supply is wonky
Yes, it is. (It also powers the long-tail.)
> starts off at -16v and then steadily climbs to -65v
WHEN?
About 10 seconds after switch-on? That may be normal.
What is the voltage on output tube grids after 20 seconds? About negative 12 Volts is reasonable.
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The voltage climbs almost immediately and stays at -65v.The amp cuts out obviously,as it is low current.
I wish it were -12v!
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What are the voltages at the long-tail? (I don't know which plan you are looking at.)
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It's a mesa studio 22.The PI is very different than a long tail pair.The schematic is blury at best,even on the mesa tech site.
The two things that are confusing is the phase inverter(different than any I've seen) and the center tap for the low voltage taps goes to the footswitch supply. ???
I can't upload the schematic with my cheap computer but it's at schematic heaven.
Thanks for your help!
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The PI is very different than a long tail pair.The schematic is blury at best,even on the mesa tech site.
The two things that are confusing is the phase inverter(different than any I've seen) and the center tap for the low voltage taps goes to the footswitch supply. ???
It's a long-tail, it's just not a typical guitar-amp long-tail.
The 2 triodes are tied together at their cathodes, with the preamp signal going into the "top triode" and the feedback signal going into the "bottom triode". The drawing is quirky, but what also makes it look wierd is that this is essentially a fixed bias version of the long-tail (you could also argue it's cathode biased...).
The real problem is that the cathode resistor is drawn over with the negative supply; it's 15k. It's connected so that its low end is at the negative supply instead of ground, and the high end of the resistor should sit at 2v. The grids of the long-tail are tied to ground through a 150k resistor for the bottom triode and the master volume for the top triode. The feedback circuit is derived and injected a little differently than typical in guitar amps. The real deviation from standard is that there is a 68k resistor on one plate and a 120k resistor on the other plate. A little difference is required to achieve balance in the long-tail, but this might be going a little far on purpose to achieve a sound.
About the footswitch supply: the blue wire on the PT is not a center-tap, just a tap. Even if it was, the red/yellow wire is the one being grounded. So that means there is simply a relatively large a.c. output feeding the negative supplies, with a smaller a.c. output feeding the footswitch supply. The footswitch and EQ supplies have their own rectifiers and filters.
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Ok,that explains where the wires go,but it shouldn't be doing what it's doing with the bias supply.Cold the power transformer be wonky?
I was thinking about just converting it to cathode bias but that PI is confusing.
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Are you trying to fix it, or just understand it?
I'd rather not try to understand it just now. I know the pieces but not how they interact.
I'm assuming it worked once, if it isn't working now and giving too MUCH voltage one place, we may find too LITTLE voltage (or voltage-drop) somewhere else.
Or: since D is wrong, maybe F is wrong too.
What are the voltages at the long-tail?
> It's a mesa studio 22.
> it's at schematic heaven.
Yes, there are TWO:
http://www.schematicheaven.com/boogieamps/boogie_22cal.pdf
http://www.schematicheaven.com/boogieamps/boogie_22calplus.pdf
I don't know which plan you are looking at.
But both plans have dubious details.... Mesa's docs are rarely accurate.
Still you expect zero volts at grids, 2V at cathodes, and 230V or 250V at plates.
Are you getting these voltages?
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It's the first schematic.I'll check the voltages at the PI and get back to you.
I am not trying to figure it out,just fix it.I'll wrap my head around it later.
So far everything Mesa designs is silly at best.
Even talking to a real Mesa Tech is kind of interesting.They can fix this stuff but they are not happy about the design either.