Happy New Year all
I am looking at fixing a solid state GK bass amp that some fool blew when he was young and foolish.
The bias pot burned out (along with a couple transistors), so I will have to replace it and bias the amp. I won't find this exact pot, but I guess it is 1k.
But my, ahem, experience is that if the bias setting is even slightly off for a second it can blow up the amp. What is the procedure for setting bias? Is it safer to dial the pot down to the minimum first and slowly dial up? Is there a way to calculate the expected right resistance value of the pot so that it's pretty close to right to begin with?
The attached schemo says it should be adjusted for "10mv across r21 & r24."
Any tips on the right practice would be helpful - the theory will be over my head.
I am looking at fixing a solid state GK bass amp that some fool blew when he was young and foolish.
The bias pot burned out (along with a couple transistors), so I will have to replace it and bias the amp. I won't find this exact pot, but I guess it is 1k.
But my, ahem, experience is that if the bias setting is even slightly off for a second it can blow up the amp. What is the procedure for setting bias? Is it safer to dial the pot down to the minimum first and slowly dial up? Is there a way to calculate the expected right resistance value of the pot so that it's pretty close to right to begin with?
The attached schemo says it should be adjusted for "10mv across r21 & r24."
Any tips on the right practice would be helpful - the theory will be over my head.
I usually start bias pot at lowest setting.
Apply sine wave to input. Monitor speaker out on scope.
I slowly turn pot until cross over distortion goes.
That works for me as that is minimum acceptable bias.
Peavey use minimal bias too in their amps.
However others would disagree and say that to get rid of more distortion the bias needs to be higher.
As you have found the amp will blow up if too much bias is given.
On my designs I put a Zener across the bias circuit to stop over biasing.
Apply sine wave to input. Monitor speaker out on scope.
I slowly turn pot until cross over distortion goes.
That works for me as that is minimum acceptable bias.
Peavey use minimal bias too in their amps.
However others would disagree and say that to get rid of more distortion the bias needs to be higher.
As you have found the amp will blow up if too much bias is given.
On my designs I put a Zener across the bias circuit to stop over biasing.
Great, thanks. If overbiasing will blow things up, but underbiasing won't, I think I'll be okay, just dialing up from the minimum. ... as long as I get the pot in there in the right direction..
Adjusting in small increments and waiting for a while to let the circuit warm up is important. Bias is often very sensitive to temperature. Replace the cover between adjustments and watch the temperature. Stop before you can't keep your hand on the sink.
 
 
Stupid question, but is this the right wiring for the primary/switch/fuse on amps in general (nevermind that this illustration is for some tube amp)?
I snipped my leads previously and have to put them back in.
I snipped my leads previously and have to put them back in.
Which are? I was unaware there was a different set of connections in EU. Or a need for a special switch? All of my switches are rated for 250VAC anyway so I didn't even think about it. I use a relay anyway.
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I'm not good with schematics yet: am I reading this right that the send jack is a simple 1/4" jack, and the return is a switch jack? I also took these jacks out and need to replace them.
