Hello,
I am going to build F5, but dont really know how to set the safe initial bias for the 1'st time. The article said to minimum P1 and P2. What minimum refers to? Since i already put the multiturn on PCB, the resistance i read between the two pin is about 1.1K Ohm at full CC turn (the P1 and P2 real value is 5K right). Should I turn the multiturn so that the resistance being 0 Ohm or the opposite being 1.1K Ohm for the safe initial setup?
Thank you.
I am going to build F5, but dont really know how to set the safe initial bias for the 1'st time. The article said to minimum P1 and P2. What minimum refers to? Since i already put the multiturn on PCB, the resistance i read between the two pin is about 1.1K Ohm at full CC turn (the P1 and P2 real value is 5K right). Should I turn the multiturn so that the resistance being 0 Ohm or the opposite being 1.1K Ohm for the safe initial setup?
Thank you.
Hi, The group of parallel resistors and thermistor set the Vgs of the output device.
If Vgs is zero then output bias is also zero.
If Vgs is < ~3V then Id is just about zero.
As Vgs rises through 4V and beyond the output FET passes substantial current.
Set the variable resistor to zero. Check this with a ohm-meter or multimeter.
As the variable resistor is increased the effective resistance of the group of parallel resistors increases.
To set up the F5 safely, you are told to set the resistance to zero.
Measure across the resistors. Set the Variable resistor so that the overall measured resistance is zero . Do this for both +ve and -ve sides.
Insert a bulb tester in the mains supply.
Stand back and switch on. Does the bulb flash briefly?
Does the bulb go out?
Check the voltage on both supply rails, are they near what was expected.
Is anything in the low voltage side of the transformer getting warm or hot?
If all these checks show that there is no fault in the wiring then remove the bulb tester and connect the amplifier to mains power.
Stand back and switch on.
Has anything blown up?
Put on your safety glasses and go and measure your two supply rails.
If all OK, then check device temperatures. Remember to keep fingers and tools and clothing well clear of all mains connections.
Now follow the instructions for setting the bias.
If Vgs is zero then output bias is also zero.
If Vgs is < ~3V then Id is just about zero.
As Vgs rises through 4V and beyond the output FET passes substantial current.
Set the variable resistor to zero. Check this with a ohm-meter or multimeter.
As the variable resistor is increased the effective resistance of the group of parallel resistors increases.
To set up the F5 safely, you are told to set the resistance to zero.
Measure across the resistors. Set the Variable resistor so that the overall measured resistance is zero . Do this for both +ve and -ve sides.
Insert a bulb tester in the mains supply.
Stand back and switch on. Does the bulb flash briefly?
Does the bulb go out?
Check the voltage on both supply rails, are they near what was expected.
Is anything in the low voltage side of the transformer getting warm or hot?
If all these checks show that there is no fault in the wiring then remove the bulb tester and connect the amplifier to mains power.
Stand back and switch on.
Has anything blown up?
Put on your safety glasses and go and measure your two supply rails.
If all OK, then check device temperatures. Remember to keep fingers and tools and clothing well clear of all mains connections.
Now follow the instructions for setting the bias.
I've also followed Tee-bag's instructions...in addition I use the same schematic but with the 2SJ200 & 2SK1529, I just posted it for easy finding!
I dont have any fancy tools, other than some bench supplies now, so I cant look at these units through a scope. I took some personal advice from others and a few post around here and wipped up a design that seems to work real well.
The Toshiba's seem to bias up more quickly, so with the 5k pots and 2.2k drain resistors, it bias too quickly. On the Caddock/PRP version, I used 1K with 5k pots and 2.2k and 2k pots, both seem to bias a little better. The Caddock PRP version is a little less smooth and a little less colored than the Takman/Mills setup.
I would encourage people to build the default unit first, it has more protection.
I would buy a variac before any fancy parts, it pays for itself in less failures!
The Toshiba's seem to bias up more quickly, so with the 5k pots and 2.2k drain resistors, it bias too quickly. On the Caddock/PRP version, I used 1K with 5k pots and 2.2k and 2k pots, both seem to bias a little better. The Caddock PRP version is a little less smooth and a little less colored than the Takman/Mills setup.
I would encourage people to build the default unit first, it has more protection.
I would buy a variac before any fancy parts, it pays for itself in less failures!
I'm well prepared , will this do ?..................... Attachments
Distortion Magnifier
http://www.diyaudio.com/forums/soli...ordell-distortion-analyser-4.html#post2071461
Something like this, plus a 24-bit sound card and spectrum analyser freeware.
But if you really want to measure to distortion to better than 70dB, you still need a really good analog sine wave generator.
Patrick
http://www.diyaudio.com/forums/soli...ordell-distortion-analyser-4.html#post2071461
Something like this, plus a 24-bit sound card and spectrum analyser freeware.
But if you really want to measure to distortion to better than 70dB, you still need a really good analog sine wave generator.
Patrick
The Cordell generator is a very close approximation of the Tektronix SG505.
I think that there's a GB on at the moment for boards for the Ultra-Low Generator from the Linear Technology AN-67. This would be a good deal for anyone interested in something approaching a lab-grade instrument.
