Yes !!!!
power up and check what the bulb tells you.
filament off = no current flow
very dim glow = very low current flow. A clear bulb and a low wattage lets you see this.
bright glow = much higher current flow
bright = very high current flow.
From that information decide what voltage measurements will tell you most about what is happening.
eg.
a bright filament AND 700mVdc across the protection diode on the power input to the power amp tells you you have wired in the wrong polarity.
So I am ready to power up tonight. Im still alittle unclear about the light bulb device and what it truely does and what it tells me. When I power up should the light stay off or be dim? Does that indicate that my pots are set to zero? Im assuming that after initial power up to check the light bulb, I would power down remove light bulb device and power back up and then proceed to make my adjustments to p1 ans p2. I prefer the only burning smell in the house be dinner tonight and not my beloved amp. 
Im really excited, as this is the final piece in my audio system to be built. It will replace the 2 chip amps.
Im really excited, as this is the final piece in my audio system to be built. It will replace the 2 chip amps.
So I am ready to power up tonight. Im still alittle unclear about the light bulb device and what it truely does and what it tells me. When I power up should the light stay off or be dim? Does that indicate that my pots are set to zero? Im assuming that after initial power up to check the light bulb, I would power down remove light bulb device and power back up and then proceed to make my adjustments to p1 ans p2. I prefer the only burning smell in the house be dinner tonight and not my beloved amp.
It's for protection against shorts, miswiring, etc. With no bias current flowing, the light should stay dim. A short would allow current to flow and light it up.
So I fired up my amp tonight with bulb device in line, the first time the bulb went bright and the burned out but I did have enough time to measure across diode and it was increasing quick towards 700mV dc, I went back to beginning of this thread and noticed that the green and blue wires from transformer to diode block where opposite of what pictures here showed, I switched them and got another bulb, this time the bulb lite up for a few seconds and then went out, I tested that bulb in a lamp in the house and it still worked so based on that im assuming that since it went outI am ready to bias up?
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Re-read this, and espically the last paragraph;
Lightbulb Mains lead notes -
If the bulb ever turn on and stays bright, you probably have a short.
Normal operation when turning on a cold amp will have the bulb glow bright for a second or two, then dim, perhaps off. (this is the capacitors charging, then full)
As you increase the bias of the amp the bulb will glow brighter, and this is linear with the bias amount. A fully biased amp can get the bulb to glow very bright.
You cannot set full bias with the bulb in place - as it increases the bulb will glow more, limiting the voltage to the amp and all the readings will be wrong compared to when the bulb lead is out.
You can, however, set the initial bias with the bulb in place. Start the procedure, turn the pots until something happens, and set, at maximum, 0.1V across the source resistors and zero offset. Getting the pots started this way is a good idea. Remember, this is with a reduced voltage, and the pots will make the circuit draw MUCH more bias when the normal mains lead in used. Expect to measure 0.2V - 0.3 or more with a normal cord. Continue biasing in small steps, always trying to keep the offset near zero.
Lightbulb Mains lead notes -
If the bulb ever turn on and stays bright, you probably have a short.
Normal operation when turning on a cold amp will have the bulb glow bright for a second or two, then dim, perhaps off. (this is the capacitors charging, then full)
As you increase the bias of the amp the bulb will glow brighter, and this is linear with the bias amount. A fully biased amp can get the bulb to glow very bright.
You cannot set full bias with the bulb in place - as it increases the bulb will glow more, limiting the voltage to the amp and all the readings will be wrong compared to when the bulb lead is out.
You can, however, set the initial bias with the bulb in place. Start the procedure, turn the pots until something happens, and set, at maximum, 0.1V across the source resistors and zero offset. Getting the pots started this way is a good idea. Remember, this is with a reduced voltage, and the pots will make the circuit draw MUCH more bias when the normal mains lead in used. Expect to measure 0.2V - 0.3 or more with a normal cord. Continue biasing in small steps, always trying to keep the offset near zero.
And this; (Which was edited for the F5T, so ignore references to PCB you don't have...)
Testing and Powerup
Adjustments (Bias and Offset, set with P1 and P2)
This is easiest with three DMM.
Shorting the input jacks is helpful, although not strictly necessary.
Before power up, dial pots P1 and P2 to 0 ohms ( check with ohmmeter across TP1-2 and TP3-4 on FE board )
With voltmeter across PSU output ( best between + and - of PSU) observe max voltage of PSU - it never hurts to make sure it’s doing what you expect.
Place one voltmeter at output - to observe DC offset
Place voltmeters across TP of output board, one on N-channel, one on P channel.
For test - slowly dial up Variac ( presuming that you have one , as man with many skills) up to full mains voltage , observing rail voltage at PSU ....... thinking about max cap voltage ( 25V as in FW ? ) , because with 0 Iq PSU is unloaded and voltage is maxed (It’s useful to have another meter for this…)
If nothing is smells bad, and the magic smoke is still in the circuit - leave Variac at full mains ;
IF you don’t have a Variac, you must build a lightbulb mains lead.
Click the image to open in full size.
Click the image to open in full size.
(see notes for mains lead below)
What's important - Iq (measured as the voltage across source resistors; the Mosfet bias) must be very low , offset is irrelevant in this moment .
Now turn one pot one turn ( assuming that you have multiturn pots) then turn other pot one turn. Continue, one turn at a time on each pot until something happens.
Observe voltage across resistors and output DC offset.
Proceed one then second pot , again just one turn
Observe Iq and offset
Again one turn + one turn
Now you are probably in range when you can see which pot is pulling offset in right direction - to 0 . It will feel like one of the pots is controlling the bias on both sides, and the other is controlling the DC offset.
It’s best to increase the bias a bit, and then zero the offset. As you zero the offset you will decrease some of the bias, so it will be two steps forward and one step back. That action is normal.
As you increase the bias and zero the offset, remember to always keep the offset near zero. If you run out of turn on the pots, determine your max bias, with zero offset. (It’s useful for troubleshooting)
Proceed iteratively with pots , while you set - say - 75% of desired bias, with zero offset.
Now - put lid on box and let it cook for a while - until you get thermal equilibrium on heatsinks
It's best to use wire/clips to leave those voltmeters in place ;
Open the lid , up bias to - say - 90% of desired one ,while maintaining offset
Put lid on , let it cook.
Check again.
If all is OK - move voltmeters for Bias and offset to other channel and repeat procedure.
Use it few days at 90% of desired bias , then check and set to 100%
Remember - temp. equilibrium with lid on is important
Testing and Powerup
Adjustments (Bias and Offset, set with P1 and P2)
This is easiest with three DMM.
Shorting the input jacks is helpful, although not strictly necessary.
Before power up, dial pots P1 and P2 to 0 ohms ( check with ohmmeter across TP1-2 and TP3-4 on FE board )
With voltmeter across PSU output ( best between + and - of PSU) observe max voltage of PSU - it never hurts to make sure it’s doing what you expect.
Place one voltmeter at output - to observe DC offset
Place voltmeters across TP of output board, one on N-channel, one on P channel.
For test - slowly dial up Variac ( presuming that you have one , as man with many skills) up to full mains voltage , observing rail voltage at PSU ....... thinking about max cap voltage ( 25V as in FW ? ) , because with 0 Iq PSU is unloaded and voltage is maxed (It’s useful to have another meter for this…)
If nothing is smells bad, and the magic smoke is still in the circuit - leave Variac at full mains ;
IF you don’t have a Variac, you must build a lightbulb mains lead.
Click the image to open in full size.
Click the image to open in full size.
(see notes for mains lead below)
What's important - Iq (measured as the voltage across source resistors; the Mosfet bias) must be very low , offset is irrelevant in this moment .
Now turn one pot one turn ( assuming that you have multiturn pots) then turn other pot one turn. Continue, one turn at a time on each pot until something happens.
Observe voltage across resistors and output DC offset.
Proceed one then second pot , again just one turn
Observe Iq and offset
Again one turn + one turn
Now you are probably in range when you can see which pot is pulling offset in right direction - to 0 . It will feel like one of the pots is controlling the bias on both sides, and the other is controlling the DC offset.
It’s best to increase the bias a bit, and then zero the offset. As you zero the offset you will decrease some of the bias, so it will be two steps forward and one step back. That action is normal.
As you increase the bias and zero the offset, remember to always keep the offset near zero. If you run out of turn on the pots, determine your max bias, with zero offset. (It’s useful for troubleshooting)
Proceed iteratively with pots , while you set - say - 75% of desired bias, with zero offset.
Now - put lid on box and let it cook for a while - until you get thermal equilibrium on heatsinks
It's best to use wire/clips to leave those voltmeters in place ;
Open the lid , up bias to - say - 90% of desired one ,while maintaining offset
Put lid on , let it cook.
Check again.
If all is OK - move voltmeters for Bias and offset to other channel and repeat procedure.
Use it few days at 90% of desired bias , then check and set to 100%
Remember - temp. equilibrium with lid on is important
you don't have to check the bulb is still working.
Instead you check the PSU voltages.
Then check the amplifier voltages.
Then check the output voltages.
When all these are close to what is expected, then power down and direct to mains power up.
one side ok but other... well
So powered up and cooking the left channel bias at .479v with offset of .2mV problem though on right channel when I powered up , th1 started smoking and then started sparking, I unhooked power from that side for now and clipped th1 off, so I need to get another.
So powered up and cooking the left channel bias at .479v with offset of .2mV problem though on right channel when I powered up , th1 started smoking and then started sparking, I unhooked power from that side for now and clipped th1 off, so I need to get another.
Frustrated
So working on left channel, both pots have plenty of movement left, so I get bias to .600 but offset seems to bounce around as in goes up and down in value , then when I put the top on the bias starts dropping and offset moves as well so I having a problem with offset staying set when at .600, am I doing something wrong here, my thimisters are touching the washers for heat, is the jumping due to them being 5% should I pick up some 1%. I do have the inputs shorted.
So working on left channel, both pots have plenty of movement left, so I get bias to .600 but offset seems to bounce around as in goes up and down in value , then when I put the top on the bias starts dropping and offset moves as well so I having a problem with offset staying set when at .600, am I doing something wrong here, my thimisters are touching the washers for heat, is the jumping due to them being 5% should I pick up some 1%. I do have the inputs shorted.
Well long story short, yes I am stable now. Both left and right channel are biased at .600 v with offset of .001 v . The problem it seems is one of my meters is messed up so I went to harbour freight and picked up two of the $6 meters and I dont get bouncing offset numbers any more. Im doing the happy dance and since im off tomorrow my wife gave me permission to rock into the night. Thank you for your help.
Fantastic news!! Congratulations!
Photos please.
BTW, your old meter probably just needs a battery. Also, run to radio shack, get some small alligator clips, snip the probes off the Harbor Freight meters and solder on the alligators. It's the best thing I ever did to mine. (I have 2 as well.)
Photos please.
BTW, your old meter probably just needs a battery. Also, run to radio shack, get some small alligator clips, snip the probes off the Harbor Freight meters and solder on the alligators. It's the best thing I ever did to mine. (I have 2 as well.)