put 1K multiturn trimpot there , connected as variable resistor - wiper leg connected to one outer leg
start from middle ( as delivered from factory)
if output offset is on positive side - you need to increase it's value (open lower mosfets more )
if offset is on negative side , you need to decrease it's value (close lower nosfets more)
later you can pull trimpots ouit , measure and put fixed resistor , even f I'm not loosing my sleep about that (neither do Papa , in J2 , for example )
start from middle ( as delivered from factory)
if output offset is on positive side - you need to increase it's value (open lower mosfets more )
if offset is on negative side , you need to decrease it's value (close lower nosfets more)
later you can pull trimpots ouit , measure and put fixed resistor , even f I'm not loosing my sleep about that (neither do Papa , in J2 , for example )
I have found some vishay trimpots (10% tolerance) 0.5w near me, I think I will go for them, and if it's not a problem I'll let them in place instead of changing them for fixed ones.
As for the 100k I'll buy the 5mm 0.5w I can find near my place
For R13, I'll buy some values between 40 and 46K
As for the 100k I'll buy the 5mm 0.5w I can find near my place
For R13, I'll buy some values between 40 and 46K
You measure voltage, and calculate the current. Otherwise you would need to lift one end of the resistor.
Vgs seems reasonable. Now you need to figure out why the current source is so low. R13 is the usual adjustment mechanism. Some Aleph versions included a 100K pot there. Higher resistance means more current. You could just lift one end and see how hot you get. With it lifted you should see .6-.7V across R39/40. Probably a bit too much bias, for ~32V rails, though. Maybe lift one end and connect it to a 10K to make a 46K resistor. You do have a stash of 10K don't you?
Vgs seems reasonable. Now you need to figure out why the current source is so low. R13 is the usual adjustment mechanism. Some Aleph versions included a 100K pot there. Higher resistance means more current. You could just lift one end and see how hot you get. With it lifted you should see .6-.7V across R39/40. Probably a bit too much bias, for ~32V rails, though. Maybe lift one end and connect it to a 10K to make a 46K resistor. You do have a stash of 10K don't you?
R40 (on 2V caliber ) = -.203
R39 (on 2V caliber ) = -.202
I'm still with my 39k res on R13
If I understand well, to find current from those measurements, I have to divide Voltage by Resistance:
.203/0.47 = 0.43A for one fet
For total bias do I have to multiply by the number of fets? witch would lead to a total 2.58A Bias for this channel (0.43x6).
PS: I put my sink on vertical position instead of lying on the table (witch create convection phenomenon) Temp dropped to 45°C
DC offset between Out and Gnd is 5.3mV
R39 (on 2V caliber ) = -.202
I'm still with my 39k res on R13
If I understand well, to find current from those measurements, I have to divide Voltage by Resistance:
.203/0.47 = 0.43A for one fet
For total bias do I have to multiply by the number of fets? witch would lead to a total 2.58A Bias for this channel (0.43x6).
PS: I put my sink on vertical position instead of lying on the table (witch create convection phenomenon) Temp dropped to 45°C
DC offset between Out and Gnd is 5.3mV
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Didn't we go over how to calculate fet junction temperature? Could have been another thread. Calculate rise above sink temp by multiplying power dissipated (Ifet x Vrail) by ~1.5 if mica and goop, .93 if Keratherm. Junction temps under 100C are generally considered reliable, although it's not a hard and fast rule.
Sinks at 55°C is a domestic environment limit - not so hot that you can't pull your hand off without getting burned.
Sinks at 55°C is a domestic environment limit - not so hot that you can't pull your hand off without getting burned.
(0.43 A x 32 x .93)+ 50°C sink temp = junction temp in your current setting. Or solving for 100°C junction on a 55° sink, I get 1.5A max bias per FET. It doesn't look like you need to worry about junction temperature unless you allow the sinks to get hotter than 55°C.
Any DC offset under 50 mV is good.
Any DC offset under 50 mV is good.
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I've just substitued the 39k with a 47.5K on R13, I let y amp burn before taking measure, it's already at 43°C after 15min
I feared myself quite a bit at first power up, I reversed V+ and V- , hopefully the fuse burnt and nothing has been damaged ...I have to prevent myself from rushing things and triple check everything before firing up.
I feared myself quite a bit at first power up, I reversed V+ and V- , hopefully the fuse burnt and nothing has been damaged ...I have to prevent myself from rushing things and triple check everything before firing up.
I was able to let my hand on for 4 seconds, and I have a thermometer on my multimeter.....You know, the thing for chicken hearted like me
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