when I start moving up the volatge on the variac it looked like a shortciruit. At about 30 V it drew 3 A on the primary and I stopped right away.
I will start fresh with new Jfets and Mosfets.
The LM329 works at 0.6-15mA with same performance, according to datasheet.
10 kohms gives about 1.7 mA.
It will be less than that cause you have 1.38 mA going through the 5k trimpot. So you only have around 0.32mA for the Zener ie below the recommended data sheet minimum.
Best to shoot for 3mA across the feed resistor this will give 3-1.38= 1.62mA for the Zener. Say a 7k resistor instead of 10k.
Best to shoot for 3mA across the feed resistor this will give 3-1.38= 1.62mA for the Zener. Say a 7k resistor instead of 10k.
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With zenners Z1,2 1N4739 9.1V
10k on R 7,8.
Mine biassed up no problem.
However the trim pots worked anticlockwise, (backwards) although installed as the print on the pcb.
So when first turned on I was at full bias!
10k on R 7,8.
Mine biassed up no problem.
However the trim pots worked anticlockwise, (backwards) although installed as the print on the pcb.
So when first turned on I was at full bias!
eyes and brain need to work in combo with DMM
needing DMM anyway connected to circuit to set things , remembering trimpot twiddle orientation is just waste of time
in other words - do initial preset of trimpots with above mentioned combo , not reckoning on some silly graphic convention
so many parts were burn because of that mistake
edit : even when making my own pcbs , I'm just not taking care of trimpots twidle orientation .....
needing DMM anyway connected to circuit to set things , remembering trimpot twiddle orientation is just waste of time
in other words - do initial preset of trimpots with above mentioned combo , not reckoning on some silly graphic convention
so many parts were burn because of that mistake
edit : even when making my own pcbs , I'm just not taking care of trimpots twidle orientation .....
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ColinA, if you work out the situation with 10K resistor and 5K pot, assuming
nominal rail voltage not significantly higher than 25 volts, there is not
enough current for a 9.1V zener to be operating correctly
and you end up with a voltage divider giving ~ 1/3 rail voltage across
the 5k pot.
You can certainly adjust up the bias since there is adequate voltage
across the pot. However, because the zener is not regulating, the
voltage across the pot will drift with, say, any AC variation causing
a change in the rail voltage.
Dennis
Please, this gets very confusing for some of us that are not too mathematical.
There is an official guide to building this amplifier with the boards and parts from the store.
http://www.diyaudio.com/forums/pass-labs/277850-f6-illustrated-build-guide.html
In that is a B.O.M. which I followed.
Then someone says "You built it wrong!" 😕 😕
Either Mr 6L6 is correct or others are.
Please guys get and talk to each other!!!
Many others, like me, will have followed the guide.
If it needs changing then it should be done!
Very confused,
but with an F6 that sounds wonderful!!! 🙂
Yes, you're right about the 10K that feeds the Zener and the trimpot.
Here is t he math : if Zener is 6.9 V and rail is 23V, than the voltage drop on that resistor is 23-6.9= 16.1V. The trim pot is 5K and takes 6.9:5= about 1.4mA.
So one would need at least 2 mA if using a LM329, which gives a 16.1:2= 8.05 Kohms Max. To be on the safe side 7 k works, or even 5 K for more demanding Zener.
Thanks.
Some builds here have been with SemiSouth or Cree outputs, and the reporting of Zener/Resistor combinations have been all over the place.
3 possibilities, the guy is too busy, down right laziness, or ego.
You can decide which one.
I have no idea.
It's a combination of the first two. The third doesn't come into play - the entire point of doing this is to help people have successful builds, not to be the coolest guy on the block... (Trust me, that's never going to happen... 😀 )