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Old 30th December 2012, 02:57 PM   #11
Zen Mod is offline Zen Mod  Serbia
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what ?

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Old 30th December 2012, 02:59 PM   #12
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He, he...

good morning master Zen Mod, please drink the small little black coffee standing on the transformer in your amp.....
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Old 30th December 2012, 03:33 PM   #13
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chasing proper Iq , while still having proper Ids voltage for Lu , is ballancing act of adjusting values of both source resistor groups - one of Q3 (dominant for Iq ) and of Q1 (dominant for Uds of it)

then - setting of output node potential , is just matter of P1 position

everything else - ZAP (P2) position , same as H2/H3 , is matter of either ear wax , or measurement

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Old 30th December 2012, 03:41 PM   #14
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thanks ZM..... if you are ZM, the other guy is still drinking the small coffee...

...have nice days and thanks for your help!

with you the new year will be a good year again......!
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Old 30th December 2012, 04:22 PM   #15
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...... to serve ...... and protect


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Old 30th December 2012, 04:26 PM   #16
generg is offline generg  Germany
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and to be amused from time to time....
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Old 30th December 2012, 05:39 PM   #17
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Quote:
Originally Posted by yup View Post
I have rechecked the voltage across R6/7/8 since my earlier measurements looked a little bit off (taken over a short time period, so voltage probably drifted slightly).

The voltage across R6/7/8 is 0.654Vdc, more in line with the 0.66 that it is supposed to be.

This puts my idle dissipation at 75 watts per mono block (including regulation). This makes me wonder if Nelson reduced the dissipation of the F3 from the ZV9 due to the thermal limit of his chassis, or to reduce distortion, or I am totally confused

Either way, the voltages across the Jfet still look odd (Drain 2.526v, Source 0.979v) - the Jfet is dissipating 2.55 watts - 1.55 volts/1.65A (much lower than the 4 to 6 watts in the ZV9 article).
Nice looking amplifier.

The 100 watt figure comes from the AC line, and includes numerous losses
before it gets to the heat sink. 75 watts on the heat sink sounds about
right.

I usually see more voltage across the Jfet, but this will vary from
transistor to transistor - also my lot codes could be different from yours.

Did you go through the Idss exercise in selecting the resistor value R5?

In any case, it is the actual performance that matters, and you will likely
need a distortion analyzer if you want to play with the values.

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Old 30th December 2012, 06:35 PM   #18
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Quote:
Originally Posted by Zen Mod View Post
chasing proper Iq , while still having proper Ids voltage for Lu , is ballancing act of adjusting values of both source resistor groups - one of Q3 (dominant for Iq ) and of Q1 (dominant for Uds of it)

then - setting of output node potential , is just matter of P1 position

everything else - ZAP (P2) position , same as H2/H3 , is matter of either ear wax , or measurement

Zen Mod,
Thanks for confirming the adjustment method. The only remaining question (I have read every F3/ZV9 thread I can find) is... What is the "proper" voltage/current in my F3 configuration (42V after regulation & plenty of heat sink). I know the official answer is to use a distortion analyzer and find out, but in the end, this is a hobby for me so I can't warrant the cost of a useful DA.
Based on comments in the ZV9 article NP states "Vds of the JFET in the amplifier is 2.5 volts with the output DC figure of 23 volts" - curiously the associated schematic in figure 8 shows just 2.0 Vds/2A, while the F3 schematic shows 2.4 Vds/1.65A

My F3's are currently about 1.5 Vds/1.65A with 21Vdc at the output. This seems too far off of 2.5Vds to be right.

I know, without a DA, we are all just guessing, but I was hoping that someone with a DA has gone through this so they could point me in the right direction. I have expended enormous time and resources (and enjoyed every minute of it), and I could use some guidance/reassurance.

My two main questions are:

What is a good target for my Vds/I given my voltages shown above? (1.5Vds/1.65A, 2.0Vds/1.65A, 2.5Vds/1.65A, 2.5Vds/2.0A, or even higher current due to available heat sinking)
My current thinking is to go with 2.5Vds/2.0A based on Nelson's comments in the ZV9 article (the jfet shouldn't care what voltages the rest of the circuit sees???)

Why can't I get my gain up to the 12.75db listed in the F3 spec? Do I just need to reduce the value for R18? Or is this related to the low voltage across the jfet?

Thanks for everyone's help.
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Old 30th December 2012, 07:13 PM   #19
Dzius is offline Dzius  Poland
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Since you have got nice heatsinks go for 2A bias witout hesitation. So as to wrong voltage try changing Q2 then. I built Zen v9 with 2A bias and F3 with 1.75A bias. In both cases voltages were similar.

Last edited by Dzius; 30th December 2012 at 07:15 PM.
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Old 30th December 2012, 07:48 PM   #20
Zen Mod is offline Zen Mod  Serbia
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just go for higher Iq

decrease of source resistor group value will rise gain , too
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