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Old 26th March 2012, 09:15 AM   #811
protos is offline protos  Greece
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I checked each transistor twice under the same conditions. The results were the same. There were a couple at around 3.75-3.85 vgs , a couple around 4.20-4.40 then it was 6 , 8, 9.5, 11.5.
I checked also with a higher bias setting and it seems the first four have the same gain i.e. the increase in Vds between one bias setting and the other higher bias were similar between the four. That is good. The other four seem to be of a higher gain but I need to test them more because they were overheating because of the high dissipation(high VdsXhigh bias) at the higher bias setting and so I could not get more stable measurements at the higher setting.
I must say it is a bit of a hassle trying to find ways to temporarily test TO-3 units on a heatsink compared to the to-247 type I am used to.
I guess the gain characteristics are more important than small differences in Vgs which means the curves are slightly shifted in parallel in the graph but still remain the same relative to one another however when there are huge differences in Vgs of more than double between fets then even the gain might suffer. In any case one will need a bias supply for sure unless you want a different Power supply for each channel.
Still having played with tubes which have obvious variations but which are still far far closer than what I am seeing with these fets.
I am surprised nobody had such variations in this forum since we mostly bought from the same source.
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Last edited by protos; 26th March 2012 at 09:19 AM.
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Old 26th March 2012, 09:27 AM   #812
protos is offline protos  Greece
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Quote:
Originally Posted by Vix View Post
Protos, I am using the F3 (or more correctly, ZEN V9) without feedback, with "ordinary" CCS-no Aleph modulation), driving a pair of Visaton B200 above 150 Hz on Open baffle. If you can't do L'amp, play with the F3, its a heck of a good amp
I am thinking of either the F3 or the L'amp since I have both transistors. I am still trying to get some feedback from anyone who has heard both. I can do L'amp of course- it is such a simple build after all. In the end I will probably do both. Just that I would like to find some F3 boards to make life easier.
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Old 26th March 2012, 10:16 AM   #813
generg is offline generg  Germany
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Quote:
I am surprised nobody had such variations in this forum since we mostly bought from the same source.
maybe nobody checked so many SITs like you....

most of us bought only one or two pairs and speaking only for me, I did not check before building like you the values separate ....

I only remarked that I had an abbreviation of about 0,5V of Vgs compared with Mikes values to get the same Vds and current for the SIT....
and because the Vgs difference was not so big I did not care much for the difference..

For F3 pcb look here...

Reissue: F3 and F5 PCB's Group Buy

and contact suds... and be patient.....
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Last edited by generg; 26th March 2012 at 10:25 AM.
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Old 26th March 2012, 12:40 PM   #814
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Are you using a self-bias arrangement to set the gate? Did you measure the voltage dropped across the input resistors of your samples? If you're dropping a significant voltage, try a 10K.

Last edited by Michael Rothacher; 26th March 2012 at 12:42 PM.
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Old 26th March 2012, 01:02 PM   #815
protos is offline protos  Greece
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Yes I am using self bias.Input resistor? You mean the 47K gate to ground ? I will see if this helps but if you are right there must be a lot of leakage from the gate to affect such a high Vgs variation.
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Old 26th March 2012, 01:22 PM   #816
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Yes, the 47K. I've seen SITs with 100uA or more gate leakage current. With 100uA, you'd be dropping 4.7V across a 47K resistor. It's worth a look anyway. Being aware of it is half the solution.
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Old 26th March 2012, 01:28 PM   #817
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Quote:
Originally Posted by protos View Post
which means the curves are slightly shifted in parallel in the graph
Your understanding is correct and that's been the case with every variety of SIT I've tried. It must be a difficult parameter to control in the process, hence the various grading schemes for Vp.
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Old 26th March 2012, 05:46 PM   #818
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Quote:
Originally Posted by generg View Post
maybe nobody checked so many SITs like you....
at first i have ordered only 2 2SK82 , in fact ( thanks to michael and god N.P) it"s first a no-brainer amplifier. sounds good at 1.6amp 19V with inductor or at 14V 2.4amp with res load
in my system i could hear more difference changing source resistor type and power supply Cap or source bypass cap than changing bias... perhaps i'm still an audiofool.... sure i'am
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Old 26th March 2012, 06:33 PM   #819
generg is offline generg  Germany
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Hi SIT-Men,

I am onboard again, my experimental assembly is working.....

so my normal amp can work parallel for comparison (BJ2)....
and no more destroying the case by the 120th change...

look my newest modification of "nearly" CCS (tamed Mufollower) and my last circuit change.

I simply put together two pictures from Mike, the upper part is his suggestion for a CCS with SJEP120R100,
the lower part is Mike´s normal suggestion for the SIT.....

the changes for the Mu are the drain resistor of 0R3 for the SIT and the 500R pot going to the 1000uF going to the drain of the Semisouth.

With the pot I could make at a certain position the k2 dominant for for lower and higher volume levels, followed by the k3 in a constant distance.
Without the pot the k3 got very dominant putting the volume up....

So it has the Zen factor!

Next experiment will be the inductor solution.....
Attached Images
File Type: jpg SIT MU2.jpg (54.7 KB, 384 views)
File Type: jpg 106_0108.JPG (332.7 KB, 382 views)
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Old 26th March 2012, 06:40 PM   #820
Zen Mod is offline Zen Mod  Serbia
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