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Old 25th March 2013, 03:22 PM   #461
tinitus is offline tinitus  Europe
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or look at this way

lets say we have two amps, A and B

amp B is special, and 'might' be 2% better than amp A
but only when built by an expert
or used under 'certain conditions'

means 99% of all builders will still achieve better result with amp A

or in worst case
how good is the best amp if its gone in a month
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Old 25th March 2013, 03:27 PM   #462
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1% will also achieve a better result with A.
__________________
BOXES for crying out loud
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Old 25th March 2013, 03:32 PM   #463
tinitus is offline tinitus  Europe
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touche'

follow the master then, and hope for the 99%
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Old 25th March 2013, 05:26 PM   #464
juma is online now juma  Serbia
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Quote:
Originally Posted by NicMac View Post
Then I'm in trouble
No, you are not - but you might get yourself into one...
Go ahead with lower PSU voltage and higher/lower bias, just leave the biasing scheme alone (don't omit the pots, NTCs, etc...). Read carefully the F5 service manual and understand the temperature/DC offset dependency i.e. how MOSFETs work and amp as a whole.
Joggling with things you don't completely understand can leave you with burned amp, speakers, house...
Zero tempco is not guaranteed at 0.6A - it's just written in the datasheet (which may not be absolutely true for all devices - some experimenting and measuring is always needed).

Last edited by juma; 25th March 2013 at 05:31 PM.
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Old 25th March 2013, 06:10 PM   #465
NicMac is offline NicMac  Italy
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Quote:
Originally Posted by juma View Post
Joggling with things you don't completely understand can leave you with burned amp, speakers, house...
Zero tempco is not guaranteed at 0.6A - it's just written in the datasheet (which may not be absolutely true for all devices - some experimenting and measuring is always needed).
Experimenting and measuring is my day-time profession so this is indeed also where I planned to start with this version of the "F5J".
I have been tracing a few thousand FETs over the last year or so and having a good (experimental) look at the zero tempco point can only increase my understanding of these devices.
Thanks for the help and encouragement
Nic
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Old 25th March 2013, 08:55 PM   #466
juma is online now juma  Serbia
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Quote:
Originally Posted by NicMac View Post
Experimenting and measuring is my day-time profession ....
Then you know how cruel reality can be
I'm sure you'll be presenting us new insights
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Old 25th March 2013, 11:04 PM   #467
NicMac is offline NicMac  Italy
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Quote:
Originally Posted by juma View Post
I'm sure you'll be presenting us new insights
Not much insight - just some data
I slow-fried a 2SK2013 and recorded traces every 6 degrees of temperature rise.
0.6A does not look particularly special to me - in fact 1.1A seems more like the right drain current for zero tempco, but unfortunately beyond the device rating
The measurements are not really the same as data sheet......
The zero tempco occurs at the correct Vgs (about 3V), but I measure higher current
Could the difference in Vds (15V vs. 10V in data sheet) explain this?
Attached Images
File Type: png 2SK2013temp.png (38.2 KB, 237 views)
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Old 26th March 2013, 10:31 AM   #468
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NicMac, that's what I was talking about - not all 2sk2013s are born equal. The one batch I got was in compliance with datasheet, the other one was not (zero tempco was in 800-900mA area).
That's why it's good to leave the NTCs and pots in the circuit.

OTOH, that picture of yours shows a rise of Id of about 170mA at Vgs=2.5v for a temperature change of 50 degrees Cels. and that's not a big deal anyway...
Bias it at about 450mA (cold) and the bias will rise to about 600mA when hot, and everything's OK.

Last edited by juma; 26th March 2013 at 10:38 AM.
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Old 26th March 2013, 08:39 PM   #469
NicMac is offline NicMac  Italy
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I have attached the data plotted in a different way to better illustrate how current change with temperature for selected values of Vgs. Black dots are the measured values and curves are quadratic fits.

I will have 3 different batches of 2013/313 to look at so maybe I will find some with lower zero tempco. I'm not really counting on this as the increase in current is indeed fairly limited and unlikely to cause thermal runaway. It will work just fine (even without NTC).

Also I have no intention of matching FETs for three parameters

Cheers,

Nic
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File Type: png ZeroTempco2.png (46.6 KB, 193 views)
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Old 26th March 2013, 10:03 PM   #470
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Quote:
Originally Posted by NicMac View Post
...the increase in current is indeed fairly limited and unlikely to cause thermal runaway. It will work just fine (even without NTC)....
Of course. Just make sure to use decent heatsink that will keep the Tj below 100 degrees Cels. and everything will be fine.
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