F5 power amplifier - Page 450

tinitus · 12th July 2009, 03:19 PM

4U may work, depending on air temperature
Heat sink is 300x165x40

My guess is that its Fischer heatsink
Sk501 sem to fit the specced size used in modushop box
Increasing from 150mm to 200mm height doesnt seem to make significant improvement
Its about 0.3K/W in both sizes
Curve is almost horisontal at 150mm height, so no further improvement with increased size, it seems

But 40mm fins is on the small side and not the most effective
SK157 from Fischer has 83mm fins, that seem to be very effective
Its close to 0.2K/W at 200mm, and still more to gain, it seems

massimo · 12th July 2009, 03:23 PM

Quote:

Originally posted by pro
Hi Massimo, I think i'll go for 4U 400, so I'll have a bit more room for the PS that is a bit overkill (ingombrante è dir poco)

The only problem with the 400 mm unit is that each side/heatsink is devided into two 200 mm pieces mounted side by side.

tinitus · 12th July 2009, 03:31 PM

Quote:

Originally posted by massimo

heatsink is devided into two 200 mm pieces mounted side by side.

good post

And I obviously only looked at 300mm case

pro · 12th July 2009, 03:33 PM

Quote:

The only problem with the 400 mm unit is that each side/heatsink is devided into two 200 mm pieces mounted side by side.

So, just one piece per mosfet

udailey · 12th July 2009, 03:39 PM

With one side split into two heatsinks you actually get a tiny bit more efficiency out of the heatsinks.
Uriah

massimo · 12th July 2009, 05:45 PM

To get the max efficiency, one should fix the two mosfet exactly at the center of each heatsink. That means about 200 mm apart and at 82 mm high

pro · 12th July 2009, 06:25 PM

A question regarding j-fet.
Is it normal that while k170 at 6ma bias, on 30 units, I have found a Vgs which varies between 20 and 80 mV while the J74 varies between -500 and -1500 mV?

EUVL · 12th July 2009, 06:37 PM

> while the J74 varies between -500 and -1500 mV?

Did you mean between -500 and -1500 mV or between 50 and 150 mV ?

If you meant -500 and -1500, then it is not normal. Firstly I expect your Idss to be larger than 6mA, in which case Vgs should be POSITIVE and not negative for 2SJ74, and vice versa for 2SK170.

But suppose that Vgs is positive, and even with the V-grade at 16mA Idss, the Vgs should not be more than +400mV at 6mA bias. Please refer to Page 2 of datasheet.

If you indeed measured -1500mV (i.e. the gate voltage is lower than source), you are for certain forward biasing the JFET, and I would have expected a much larger current than 6mA.

Or I misunderstood you completely.

Patrick

pro · 12th July 2009, 07:02 PM

Hi Patrick. Thank you for the answer.
I arranged this small schematic, and the values I have read between ground and source are from -500 to -1500 with the most, on 30 units, between -800 and -1300 mV. Only one is -60 mV.

bobodioulasso · 13th July 2009, 11:16 AM

I can't understand (may be the warm weather) your schematic: gate-drain is 50v? No load? Source resistor = 3870R? Source at 0V ?

12th July 2009, 03:39 PM	#4495
udailey Passive Aggressive diyAudio Member Join Date: Mar 2008	With one side split into two heatsinks you actually get a tiny bit more efficiency out of the heatsinks. Uriah __________________ You can purchase LDRs anytime to build a standard LDR attenuator or to build my new LDR Attenuator "A Lighter Note". Email me. diyldr@gmail.com

12th July 2009, 06:25 PM	#4497
pro diyAudio Member Join Date: Feb 2004 Location: Milano	A question regarding j-fet. Is it normal that while k170 at 6ma bias, on 30 units, I have found a Vgs which varies between 20 and 80 mV while the J74 varies between -500 and -1500 mV? __________________ Michele

12th July 2009, 07:02 PM	#4499
pro diyAudio Member Join Date: Feb 2004 Location: Milano	Hi Patrick. Thank you for the answer. I arranged this small schematic, and the values I have read between ground and source are from -500 to -1500 with the most, on 30 units, between -800 and -1300 mV. Only one is -60 mV. __________________ Michele

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12th July 2009, 03:19 PM	#4491
tinitus diyAudio Moderator Join Date: Nov 2005	4U may work, depending on air temperature Heat sink is 300x165x40 My guess is that its Fischer heatsink Sk501 sem to fit the specced size used in modushop box Increasing from 150mm to 200mm height doesnt seem to make significant improvement Its about 0.3K/W in both sizes Curve is almost horisontal at 150mm height, so no further improvement with increased size, it seems But 40mm fins is on the small side and not the most effective SK157 from Fischer has 83mm fins, that seem to be very effective Its close to 0.2K/W at 200mm, and still more to gain, it seems

12th July 2009, 05:45 PM	#4496
massimo diyAudio Member Join Date: Dec 2002 Location: Milan, Italy	To get the max efficiency, one should fix the two mosfet exactly at the center of each heatsink. That means about 200 mm apart and at 82 mm high

12th July 2009, 06:37 PM	#4498
EUVL diyAudio Member Join Date: Oct 2003	> while the J74 varies between -500 and -1500 mV? Did you mean between -500 and -1500 mV or between 50 and 150 mV ? If you meant -500 and -1500, then it is not normal. Firstly I expect your Idss to be larger than 6mA, in which case Vgs should be POSITIVE and not negative for 2SJ74, and vice versa for 2SK170. But suppose that Vgs is positive, and even with the V-grade at 16mA Idss, the Vgs should not be more than +400mV at 6mA bias. Please refer to Page 2 of datasheet. If you indeed measured -1500mV (i.e. the gate voltage is lower than source), you are for certain forward biasing the JFET, and I would have expected a much larger current than 6mA. Or I misunderstood you completely. Patrick

13th July 2009, 11:16 AM	#4500
bobodioulasso diyAudio Member Join Date: May 2008	I can't understand (may be the warm weather) your schematic: gate-drain is 50v? No load? Source resistor = 3870R? Source at 0V ?

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