How hot can I bias my F5? - Page 6 - diyAudio
 How hot can I bias my F5?
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 7th September 2013, 11:47 AM #51 diyAudio Member   Join Date: Jan 2006 Location: Linz hallo andrew, thanks for your answer. but i donīt understand "Rth c-s". please, can you explain? thanks, mario
 7th September 2013, 03:42 PM #52 diyAudio Member   Join Date: Jul 2004 Location: Scottish Borders Rth c-s = Thermal Resistance from Case to Sink. __________________ regards Andrew T. Sent from my desktop computer using a keyboard
 7th September 2013, 04:04 PM #53 diyAudio Member   Join Date: Jun 2004 Location: Poland Poniatowa qwidolek Thermal resistance between mosfet and heatsink. What can mean no or wrong paste,wrong type of insulator, loose screws, convex or concave surface of heatsink near mounting point or broken thermometer ;-D
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Join Date: Feb 2009
Quote:
 Originally Posted by Weymario Hi! i am confused. hope someone can help. i canīt bias my F5 to 0.6 v (R11/R12). when i have .36 v the voltage at R3/R4 is 4.6 v. what is wrong? the temperature of the heatsink (which is 300 x 150 mm per channel) is after (2 hours) 46 celsius, the transistors about 72 celsius. thanks, mario
regarding the bias. are you sure that R11/R12 are 0.47ohm?

 8th September 2013, 05:13 PM #55 diyAudio Member   Join Date: Jan 2006 Location: Linz yes, R11/12 are 0.47 ohm (= R47 ohm, right?). other idea? i think, the insulator between transistor and heatsink is a (pink) silicon-pad, no paste. if the surface is ok, i have to check in the next days. thanks, mario
 10th September 2013, 07:08 PM #56 diyAudio Member   Join Date: Jan 2006 Location: Linz Hi! meanwhile, i tried some things: a) decrease/increase R3/4 from 1.1 k to 9k: 1.1k was worst (bias: 0.07v), 9k also bad (bias: 0.12v); best 2.2k with bias o.24v and 4.3v over R3/4 b) decrease/increase R19/20: nothing happened c) decrease/increase R13/14: nothing happened d) power supply is +/- 25.7 v maybe the jfets (sj74/sk170) or transistors (IRFP240/IRFP9240) are the problem? any ideas are welcome thanks mario
 10th September 2013, 07:24 PM #57 diyAudio Member   Join Date: Feb 2009 just measured some temps on my F5. sinks baseplates run at 45c right at the middle outputs (2pair version) transformers run at 34c. ambient is 25c and bias is 1.7A (0.85A pr Device) looks like i need to pull up the bias some more
 12th September 2013, 05:50 PM #58 diyAudio Member   Join Date: Jan 2006 Location: Linz so you are a luky one ;-) have you tried a higher bias? any changes in sound?
 12th September 2013, 06:11 PM #59 diyAudio Member   Join Date: Feb 2009 no. i have not tried any higher bias yet.
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Location: Southeastern Pennsylvania
Quote:
 Originally Posted by AudioSan just measured some temps on my F5. sinks baseplates run at 45c right at the middle outputs (2pair version) transformers run at 34c. ambient is 25c and bias is 1.7A (0.85A pr Device) looks like i need to pull up the bias some more
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