How hot can I bias my F5?

[McQuaide]

Impressive workmanship.

Thank you sir. Where in PA are you?

 

[Antoinel]

Thank you sir. Where in PA are you?

I am in Spring Mount which is close to Collegeville where Ursinus college is.

 

[McQuaide]

.

Maybe I'll crank it up this afternoon and see what happens.

 

[Nelson Pass]

Maybe you did....

 

[vdi_nenna]

I am in Spring Mount which is close to Collegeville where Ursinus college is

I'm in Wayne, PA!

 

[Antoinel]

I'm in Wayne, PA!

We are close. Hopefully we'll get together someday.

 

[vdi_nenna]

Sure, let me know.

 

[McQuaide]

Maybe you did....

Hi Nelson, no I didn't! The weather was too nice here on the east coast to sit inside and play with potentiometers. Definitely will do it though.

 

[Weymario]

correct bias

Hi! i am confused. hope someone can help.
i can´t 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

 

[AndrewT]

...........the temperature of the heatsink.......is .....46 celsius, the transistors about 72 celsius.........

26Cdegrees between transistors and heatsink indicates a problem with Rth c-s

 

[Weymario]

hallo andrew,

thanks for your answer. but i don´t understand "Rth c-s". please, can you explain?
thanks, mario

 

[AndrewT]

Rth c-s = Thermal Resistance from Case to Sink.

 

[wrath]

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

 

[AudioSan]

Hi! i am confused. hope someone can help.
i can´t 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?

 

[Weymario]

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

 

[Weymario]

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

 

[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

 

[Weymario]

so you are a luky one ;-)
have you tried a higher bias? any changes in sound?

 

[AudioSan]

no. i have not tried any higher bias yet.

 

[McQuaide]

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

Take it up...you won't be sorry