F5-T V3 - monoblock - bias setting

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5k7

Member
Joined 2019
#1
Hello guys,

I'm building v3 mono in separate chassis. I connected everything and it seems that everything is fine. Amp is responding to P1,P2 etc. I came across many threads and messages with different answers and now I'm bit confused.

Before for F5 i should have 0,6V on Mosfet or R7-R8, R9-R10 to get 1.3 A bias. Is it the same for mono V3? Calculation is based on R=0.47Ohm which is fine, but actually I cannot find such resistor on the frontend board to verify this. Additionally there is a lot of different stories on forum. From checking voltage at R11-R12 to R7/R9. I know that differences are because of many F5/F-5T variations, but I'm kind of lost here.


What will be the correct voltage for F5-T v3 mono and based on which resistor this calculation is made ?


Thanks
 
#4
As requested, please post or link to the precise schematic you are using. We cannot help if you won't. BoM does not matter, the schematic is key.

See the build thread. V3 has same bias procedure and test points. Posts #1 and Post #2 are particularly helpful. You choose the Iq that makes you most comfortable to start based on your access to a fire extinguisher 😉 and/or with your heatsinks etc. I'd start low....

F5Turbo Illustrated Build Guide

As you read the thread, you'll see a few cautionary tales of parts going up in flames. Dennis mentioned being careful. I second that... you may note that at least two people found out just exactly what happens when you go a bit too far.
 
#5
Measure DC volts across TP2 and TP3 on the output boards. This puts the meter across the output mosfet source resistors.

Nelson Pass from the F5T article said:
The point at which the diodes conduct is temperature dependent, so you will want to set the bias so that it makes a nice transition above the bias point and doesn't run away when the amplifier gets hot.
If you are competent, fearless and also own a fire extinguisher, you can find this point. Just run the amplifier into a reasonably low impedance until it gets good and hot – as hot as you plan to let it get - ever. Then adjust the bias to a point below where the idle current starts to really take off. You should find that this point is around 0.4 volts across the 1 ohm resistors. If you are a fraidy-cat, then just set it at 0.3 volts, and conservatively fuse the AC line.
Click to expand...

Original here - https://www.firstwatt.com/pdf/art_f5_turbo.pdf


My suggestion is never go past 0.35v if you have the diodes installed. Higher bias risks runaway.
 
#6
Hello F5T-V3-builders,


I fully agree with ItsAllInMyHead and 6L6. Bias up this amp slowly!
Always wait around 20 minutes till the MosFets get their higher temperature caused by higher bias. Allways keep an eye on your 3 DMMs whats going on
with your voltage measured over the bias resistors of one N-ch and one p-ch-MosFet.
Also keep an eye on the offsetvoltage at your speakeroutput - keep it below 50mV.
Patience is your friend!!! I needed a few hours.

My F5T-V3 - Monoblocks are running on rails of +-42.7 V. The maximum bias over one bias resistor is 330mV - in my case. I have 4N-ch and 4 P-ch-MosFets per Monoblock. I would have to open the case again to check what
value of bias resistor I have used? I am pretty sure I used 2 pieces of 1 Ohm -
paralleled = 0.5 Ohm per Mosfet (as in the schematic of V3). But I will check.
Cheers
Dirk 😀
 
#7
Thanks guys now it's a little bit more clear ;-) I followed bias procedure once gain with meters across TP2,TP3. First with 0,3mV for first 20 minutes and after i set it to 0,35mV as suggested for next 20 minutes.

I made a photo with results.

So:
1) first two from left are across N/P ->TP2/TP3
2) third one -> DC offset - i was able to set it to 0,000 without problems
3) Current from N side - O/P <==> Frontend board O/P output. 2,8 A. Is it ok ?



F5-T-bias.jpg






Temperature on every radiator around 46C, on Mosfet 50C
 
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