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An illustrated guide to building an F5
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Old 13th October 2021, 11:19 PM   #1891
asilker is offline asilker  United States
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Hey folks me again. Thanks again in advance.

Here's my amp, its all wired up and looking pretty. I brought up the power supply on a variac and it measured +25v and -25 and made me feel like the deft success I'd always hoped to be. All of my LED's are wired the right way too, which always feels good.

Click the image to open in full size.

I'm having trouble getting the amp to bias. I'm measuring DC voltage across the 47r 3w resistors that are in the N and P channels. I carefully measured my trimpots and made sure that P1 + P2 were set to their minimum (0.5 ohms). A third voltmeter is across the speaker outs as per @ZenMod 's instructions in another thread.

Click the image to open in full size.

I cannot get the voltage above the neighborhood of 50mV.

I had to remind myself that the V2 build guide is not 100% pertinent to the v3 board because parts numbers had changed. Once I figured that out, I was still befuddled.

So, here's where I am: no heat on the sinks, no burned or visibly damaged parts, hardly measuring above 50mV. What do I do?

Last edited by asilker; 13th October 2021 at 11:26 PM.
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Old 13th October 2021, 11:43 PM   #1892
ItsAllInMyHead is offline ItsAllInMyHead  United States
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An illustrated guide to building an F5
How many turns on P1 and P2 have you tried from the minimum point? If I recall correctly, it took mine quite a few turns before the voltage started to creep up.
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Old 13th October 2021, 11:48 PM   #1893
Hikari1 is offline Hikari1  United States
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Quote:
Originally Posted by asilker View Post
Hey folks me again. Thanks again in advance.

Here's my amp, its all wired up and looking pretty. I brought up the power supply on a variac and it measured +25v and -25 and made me feel like the deft success I'd always hoped to be. All of my LED's are wired the right way too, which always feels good.



I'm having trouble getting the amp to bias. I'm measuring DC voltage across the 47r 3w resistors that are in the N and P channels. I carefully measured my trimpots and made sure that P1 + P2 were set to their minimum (0.5 ohms). A third voltmeter is across the speaker outs as per @ZenMod 's instructions in another thread.



I cannot get the voltage above the neighborhood of 50mV.

I had to remind myself that the V2 build guide is not 100% pertinent to the v3 board because parts numbers had changed. Once I figured that out, I was still befuddled.

So, here's where I am: no heat on the sinks, no burned or visibly damaged parts, hardly measuring above 50mV. What do I do?
F5 Bias Problem
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Old 14th October 2021, 12:24 AM   #1894
asilker is offline asilker  United States
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I've maxed the trimpots out in the right channel and still not near where I need them to be. I'm tired tonight but I'll repeat testing both sides of the amp again tomorrow when I'm fresh.

I've read through the above thread, looks like it's worthwhile to adjust the resistor that's in parallel with the trimpots to 3k3. I can try that this week too.
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Old 14th October 2021, 12:27 AM   #1895
rayma is online now rayma  United States
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If you do that, be sure to reset the pots to minimum before applying power.
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Old 14th October 2021, 12:30 AM   #1896
asilker is offline asilker  United States
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Rayma, roger that. They have been reset to minimum after each test as a matter of course.
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Old 14th October 2021, 12:32 AM   #1897
elwood625 is offline elwood625  United States
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You forgot to populate P3 on the circuit board.
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Old 14th October 2021, 12:39 AM   #1898
rayma is online now rayma  United States
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It's not necessary to install P3 unless tweaking the distortion.
P3 just unbalances the input stage source resistors.

Last edited by rayma; 14th October 2021 at 12:44 AM.
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Old 14th October 2021, 01:26 AM   #1899
Hikari1 is offline Hikari1  United States
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Originally Posted by rayma View Post
If you do that, be sure to reset the pots to minimum before applying power.
Also, the amp will come up to bias much fasterůso, go slow with your turns.
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Old 14th October 2021, 02:00 AM   #1900
rayma is online now rayma  United States
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Yes, undershoot when adjusting, and also allow an hour or so for thermal stabilization.
Put the cover on while waiting.
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