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Initial setting of F5 bias--help, please!
Initial setting of F5 bias--help, please!
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Old 29th March 2020, 07:36 PM   #41
Zen Mod is offline Zen Mod  Serbia
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Initial setting of F5 bias--help, please!
P3- leave it out or strictly set to exact middle value

these are for deliberate setting of THD spectra, and if you need to ask, you are still not prepared to use them
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Old 29th March 2020, 07:39 PM   #42
ItsAllInMyHead is offline ItsAllInMyHead  United States
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Initial setting of F5 bias--help, please!
I'd recommend reading 6L6's build guide - the functions of all three pots and how to set them prior to the first power-up will save you some time and effort. It's all in the first page or two. If you don't set the bias pot properly prior to power up... you could blow another set of FETs.

Edited to add... and what ZM said
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Old 2nd April 2020, 12:38 AM   #43
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I have built a F5 turbo for another with purchases from the Diyaudio shop. Higher quality components and matched jFETs We’re used. But I can’t get the amp brds to bias.

Using a variac with only 40VAC going into the amp for 2.65VDC on the brds the 0.47 resistors have 1V across them. It doesn’t matter which of the two trim pots (10 turn) are on full anti-clock or fully clockwise turned in any or all combinations the bias doesn’t change. Also there is 0.33V DC offset stress the speaker posts. The resistors are slightly warm.

I have checked all components resistors and jFETs a number of times and all appear to be correct. At assembly each component was quad checked against the schematic at assembly.

Any clues why I cannot bias off the power FETs. They appear to be fully on and conducting? Anyone else experienced this?
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Old 2nd April 2020, 12:47 AM   #44
Zen Mod is offline Zen Mod  Serbia
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Initial setting of F5 bias--help, please!
any pictures to share with us?

while powering amp for first time , you don't have any right or freedom to use any other position/setting of trimpots than total zero, confirmed with ohmmeter across parallel resistors (parallel to trimpots)
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Last edited by Zen Mod; 2nd April 2020 at 12:49 AM.
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Old 2nd April 2020, 12:58 AM   #45
Dennis Hui is offline Dennis Hui  Canada
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Initial setting of F5 bias--help, please!
Why the variac and what rail voltage were you getting? And what voltages do you get across R3 and R4?

As ZM said, you should start with P1 and P2 at minimal value (adjust them so
that R5 and R6 measure close to zero resistance)
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Old 2nd April 2020, 01:45 AM   #46
mhouston is offline mhouston  Australia
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See above for the voltages DC and AC.
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Old 9th April 2020, 02:33 PM   #47
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Default F5v3 build guide link pls

Can someone please send me the build guide from 6L6 that describes the 3 pots and how to set them. Thank you very much!
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Old 9th April 2020, 08:40 PM   #48
ItsAllInMyHead is offline ItsAllInMyHead  United States
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Initial setting of F5 bias--help, please!
Use this one to start...

Firstwatt F5 amplifier v3 - diyAudio Guides

Then for more depth re: P3, you can see this and his YouTube video.

An illustrated guide to building an F5
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Old 16th April 2020, 03:03 PM   #49
b4bz is offline b4bz  Canada
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Quote:
Originally Posted by ItsAllInMyHead View Post
Use this one to start...

Firstwatt F5 amplifier v3 - diyAudio Guides

Then for more depth re: P3, you can see this and his YouTube video.

An illustrated guide to building an F5
There is a YouTube video? Would love to have a link since I can't find it in the thread.
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Old 16th April 2020, 03:28 PM   #50
ItsAllInMyHead is offline ItsAllInMyHead  United States
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Initial setting of F5 bias--help, please!
LMGTFY -

setting P3 pass amp - Google Search

First result -

Adjusting P3 - a video
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