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 Post subject: Is 75mA Per Device Okay?
PostPosted: Sat Nov 07, 2015 10:41 pm 
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Joined: Sun Dec 01, 2013 8:44 pm
Posts: 11
I finally got around to resetting the bias to the new recommended 150 mA per 'FET, and this is when I discovered my old Craftsman meter only goes up to 320 mA! (or I can switch the leads to the 20 amp jack and get much lower resolution.)

I decided to compromise and set the bias at 75 mA per device for a total of 300 mA. I hope this is acceptable?

I have gone cross-eyed looking for a suitable budget meter that will read up to at least 600 mA at a sufficient resolution, and I've seen a few out there in the $75 range... I suppose I need a nice ~$500 Fluke for the auto shop but I have always seemed to get by on meters that I'm not afraid of dropping, spilling chemicals on, slamming the hood on, backing over with a car, dropping, dropping, dropping, or backing over with a car after dropping and slamming the hood on after soaking in gasoline. Not that I have EVER done any of these things. But a $500 Fluke would be a magnet for catastrophe.

Anyway SN #1189 is back on the air and doing a FB job. Thank you Jim and team!

Tim


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PostPosted: Sun Nov 08, 2015 10:58 pm 
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Joined: Mon Jul 23, 2012 6:25 am
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Hi Tim,

If your amp is working to your satisfaction then 75mA is perfect. The only advantages of higher bias is a bit more gain and better linearity. If you want to run 150 mA per MOSFET with your existing measurement capabilities here's what you do:

1. set all pots (VR1-VR4) fully CCW.
2. Turns VR1 CW until you get 150 mA, record the voltage at the gate of Q1, then turn VR1 back down.
3. Repeat this step for VR2 - VR4 ( leave VR4 at 150 mA and don't turn it down)
4. Reset VR1 - VR3 to the recorded voltages for 150mA bias current.

But again, if your amp is working the way you want it to, leave it alone. If it ain't broke, don't break it!

73,
Jim WA2EUJ


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PostPosted: Sun Dec 06, 2015 7:40 am 
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Joined: Mon Feb 10, 2014 4:50 pm
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Location: Salem, Illinois
I find the Vichy VC99 a very good, inexpensive substitute for a high cost Fluke.

John - W9JSW

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John - W9JSW (formerly AC9HY and KE5SSH)


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PostPosted: Sun Dec 06, 2015 9:55 pm 
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Joined: Mon Jul 23, 2012 6:25 am
Posts: 1143
When I set the bias during 'Factory Tests' I use a homebrew meter that can handle the full current of the amp and provide 250uA resolution for setting the bias. That way I only have to plug in the amp once. That kind of range translates to over 90 dB of dynamic range. Since my A/D only has between 70 and 80 dB, I had to make the thing autorange. I'm not sure that it's all that accurate in an absolute sense but that's not important for setting the bias. The key is matching the current between devices so relative accuracy is all that's required.

73,
Jim WA2EUJ


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PostPosted: Sat May 07, 2016 8:22 pm 
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Joined: Sun Mar 20, 2016 4:50 pm
Posts: 6
I set my HR50 to 50 mA because that is what the current manual says and I didn't know any different (but did read at least one post here I think it was that said they used 500mA! because that was what was in the data sheet). Is 150 mA still the current wisdom?

Thanks,

Robert


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PostPosted: Sat May 07, 2016 11:07 pm 
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Yes, I need to update the manual, 150 mA is the best point for the bias, 50mA was a little light.

73,
Jim WA2EUJ


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PostPosted: Sun May 08, 2016 9:07 am 
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Joined: Sun Mar 20, 2016 4:50 pm
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Okay, got it, thanks Jim. Maybe a forum sticky with a list of (currently) undocumented changes for builders to check?

In the meantime, are there any other changes I (as a new builder) should know about?

73,

Robert, WA2T


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PostPosted: Sun May 08, 2016 10:12 am 
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Joined: Sun Mar 20, 2016 4:50 pm
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Rentwist wrote:
Okay, got it, thanks Jim. Maybe a forum sticky with a list of (currently) undocumented changes for builders to check?

In the meantime, are there any other changes I (as a new builder) should know about?

73,

Robert, WA2T


Scratch that, maybe an undocumented changes area in the builders portals on the Google site would be more appropriate?


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PostPosted: Sun May 08, 2016 12:34 pm 
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Joined: Mon Jul 23, 2012 6:25 am
Posts: 1143
Actually by Rev G on the PCB's we've got them up to date. The only thing we are fighting now is discontinued parts, our RCA jack supplier ran out and we'll have to change the footprint. But that type of stuff is transparent to the builders...

Jim.


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