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Old 13th April 2013, 08:01 PM   #1741
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There are now a limited number of F-5T boards available in the store.
Well those didn't last long...
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Old 13th April 2013, 08:55 PM   #1742
a.wayne is offline a.wayne  United States
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As expected ...............
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Old 13th April 2013, 09:00 PM   #1743
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SteveHolt -

P3 is to adjust the distortion spectra only, it has nothing to do with setting output bias.

You need to neutralize it, count turns from one side to the other and stop halfway. Don't touch it again.

P1 and P2 are where you set the bias and the offset.

The two pots work as a sort of teeter-totter to control both the offset and bias, in concert with each other. One does not do bias and the other doesn't do offset, instead you twiddle both to get both readings where you need it.

The best thing to do is get (3) voltmeters. Place one on the output to measure offset. place the other two on the N and P channel source resistors.

Turn both the pots evenly (it might take what feels like many turns) until you start seeing voltage across both source resistors. Turn them up to 0.2v. Now adjust one pot until you make the offset zero. This will certainly change the bias. That's normal.

Continue to increase the bias about .1v at a time, and then eliminate the offset. You will take two steps forward and one step back. Again, that's normal.

Once it's to 0.4, with zero offset, put the lid on and let it sit for 1/2 hour. Then adjust the bias to 0.47v across the source resistors with zero offset, and you are done.

Check it again in about a week.

Last edited by 6L6; 13th April 2013 at 09:04 PM.
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Old 13th April 2013, 09:06 PM   #1744
6L6 is offline 6L6  United States
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And reading your post again you are not really asking about the normal bias procedure...

But with no signal or load the proper DC output voltage should still be zero.
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Old 13th April 2013, 09:20 PM   #1745
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As expected ...............
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Old 14th April 2013, 05:15 AM   #1746
oiphy is offline oiphy  Norway
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thank you.... going in my f5 folder
learning something everyday here ,,,,,,,
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Old 14th April 2013, 05:48 AM   #1747
sangram is offline sangram  India
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@steveholt: P1 and P2 control the bias on the output devices, and the two devices interact with each other as their individual bias levels change. So as you increase bias on the MOSFET connected to the positive rail, the output is driven overall positive and vice versa.

P3 is to be ignored after setting halfway (ohmmeter is the best way to do this and before installation and power up, ideally). On initial powerup if you have followed procedure correctly you should get zero voltage across the source resistors, and turning the pots P1 and P2 should increase this voltage. It typically takes many turns to get this going, five or more turns, but if you are running out of range even at maximum bias (maximum resistance) you may need to tweak the drain resistors on the input FETs. An ohmmeter is good for verifying pot resistance because those little screws keep turning even after hitting the endstops.

As the bias goes up you will see that the offset will constantly change. When you hit close to the desired bias level, you may have to make tiny adjustments to the bias to get everything to level out (the exact bias will be slightly different for both halves). At this point you might want to involve P3 as it affects the symmetry between the top and bottom halves of the circuit, and tune for maximum symmetry (exactly same bias levels on both sides of the supply) once the amp is fully warmed up.

I have managed offsets below 10mV with single F5, I see no reason why same or better is not possible with the Turbo.
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Old 14th April 2013, 07:26 PM   #1748
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Here ya to V3. No diodes, but overal bias higher than my V2. No miller compensation cap for instability. No heavy testing, but small stuff checks out fine.
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Old 14th April 2013, 07:53 PM   #1749
6L6 is offline 6L6  United States
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Cascode front end? Dual or single Jfets? Give it a whirl with the Store boards as well if you can.

Looks good regardless!
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Old 14th April 2013, 08:10 PM   #1750
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Single FE. Cascode, because it was there. Will hook up store boards in V3 tonight, hopefully. ONce again plugged the damn PSU up wrong. Had positive going into ground slot. with negative rail correct, G to G, poitive rail to ground, full power, all was fine. It even had little bit of bias on outpus. No worries. Listening to music now. I cannot say that V3 is big step over V2. For those who need the power, think you would have different results. V2 gives you the signature sound of the Turbo series. From there, it is about more power.
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