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Old 15th November 2007, 03:32 PM   #501
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Mouser.
270 degree turn pots will destroy your sanity whilst trying to set bias. I use the Vishay 20 turn pots. Look at 594-64Y102 and its cousins, depending on how you want to access the set screw...side, top, etc.

Grey
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Old 15th November 2007, 04:35 PM   #502
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Quote:
Originally posted by GRollins
Mouser.
270 degree turn pots will destroy your sanity whilst trying to set bias. I use the Vishay 20 turn pots. Look at 594-64Y102 and its cousins, depending on how you want to access the set screw...side, top, etc.

Grey
If you are gonna use a 20 turn pot make sure that you get the screwdriver thing-a-ma-jiggy which wont't fall off.

I usually start with a 270 degree trimpot of a larger value than necessary -- when I have more acurately figured out the value necessary I solder in a resistor and smaller value trimpot --
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Old 15th November 2007, 05:54 PM   #503
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It's been a year or two since I set the bias on a Hafler, but I don't recall the value of the pot being too far off of what I would choose in the same position. (Make allowances if you want proportionately more class A bias.) But for whatever reason, single-turn pots always develop a notch right next to where I want to set the bias. It's either a little bit too high or a little bit too low. Once you apply enough torque to get out of the notch, the confounded thing jumps past where you were trying to go. Getting it set just right requires copious amounts of alcohol that I'd rather consume while listening...not at the bench.
Not that a ten or twenty-turn pot can't give you the same problem, but the jump represents less difference in the bias.
It would be wise to check the pin spacing to verify that the pot I suggested above will fit properly. I don't have that circuit board where I can lay hands on it and the diagram in the manual doesn't have a scale.

Grey
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Old 15th November 2007, 06:41 PM   #504
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Quote:
Originally posted by GRollins
But for whatever reason, single-turn pots always develop a notch right next to where I want to set the bias.

Try this -- the notch is probably a result of the continuous current flow heating up the junction -- I saw this mentioned in Elektor a few years back -- try paralleling a larger value across a larger value potentiometer.
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Old 16th November 2007, 01:19 AM   #505
mmerig is offline mmerig  United States
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Quote:
It would be wise to check the pin spacing to verify that the pot I suggested above will fit properly. I don't have that circuit board where I can lay hands on it and the diagram in the manual doesn't have a scale.
I have usd the Vishay 20-turn pots and they make the bias much easier to set. I had the same "notch" or jumping problem Grey mentioned.

The pins on the Vishay pots are thinner and spaced a little narrower (bending them) than the stock pot's, so they don't sit as firmly, but other than that, they work fine. The little screwdriver thingy that Jack mentioned is well worth it. I have had them in for years, and they are stable.
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Old 16th November 2007, 03:51 AM   #506
anatech is offline anatech  Canada
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Default Semifixed resistor "notch"

Hi Jack,
Quote:
the notch is probably a result of the continuous current flow heating up the junction -- I saw this mentioned in Elektor a few years back
Nope. Fancy explanation, but the cause is simply pressure over time. Grey, try to use new controls. They don't have that "notch" - yet.

You need to have contact pressure for a reliable connection, that is one reason that multiturn controls sometimes fail in this position.

The thing that completely mystifies me is this. We have a number of people around here that can design an amplifier easily enough, but they can't figure out the adjustment range for this one simple circuit? I don't believe it. It's simple laziness, you guys know very well how to design a bias circuit. You also are smart enough to adjust the values once you have it built and running if need be.

Hi Grey,
I've spent well over 30 years setting bias currents and offsets on old audio equipment and test gear. All you need is a steady hand and some patience for the rougher ones. It ain't that hard to tell you the truth. Just put a touch of contact cleaner on the metal wiper part and turn the control back and forth some. The notch should be approximately where the control should be set. You've admitted as much yourself.

Besides, bias current is not a precision adjustment. It's going to wander with ambient and heat sink temperature. So even if you can only get it in the ballpark, you'll be fine.

So all, once again. The proper adjustment control for bias is the lowly 270 control. Same for DC offset. If you are worried about the adjustment range, then just think about it. Build it and adjust your values empirically if you must. Multi turn controls do not age well in this application, they are the wrong part for the job.

-Chris
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Old 16th November 2007, 01:40 PM   #507
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Default anatech,

Quote:
I've spent well over 30 years setting bias currents and offsets on old audio equipment and test gear. All you need is a steady hand and some patience for the rougher ones. It ain't that hard to tell you the truth. Just put a touch of contact cleaner on the metal wiper part and turn the control back and forth some.
I agree. A spritz of cleaner and carefully rotating a few times fixes any control problems I have found in ~50 amps over the last 25 years. Many of these amps have been in frequent use over these many years and no control has failed in those I have examined.
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Old 16th November 2007, 01:43 PM   #508
mmerig is offline mmerig  United States
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Quote:
Multi turn controls do not age well in this application, they are the wrong part for the job.
What is it about these multi-turn pots that makes them "not age well"?
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Old 16th November 2007, 01:53 PM   #509
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Default Re: anatech,

Quote:
Originally posted by Dick West


I agree. A spritz of cleaner and carefully rotating a few times fixes any control problems I have found in ~50 amps over the last 25 years. Many of these amps have been in frequent use over these many years and no control has failed in those I have examined.
I have found the same thing in trimmers of test equipment I have POOGED or repaired. Most recent was a Krohn Hite DA -- the trimmer for the oscillator had to be replaced and the value on the schematic wasn't correct. The OEM part was defective and the THD couldn't be brought below 0.002% -- with a new 10 turn trimmer I got it down to 0.0008% -- and I then sold the thing on EBay.

You can get mil-spec trimmers in the surplus market.
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Old 16th November 2007, 02:50 PM   #510
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People act as though single-turn pots are graven in stone. Why torture yourself with a single-turn pot when there's a simpler/faster solution?

Grey
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