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Old 12th August 2007, 10:34 AM   #1
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Default Replacing the bias adjustment pot?

As a safety idea wouldn't ot be a good idea to replace the cheap 50 cent bias pot with a fixed value resistor once the value has been figured?

I have seen good pots go bad in just a few years. There is nothing more annoying to me than to be adjusting a bias pot that appears to be good and have it go open all of a sudden.
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Old 12th August 2007, 11:04 AM   #2
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what I did in the past is to replace bias pots in the following way:

Adjust BIAS roughly with original POT.
Remove POT, measure it's adjusted value.

Then I put in a combination of fixed resistor and a
POT of smaller value. It allows for more exact
BIAS adjusting, for instance, if the new pot has
50 % of the value of the old one (plus then a
fixed resistor of again 50 %) this allows for
2 times finer adjustment (same angle, 50 % resistance
change). Also, depending on the circuit layout, it
might protect you to go for too high BIAS currents,
and at least it reduces load dissipation in the pot itself.

Sometimes the vendors put in way to large pots (in Ohm),
so jou can just go for a smaller one if you find you'll
never use say the first 50 % of the pot setting.
If you change transistors laters you might then
need to go back to original values to obtain proper
BIAS.

Good Luck,
Stephan
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Old 12th August 2007, 11:41 AM   #3
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If you have a resistor in series with the pot you haven't really accomplished anything other than possibly making the adjustment easier. If and when a problem would occur and the pot went open then having the resistor there wouldn't accomplish anything. The circuit would still be open and we know what could happen then.
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Old 12th August 2007, 11:52 AM   #4
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Transistors and diodes change characteristics through the years, and resistors drift, and caps age. You need the ability to re-adjust idling current to compensate for those changes.

Replace the trimpot with a good one.
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Old 13th August 2007, 11:17 AM   #5
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Quote:
Transistors and diodes change characteristics through the years, and resistors drift, and caps age. You need the ability to re-adjust idling current to compensate for those changes.
I believe that once the bias has been set correctly there is not going to be a need to adjust it except for replacement of the output transistors or emitter resistors. Amplifiers that I rebuilt 20 years ago are still spot on their bias value.

The point being that pots can and do go open. Older pots can cause problems when trying to adjust bias. I have seen many good rebuilds go up in smoke because of a questinable pot. Pots can get hot and change value.
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Old 13th August 2007, 11:34 AM   #6
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Old carbon composition resistors would change value by tens of percent (or more) over the years, but modern parts don't move at all unless damaged or operated at their thermal limits. Semiconductors should also be stable unless damaged. I only use good quality pots in my projects, and it's always good practice to use a series-shunt combination of resistors around the pot so that if the wiper goes open or noisy, the bias is limited to a fairly narrow range. There's nothing wrong, and a lot right, by setting the bias with a fixed resistor. Those split leaf type Vector pins are perfect for that, as the resistor can be dropped in the slots and soldered, but removed and changed quickly without board damage. I usually give the bias pots in an old amp a shot of contact cleaner and rotate them over the full travel a dozen or so times, before firing up the amp, and I have seen more than one amp destroyed by intermittent bias pots.
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Old 13th August 2007, 11:43 AM   #7
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Just wondering: Do you right now have a problem with your pot? If not, don't do anything. If yes, replace it with a similar and wait 10 years until the next time or simply choose a normal industrial quality pot and wait forever.

Don't fix an unbroken amp, my advise.
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Old 13th August 2007, 09:20 PM   #8
anatech is offline anatech  Canada
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Hi Joe,
I do the same thing that Conrad does. No surprises for me. I check them if the outputs shorted as they can be burnt.

Hi P-A,
What can I say? I do put some contact cleaner on the control and at least look a the bias current. I find many off.

-Chris
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Old 5th November 2007, 07:11 AM   #9
Unique is offline Unique  Greece
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I know that you talk about an other amplifier, but i saw that you were talking about biasing and thought that this was a nice opportunity to ask for a little help. I own a Bryston 3B power amp on which i am replacing the transistors on it's right channel that were fried ( ordered them and waiting for their arrival ). Specificaly the 2N6609 and 2N3773 ones ( the problem was with the one pair of them but i ordered two pairs in order to have fresh ones ). At the time that i was troubleshooting i did some measurements on my amp's outputs and found that on the left channel ( since right was blowing fuses because of the problem ) the multimeter shows some dc. So, i think i have to do some adjustments anyway? Is there a standard way to do this? And if so, should i replace it's 30 years old pots as well? I am cabable of doing that kind of replacements if necessary and taking all of the precautions needed.

Thanks in advance.
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Old 5th November 2007, 04:25 PM   #10
anatech is offline anatech  Canada
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Hi Unique,
Quote:
the problem was with the one pair of them but i ordered two pairs in order to have fresh ones
The only way to properly repair one channel is to replace all those outputs with new. You should have bought extra so you could match them. You should also replace the driver transistors. Bear in mind that the originals have extra long leads.
Quote:
So, i think i have to do some adjustments anyway? Is there a standard way to do this? And if so, should i replace it's 30 years old pots as well?
Yes. However, before this even, you need to replace the older cheap electrolytic caps in both channels. Now if you want to improve the sound further, replace all those ceramic caps with mica or polystyrene capacitors. Once you are done that, you can clean the old trimmer controls or replace them. Do not use multi turn controls!
Quote:
At the time that i was troubleshooting i did some measurements on my amp's outputs and found that on the left channel ( since right was blowing fuses because of the problem ) the multimeter shows some dc.
Not shocking. You need to replace both the NPN and PNP diff pairs. Match the NPN types and the PNP types. It helps if they are the same beta between NPN and PNP. It also will improve the sound quality. Do both channels. They are not expensive parts and you may need 20 or 30 of each type to get matches. Buy them all at one time if you expect to get a good yield.

If you do these things, you will notice that your amp sounds much better and may be more stable as it warms up. The bias current does have a large positive temperature coefficient. The bias will be off when you first fire the amp up. You need to keep an eye on these when you run them the first time. Do one channel at a time.

-Chris
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