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.
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.
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
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
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.
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.
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.
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.
Thanks in advance.
Hi Unique,
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
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.
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!
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
The difficult part is that i can't find lots of them. Especialy for 6609's i am still looking for them. I am thinking of looking for them on the internet. Do you think this is a good idea? Any suggestions on ( reliable ) sites? I will also have to learn about matching them, but i don't worry since i saw some posts here talking about this issue.
Correct me if i am wrong, we are talking about those two pairs of 2N5400 and 2N5550?
Chris, one last thing:
Since i am refreshing my amp, i was thinking of replacing it's mains capacitors. Of course i did that a few years ago, by replacing the old 2x 7500uF per channel ( if i remember their voltage correctly ) with 2x10000uF/63V per channel. The sound was impressively got better, but as i read on a thread in this forum, it's better to replace big caps with 2 smaller in parallel connection. Since i have some unused pieces of 4700uF/100V, i am planning of using them but i am not sure i understood completely the way of connecting them. I made a drawing of the way i understand the connection and i was wondering if you could take a look and tell me if i am correct on this one. Should i also replace the bridges? If so, with same type or bigger ones?
P.S. Should i use any other caps somewhere? Something like 100n or so?
P.S.2 I hope i didn't get you tired with all these newbie questions, but i think these are the last ones.
Regards,
Nicholas
Since i am refreshing my amp, i was thinking of replacing it's mains capacitors. Of course i did that a few years ago, by replacing the old 2x 7500uF per channel ( if i remember their voltage correctly ) with 2x10000uF/63V per channel. The sound was impressively got better, but as i read on a thread in this forum, it's better to replace big caps with 2 smaller in parallel connection. Since i have some unused pieces of 4700uF/100V, i am planning of using them but i am not sure i understood completely the way of connecting them. I made a drawing of the way i understand the connection and i was wondering if you could take a look and tell me if i am correct on this one. Should i also replace the bridges? If so, with same type or bigger ones?
P.S. Should i use any other caps somewhere? Something like 100n or so?
P.S.2 I hope i didn't get you tired with all these newbie questions, but i think these are the last ones.
Regards,
Nicholas
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