Hello,
Since i m having problem finding the right value to bias my champ. I d like to replace the fixed 470 Ohm value with a 1k pot and adjust by ear. The amp sound amzaing around the fixed value but i need to get rid of some emphasis the amp has on certain frequencies which are pretty annoying when you turn the volume up. I have made a test with a 1 k resistor instead of the 470 ohm and the problem was gone but the amp sounds pretty cold and dull like this.
So I m trying to find the right spot and I can not achieve it with fixed values or at least, it would take too much time. I can always put back a resistor later.
Can I simply replace the resistor with a 1k pot?
Bias pot instead of cathode fixed bias
Moderators: pompeiisneaks, Colossal
Re: Bias pot instead of cathode fixed bias
It'll burn up the pot. The pot has to have the same wattage as Rk.
Parallel resistors to get close, then add the pot in parallel to those, to share the current (short term test only).
Parallel resistors to get close, then add the pot in parallel to those, to share the current (short term test only).
Tube junkie that aspires to become a tri-state bidirectional buss driver.
Re: Bias pot instead of cathode fixed bias
Sure. But in order to avoid adjusting the pot too low and burning up a tube, I would suggest putting a 500Ω pot in series with the 470Ω resistor. Oh, and use a 5 watt pot.
Re: Bias pot instead of cathode fixed bias
Ok, so also the pot needs to be watt rated like the resistors.
I already had in mind to leave the 470ohm but I didn t know how to wire it because there is also the 25uf in parallel with the 470 ohm.
I already had in mind to leave the 470ohm but I didn t know how to wire it because there is also the 25uf in parallel with the 470 ohm.
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martin manning
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Re: Bias pot instead of cathode fixed bias
Clip some resistor values in parallel with the 1k. If you use the common values 3k3, 2k2, 1k8, 1k5, 1k2, and 1k you will get 767, 688, 643, 600, 545, and 500 ohms.
PS You can probably get away with using 1/2W resistors for this
PS You can probably get away with using 1/2W resistors for this
Re: Bias pot instead of cathode fixed bias
It seems like I may be the odd man out on this one. I don't believe there is much to be gained, if anything, by doing this. The Champ is cathode biased. This is also known as self biased. To a large extent and within reasonable range of 470Ω the amp is going to seek it's own bias point with this type of circuit. As a result, I don't believe you'll find much tonal change, if any. I'll go as far as suggesting any difference you hear could be imaginary.
You may want to explore this a little more before you go to all that trouble. For example, this is what Aiken has to say on the subject:
What about cathode-biased amplifiers?
Do cathode-biased amplifiers need to be biased? The short answer is yes. The cathode biasing method is self-regulating, to an extent, because increases in cathode current create a larger voltage drop across the cathode resistor, which in turn, creates a larger negative grid-to-cathode voltage, which counteracts the increase in current. The tube will reach a stable point of equilibrium and stay there. However, just as different tubes from different manufacturers will draw varying amounts of current in a fixed-bias amplifier, the same is true of a cathode-biased amplifier. For this reason, the bias should always be checked, even with cathode-biased amplifiers.
Checking the bias current in a cathode biased amplifier is easy, just measure the voltage across the cathode resistor and divide by the resistance value to obtain the cathode current. Note that if the output tubes share a common cathode resistor, you must divide the current reading by the number of tubes sharing the resistor. Note also that a common cathode resistor does not allow you to determine the individual currents of each tube, so if one tube is drawing more current than the other, you would not be able to determine which is causing the mismatch, and, in fact, you would not be able to tell there was a mismatch at all. You can add individual 1 ohm resistors from the cathode of each tube to the common bias resistor, but you must then measure across the 1 ohm resistors, not from the cathodes to ground, to determine the voltage drop, and thus the cathode current. You can also use individual cathode bias resistors on each tube. The value of the resistor will be double that of the common resistor if two tubes are used, or four times that of the common resistor if four tubes are used. Each resistor would also have to be bypassed with its own electrolytic bypass cap.
The difficulty with cathode-biased amplifiers is that the cathode resistor must be physically changed for another one of different value in order to change the bias current. Although it can be done, very few guitar amplifiers have adjustable cathode bias.
Cathode-biased class AB amps are usually exempt from the "70% rule", because their cathode voltage rises when a signal is applied, effectively reducing the bias, and shifting the amp further into class AB operation. This means you can bias them hotter than a normal fixed-bias class AB amp and the tubes will still survive. Having said that, you have to experimentally determine how hot you can bias them by finding out how far the bias shifts during signal flow.
If the cathode-biased amp is "true" class A, there will be no bias voltage shift seen on the cathode when signal is applied, so you can bias at max dissipation and not worry about it. If the amp is actually class AB, you might still be able to get away with biasing at max dissipation because of the large bias shift at full power that pushes the amp into the class AB region, but you should check the tube dissipation at all signal levels. Note that max dissipation may not occur at full power, rather at somewhere between idle and full power (usually around halfway), so you have to carefully determine the safest max idle current to avoid exceeding the dissipation at any point in the tube's operation.
https://www.aikenamps.com/the-last-word-on-biasing
You may want to explore this a little more before you go to all that trouble. For example, this is what Aiken has to say on the subject:
What about cathode-biased amplifiers?
Do cathode-biased amplifiers need to be biased? The short answer is yes. The cathode biasing method is self-regulating, to an extent, because increases in cathode current create a larger voltage drop across the cathode resistor, which in turn, creates a larger negative grid-to-cathode voltage, which counteracts the increase in current. The tube will reach a stable point of equilibrium and stay there. However, just as different tubes from different manufacturers will draw varying amounts of current in a fixed-bias amplifier, the same is true of a cathode-biased amplifier. For this reason, the bias should always be checked, even with cathode-biased amplifiers.
Checking the bias current in a cathode biased amplifier is easy, just measure the voltage across the cathode resistor and divide by the resistance value to obtain the cathode current. Note that if the output tubes share a common cathode resistor, you must divide the current reading by the number of tubes sharing the resistor. Note also that a common cathode resistor does not allow you to determine the individual currents of each tube, so if one tube is drawing more current than the other, you would not be able to determine which is causing the mismatch, and, in fact, you would not be able to tell there was a mismatch at all. You can add individual 1 ohm resistors from the cathode of each tube to the common bias resistor, but you must then measure across the 1 ohm resistors, not from the cathodes to ground, to determine the voltage drop, and thus the cathode current. You can also use individual cathode bias resistors on each tube. The value of the resistor will be double that of the common resistor if two tubes are used, or four times that of the common resistor if four tubes are used. Each resistor would also have to be bypassed with its own electrolytic bypass cap.
The difficulty with cathode-biased amplifiers is that the cathode resistor must be physically changed for another one of different value in order to change the bias current. Although it can be done, very few guitar amplifiers have adjustable cathode bias.
Cathode-biased class AB amps are usually exempt from the "70% rule", because their cathode voltage rises when a signal is applied, effectively reducing the bias, and shifting the amp further into class AB operation. This means you can bias them hotter than a normal fixed-bias class AB amp and the tubes will still survive. Having said that, you have to experimentally determine how hot you can bias them by finding out how far the bias shifts during signal flow.
If the cathode-biased amp is "true" class A, there will be no bias voltage shift seen on the cathode when signal is applied, so you can bias at max dissipation and not worry about it. If the amp is actually class AB, you might still be able to get away with biasing at max dissipation because of the large bias shift at full power that pushes the amp into the class AB region, but you should check the tube dissipation at all signal levels. Note that max dissipation may not occur at full power, rather at somewhere between idle and full power (usually around halfway), so you have to carefully determine the safest max idle current to avoid exceeding the dissipation at any point in the tube's operation.
https://www.aikenamps.com/the-last-word-on-biasing
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martin manning
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Re: Bias pot instead of cathode fixed bias
BTW, what voltages are on the plate and screen, and what is the primary impedance of the OT?
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Ten Over
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Re: Bias pot instead of cathode fixed bias
A different approach:
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Re: Bias pot instead of cathode fixed bias
Thank you guys for all the suggestions
@phil_s To be honest, I have only tried two different values, 1k an 1.5k. Both of these resulted to be too extreme. With the 1k the amp was already too cold and sterile.
So today I have dropped a 2w 600 ohm carbon comp I found in a box. The difference is little but definitely there.
The stock value worked good for me and you are right. I don t see any reason to mess with the bias but my amp has some problem and I a m trying to fix it. I have tested and tried everything and right now, cooling the bias seem to be the only way.
With 600 ohm it is getting better and the amp is still sounding good just like with the stock value.
I m going to increase the value just a tiny bit more and listen.
I m not even testing the voltages because the amp was in the proper range even with a 1k resistor. So anything in between will be just a metter of taste.
For what I have learnt, it does make a difference but you only hear it when the volume is almost all the way up and yes, very subtle.
@phil_s To be honest, I have only tried two different values, 1k an 1.5k. Both of these resulted to be too extreme. With the 1k the amp was already too cold and sterile.
So today I have dropped a 2w 600 ohm carbon comp I found in a box. The difference is little but definitely there.
The stock value worked good for me and you are right. I don t see any reason to mess with the bias but my amp has some problem and I a m trying to fix it. I have tested and tried everything and right now, cooling the bias seem to be the only way.
With 600 ohm it is getting better and the amp is still sounding good just like with the stock value.
I m going to increase the value just a tiny bit more and listen.
I m not even testing the voltages because the amp was in the proper range even with a 1k resistor. So anything in between will be just a metter of taste.
For what I have learnt, it does make a difference but you only hear it when the volume is almost all the way up and yes, very subtle.
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martin manning
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Re: Bias pot instead of cathode fixed bias
I'm curious to see where your load line is.
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Stevem
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Re: Bias pot instead of cathode fixed bias
You might want to try the stock 470 ohm and instead lower the audio drive from the 12ax7.
Once you have enough drive to push the 6V6 to close to max clipping then that’s all you need and this should reduce the harshness that your likely hearing.
It will also help to give you more play range where you have a wider range of half dirty and clean, yet you will still be able to dig in harder and get the same max dirt.
It would be nice if you have a O-scope to view this on.
Once you have enough drive to push the 6V6 to close to max clipping then that’s all you need and this should reduce the harshness that your likely hearing.
It will also help to give you more play range where you have a wider range of half dirty and clean, yet you will still be able to dig in harder and get the same max dirt.
It would be nice if you have a O-scope to view this on.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Bias pot instead of cathode fixed bias
Sorry, what do you mean?
Hello, thanks for the suggestions, how do i lower it?Stevem wrote: ↑Fri Jan 06, 2023 8:42 pm You might want to try the stock 470 ohm and instead lower the audio drive from the 12ax7.
Once you have enough drive to push the 6V6 to close to max clipping then that’s all you need and this should reduce the harshness that your likely hearing.
It will also help to give you more play range where you have a wider range of half dirty and clean, yet you will still be able to dig in harder and get the same max dirt.
It would be nice if you have a O-scope to view this on.
Re: Bias pot instead of cathode fixed bias
Decrease the plate load resistor (Rp) or the cathode resistor (Rk). See attached, Chart 9. As you reduce these, the voltage gain (last column) decreases. I'd probably start with the cathode resistor.
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martin manning
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Re: Bias pot instead of cathode fixed bias
Re: Bias pot instead of cathode fixed bias
Are you talking about the preamp tube,
If so, should i reduce the value on both stages?
There are two 1.5k resistors there.
Sorry, what do you mean?[/quote]martin manning wrote: ↑Sat Jan 07, 2023 1:11 am [quote=Mikante post_id=447783 time=<a href="tel:1673043068">1673043068</a> user_id=23535]
[quote="martin manning" post_id=447779 time=<a href="tel:1673033739">1673033739</a> user_id=4984]
I'm curious to see where your load line is.
Plate and screen voltages, and primary impedance, will determine where the load line is on the 6V6 characteristic curves, which has a lot to do with he way it sounds.
[/quote]
I have 261 ohm as primary impedence. If you mean between b+ and the power tube pin 3.
I don t remember each pin voltages, i got 21v on the cathode.