Dear forummembers,
I am trying to design my first tube amp.
And i am trying to make the bias for my end tubes (EL34 PP).
Since this is my first amp, i was thinking about making the bias adjustable.
I came up with the following:
The 10 ohm resistors are there to easily measure the current through the tube.
With the 1K potentiometer I can then play a bit with the bias setting.
And the 500 ohm potentiometer, I can use to make sure both tubes are tuned equally.
What do you think about this idea??
I am trying to design my first tube amp.
And i am trying to make the bias for my end tubes (EL34 PP).
Since this is my first amp, i was thinking about making the bias adjustable.
I came up with the following:
The 10 ohm resistors are there to easily measure the current through the tube.
With the 1K potentiometer I can then play a bit with the bias setting.
And the 500 ohm potentiometer, I can use to make sure both tubes are tuned equally.
What do you think about this idea??
the input on the 500R pot should be your negative bias voltage not ground. I would use a higher value here maybe 10k. And also a higher value for the 100R resistors.
Both 10R resistors should go to ground and the 200u capacitors should be in parallel with them
Grid leak resistor maximum values vary based on fixed bias or cathode bias circuit. 100k might be more appropriate in this circuit.
I have done something similar with a KT88 amp but I do not know what values to actually use with an El34 amp.
Hopefully someone with more experience will provide and answer
Steve
Both 10R resistors should go to ground and the 200u capacitors should be in parallel with them
Grid leak resistor maximum values vary based on fixed bias or cathode bias circuit. 100k might be more appropriate in this circuit.
I have done something similar with a KT88 amp but I do not know what values to actually use with an El34 amp.
Hopefully someone with more experience will provide and answer
Steve
I think he is trying to make an adjustable self bias, not adjustable fixed bias.
If so:
Move the 500R balance pot to between the 10R from the two cathodes. 100R WW power is necessary here due to cathode current. You are looking for something around 150R total for the cathode bias resistor network. Change the 1K pot to a 100R pot. Both 200K grid resistors go directly to ground.
If so:
Move the 500R balance pot to between the 10R from the two cathodes. 100R WW power is necessary here due to cathode current. You are looking for something around 150R total for the cathode bias resistor network. Change the 1K pot to a 100R pot. Both 200K grid resistors go directly to ground.
I have found that my calcculated values are not correct.
I will change them, and repost the updated idea again.
Yes, i'm try to make adjustable self bias.
Since this is my first amp, i can play arround a bit with the bias
I will change them, and repost the updated idea again.
Yes, i'm try to make adjustable self bias.
Since this is my first amp, i can play arround a bit with the bias
The grid resistors should not go to ground but to the pot as drawn. This is a basic Williamson balance control, and it works very well, better than a simple balance pot on the cathodes. It actually imposes a small amount of "positive" voltage (with respect to the cathodes) on the grids and acts as a kind of "fixed" bias, if you will. But the values are off, I think. The balance pot should be 100 ohms/5 watts like the Williamson. You don't want too much variation on the grids, and you don't want too much current flowing through that pot. Then the fixed cathode resistor should be 150 ohms/ 10 watts, and for the "variable" cathode resistor 100 ohms/5 watts is sufficient. Depending on your output transformer, about 250-300 ohms total cathode resistance would be appropriate. Also, IMO, a single 220uF/100vdc cathode bypass cap is sufficient, from the top of the main cathode resistor to ground. The 10 ohm current-measuring resistors don't really add appreciably to the cathode resistance.
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For better AC balance, remove the cathode capacitors if you want to use the final at class A.
Let me briefly explain the whole project.
The intention is to build a Push Pull amplifier.
This is my first tube amp, so much of the technology is new to me.
I have come up with the following load line for my EL34:
And the following diagram I came up with for the power tubes.
The previous one I miscalculated.
The 10 ohm resistors, are there to easily measure the current through the tube.
With the 1K potentiometer (set to 307.5 Ohm for the 45mA) I can then play a bit with the bias.
And the 500 Ohm potentiometer, I can use to make sure both tubes are tuned equally.
Did this turn out to be a good design?
Or do you have any comments?
The intention is to build a Push Pull amplifier.
This is my first tube amp, so much of the technology is new to me.
I have come up with the following load line for my EL34:
And the following diagram I came up with for the power tubes.
The previous one I miscalculated.
The 10 ohm resistors, are there to easily measure the current through the tube.
With the 1K potentiometer (set to 307.5 Ohm for the 45mA) I can then play a bit with the bias.
And the 500 Ohm potentiometer, I can use to make sure both tubes are tuned equally.
Did this turn out to be a good design?
Or do you have any comments?
As I said above, the pots are unnecessarily large in value. A small adjustment means wild swings in the cathode current or balance. The 500 ohm balance pot is adding too much to the overall cathode resistance and is carrying too much current. Remember that it is part of the cathode-to-ground resistance. And I would not trust a single pot as the main cathode resistor. A failure would not be nice.
You have the right idea, but look at the original Williamson design. The pots are sized to give just enough range of adjustment without having to carry an unneccesary amount of current. You don't need more than a 50 ohm range to fine tune the cathode current, or 100 ohms to set the balance between tubes.
https://dalmura.com.au/static/Williamson 1952 The Williamson Amplifier.pdf
The Williamson is set for 65mA current per tube for Class A operation. If you want to reduce the current to AB operation, increase R22 accordingly but don't make the pots themselves larger in value.
You have the right idea, but look at the original Williamson design. The pots are sized to give just enough range of adjustment without having to carry an unneccesary amount of current. You don't need more than a 50 ohm range to fine tune the cathode current, or 100 ohms to set the balance between tubes.
https://dalmura.com.au/static/Williamson 1952 The Williamson Amplifier.pdf
The Williamson is set for 65mA current per tube for Class A operation. If you want to reduce the current to AB operation, increase R22 accordingly but don't make the pots themselves larger in value.
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Looks fine, though you might drop the 270R to 100R. Some would suggest increasing the caps to at least 470u, or more.
With 270 Ohms there, Each EL34 cathode effectively sees:
10 Ohms + 540 Ohms + 2x the 100 Ohm potentiometer Ohms setting + 100 Ohms + 50 Ohms.
You probably will not get very much quiescent current in the EL34.
Yes, replace the 270 Ohm resistor with 100 Ohms.
And, you may have to reduce some of the other resistors to get enough current.
Otherwise, you may get a bit of crossover distortion.
Just my opinions
Note: I had to completely edit out my original text in this posting.
The 200uF caps are bypassing the complete resistance of the cathodes to ground.
They are not only bypassing the 10 Ohms, but they bypass a much higher total resistance.
10 Ohms + 540 Ohms + 2x the 100 Ohm potentiometer Ohms setting + 100 Ohms + 50 Ohms.
You probably will not get very much quiescent current in the EL34.
Yes, replace the 270 Ohm resistor with 100 Ohms.
And, you may have to reduce some of the other resistors to get enough current.
Otherwise, you may get a bit of crossover distortion.
Just my opinions
Note: I had to completely edit out my original text in this posting.
The 200uF caps are bypassing the complete resistance of the cathodes to ground.
They are not only bypassing the 10 Ohms, but they bypass a much higher total resistance.
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I agree with the others. Reduce 270 ohms to 100 ohms. Also, as I said above, a single cathode bypass cap from the top of 100R cathode resistor is sufficient and will save space and expense. Bypassing the 10 ohm resistors is really not necessary. Otherwise, looks good! So what will drive them? You're halfway to a Williamson already. ;-) How many watts are you looking to get?
I don’t think the pot going to ground is going to do anything the pot connected to the 270ohm is already doing. It will not balance the two tubes as you were thinking as it is common to both output tubes. Just bump the 270 up to 430 or 390 ohm and get rid of the whole 100 ohm and pot to ground part it acts as just an adjustable 75 ohm resistor. I would make the 200uF caps up to 470uf at 63v or higher. If you want to balance the tubes use a 100 ohm 3watt pot fed by the 10 ohm resistors the center tap feeds the 390 ohm resistor. That way you can adjust each tubes current relative to the other.
Those pots need to be high power! Not the common one-buck 1/4W or 1/8W parts, even if you pay $99 "audiophile". Do Ohm's Law. Look for wirewound pots. Do not get them on eBay-- I did and I could hardly turn them they were so crudely made.
There ARE other ways to do this.
But IMHO
There ARE other ways to do this.
But IMHO
No, because you are just starting, you use factory-suggested cathode resistors and trust the Old Guys who wrote datasheets like their jobs depended on happy customers.
Actually, the 100 ohm balance pot is doing something important. In the Williamson scheme it actually controls the grids of the output tubes in relation to the cathode voltage. It's a very clever design and it's a more secure way to balance the output tube currents than a simple pot at the cathodes.
A 5W wirewound pot from Digikey or another reputable electronics house will work fine in both of those positions.
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Lol, I didn’t look at the rest of the schematic! I thought is was solely auto bias. So yes the pit is needed. you definitely want quality 5 watt or better pots in this circuit.
But if you use "fixed" bias, you gain power and don't make as much waste heat.
I mean I do both depending on power, but the high power pots for cathode current adjustment will cost more than the grid bias system.
Just saying 🙂
I mean I do both depending on power, but the high power pots for cathode current adjustment will cost more than the grid bias system.
Just saying 🙂
5 watt wirewound pots are $7. ;-) Cheaper than an extra bias supply! But more to the point, I am fully in favor of this type of output stage. Yes, you can get more power/lower distortion etc etc with fixed bias, but the Williamson-style output stage is dead quiet, good harmonic balance and cancellation, and it sounds lovely if you only need 20-30 wpc. I've built both fixed- and cathode-bias Williamson amps and I vastly prefer the cathode bias versions from a musical standpoint.