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In the Forum: Melquiades Amplifier
In the Thread: The Melquiades bass channels: the coupling capacitor.
Post Subject: BiasPosted by deemon on: 8/14/2007
Hi all !
I thought about this resistor bias idea that Chris proposed , and found some some problems here . One of them discovered Chris himself - it is a big serial resistor in the grid chain , and it can cause not only HF rolloff ( it isn't a big problem in a bass channel ) but a problem with grid current . Big resistor can cause a signal distortion when we apply a big signal to the tube and signal peaks goes closer to zero voltage . But of course we can use a cathode follower after this bias resistor ....
Another issue is an additional current that this bias circuit sucks from the input tube plate . We need to decrease the plate load resistance in order to keep the same plate current , so we must loose some gain here .
And the third problem is the most dangerous one - it's DC instability ! When you have two tubes DC coupled , you will get all variations of the first tube operation point being amplified by the second tube , it's bad ...... it might be not so bad if we have a resistors in the cathode chains of the tubes , but in our circuit we have both cathodes directly connected to the ground , and it means big DC amplification factor of the amp . You see - if we have only 3V DC variation on the first tube plate , we'll have 120 ma variation of the output tube current ! It's too bad of course ....
By the way , Chris , what do you think about your current source ? How do you want to implement it ? I have some idea how to use it and solve this instability problem . It seems to me that the best current source is a pentode . It has very big plate resistance , and it seems to be a good solution . But we can make this current source VARIABLE , applying some voltage on the pentode control ( or maybe screen ) grid , and make this voltage depending of the mean current of the output tube . It will be a kind of automatic bias , or servo system . For example , if the plate voltage of the first tube increase - it will cause increased output tube current , after it the voltage on our pentode grid will increase too , so our current source will produce MORE current , bias will go to negative and the output tube current will return to its normal value . Maybe we'll design a regulation circuit that won't be too complicated . But how this servo system will affect the sound ? I don't know ......
Best regards
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