I have a few extra circuit boards if anyone wants to build one. Schematic and board image attached. shoot me an address and ill send you one. I have 2 0r 3 I can spare.
IMHO it would be more accurate to call it "fixed bias", because it as.
There are no automatic bias control.
There are no automatic bias control.
I thought this type of bias adjust itself when you change tubes. or in other words doesnt have to be adjusted. That is how I have always seen it described.
I was superficial, it just a simple cathode bias.
The operating point sets to near to desired Ua, Ia point (at specified B+, for example take into consideration WE datasheet), but it depends of the 300B type and condition.
There are no auto-bias function, i.e. it will change -not so large, but changing- from tube to tube.
The real auto bias measuring desired anode current, and changing bias voltage to hold it stable.
The operating point sets to near to desired Ua, Ia point (at specified B+, for example take into consideration WE datasheet), but it depends of the 300B type and condition.
There are no auto-bias function, i.e. it will change -not so large, but changing- from tube to tube.
The real auto bias measuring desired anode current, and changing bias voltage to hold it stable.
I have always heard the cathode bias called auto bias. I can see it making sense both ways. depends on perspective I guess.. 🙂
🙂
🙂
A fixed resistor from the cathode to ground is 'Cathode Bias'.
A fixed resistor from the cathode to ground is also 'Self Bias'.
One feature is that it 'tends' to automatically adjust the cathode current, and make that current somewhat constant from tube to tube.
That is all true, whether it has a bypass capacitor, or does not have a bypass capacitor.
This form of bias has the very Least change in cathode current from tube to tube to tube.
True 'Fixed (Grid) Bias' does not self adjust; and it does not adjust at all (True Fixed has no potentiometer to adjust).
This has the Most change of Cathode current from tube to tube.
'Adjustable Fixed (Grid) Bias' does not self adjust; it can be adjusted because it has a potentiometer that can be adjusted.
If the potentiometer is always left at one position, this also has the Most change of Cathode current from tube to tube (because if you do not change the potentiometer, it effectively is 'Fixed Bias').
If the potentiometer is properly set, the correct cathode current, or desired cathode current can be set (as long as the potentiometer voltage range allows for the variance of different tubes).
Sometimes, the terms of Adjustable Fixed Bias, and Fixed Bias are used interchangeably.
But they are not the same. However, many choose to say Fixed Bias when they actually mean Adjustable Fixed Bias (it is so easy to not have to type the word Adjustable).
Cathode Bias and Self Bias are often used interchangeably.
But, if you use an LED, Diode, or a Battery from the cathode to ground, that could also be called Cathode Bias, or even Self Bias.
But it probably should not be called Cathode Bias or Self Bias; it is better to respect the historical use of the 'Self Bias' and 'Cathode Bias' terminology.
Instead, it should be called LED, Diode, or Battery bias.
One problem is if you try to use only one of the following: resistor, LED, Diode, Battery, Fixed Bias, or Adjustable Fixed Bias potentiometer, for all tubes (2 tubes, 4 tubes, etc.).
That requires extreme matching of the tubes in order to get the optimum performance.
Lazy use of terminology does not add anything to the art of communication.
If I said anything in-correct in the above, I apologize in advance.
And let me know of any items above that need correcting.
Thanks!
A fixed resistor from the cathode to ground is also 'Self Bias'.
One feature is that it 'tends' to automatically adjust the cathode current, and make that current somewhat constant from tube to tube.
That is all true, whether it has a bypass capacitor, or does not have a bypass capacitor.
This form of bias has the very Least change in cathode current from tube to tube to tube.
True 'Fixed (Grid) Bias' does not self adjust; and it does not adjust at all (True Fixed has no potentiometer to adjust).
This has the Most change of Cathode current from tube to tube.
'Adjustable Fixed (Grid) Bias' does not self adjust; it can be adjusted because it has a potentiometer that can be adjusted.
If the potentiometer is always left at one position, this also has the Most change of Cathode current from tube to tube (because if you do not change the potentiometer, it effectively is 'Fixed Bias').
If the potentiometer is properly set, the correct cathode current, or desired cathode current can be set (as long as the potentiometer voltage range allows for the variance of different tubes).
Sometimes, the terms of Adjustable Fixed Bias, and Fixed Bias are used interchangeably.
But they are not the same. However, many choose to say Fixed Bias when they actually mean Adjustable Fixed Bias (it is so easy to not have to type the word Adjustable).
Cathode Bias and Self Bias are often used interchangeably.
But, if you use an LED, Diode, or a Battery from the cathode to ground, that could also be called Cathode Bias, or even Self Bias.
But it probably should not be called Cathode Bias or Self Bias; it is better to respect the historical use of the 'Self Bias' and 'Cathode Bias' terminology.
Instead, it should be called LED, Diode, or Battery bias.
One problem is if you try to use only one of the following: resistor, LED, Diode, Battery, Fixed Bias, or Adjustable Fixed Bias potentiometer, for all tubes (2 tubes, 4 tubes, etc.).
That requires extreme matching of the tubes in order to get the optimum performance.
Lazy use of terminology does not add anything to the art of communication.
If I said anything in-correct in the above, I apologize in advance.
And let me know of any items above that need correcting.
Thanks!
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