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In Reply to: RE: Cathode bypass cap removed and I like it posted by banpuku on January 11, 2017 at 08:57:43
"Fixed bias" is a funny term.
Fixed bias is when you have a negative supply, that is usually adjustable, feeding the bottom of the grid resistor placing a negative voltage at the grid and the cathode is connected directly to ground (in your case the CT of the filament transformer would be connected directly to ground)
Self (or "auto bias", or cathode bias) is what you are doing (with or without a bypass cap).
The DC plate current flowing through the cathode resistor places a positive DC voltage on the cathode while the grid is at zero voltage DC.
The tube is now biased because the grid is negative WRT the cathode.
In the end the tube doesn't really care how the bias is accomplished as long as the grid is negative WRT the cathode by the correct amount.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
Tre,
I found this schematic to help me understand fixed bias. What do you think of using a simple battery to set the negative bias voltage? I currently use a battery on my driver tubes, so I have no adversity to batteries. Unless you advise otherwise.
I am intrigued by the fixed bias method and want to hear what it sounds like. I just need to make sure I understand where the negative voltage should come from and how much negative voltage I should be using for the 2A3 tubes.
Pat
The amount of negative bias is dependent on how much plate to cathode voltage you will have and how much current you want flowing through the tube at idle.The schematic left out the grid resistor. The grid resistor would go between the grid and the top of the battery.
If you change from cathode bias to fixed bias on your 2a3 there will be more plate to cathode voltage because the cathode will be at zero volts where as now it's at +45 volts or so.
That will change the amount of negative voltage you will need to reach the idle current that you want.
If you use a negative supply (instead of batteries) that delivers more negative voltage than you need then you can make it adjustable and you can set it where you need it.
I hope that made sense.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 01/11/17
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