Fixed bias question

[xcortes]

How does one know how much current flows through the nagative bias in a fixed bias amp? In other words, how does one design the ps for the bias?

Thanks

[chard]

To measure the current in a fixed biased amp, install a 1 ohm resister between the cathode and ground. Set you meter on the millivolt setting and measure how many millivolts are dropped across the 1 ohm resister. The millivolt reading across the 1 ohm resister will correspond to the milliamps flowing through the cathode. This only works with a 1 ohm resister . As long as you know your plate voltage you can design for fixed bias. Lets say you want to bias you amp at 90% and your using a 2a3 which is a 15 watt tube. 15w x 90%=13.5w. Divide your plate voltage into 13.5 to get your target current. In fixed biased amps there is no cathode voltage drop to factor in. Fixed biased is usually ajustable with a pot. With your meter across the 1 ohm resister ajust the pot until you reach your desired current draw. Be sure to start your pot adjustment on the end where zero current flows and then slowly turn the pot until you reach the your target current. 

[xcortes]

Thanks for the response but what I need to know is not how to measure current but how to design the power supply for the bias voltage.

[Paul Joppa]

In a typical Class A1 tube, no DC current flows in the grid, so the bias supply can have a high source impedance. Plate current is a strong function of grid voltage, so the grid bias voltage must be adjustable, and monitored frequently enough to maintain the current at the intended value as the tube ages.

Consequently, a fixed bias supply is an adjustable voltage, often with a resistor or choke or transformer winding between it an the grid (in order to maintain a high AC impedance presented to the driver), plus a method for monitoring the tube's current.

Cathode bias is a form of servo bias, using negative feedback to maintain the tube's current as it ages. Without that feedback, you must either monitor and adjust, or build in a substantial safety margin and be willing to tolerate wide variations in the operating current. That's why the WE 300B specs say the maximum DC current is 70mA for fixed bias, but 100mA for cathode bias.

[2wo]

If you are asking how much current you need for a bias supply. The answer is very little DC current flows in the gird circuit. If you design for 10ma or so, you will be good...john

[chard]

First determine the aproximate - grid voltage you want. Go to tube amp grid charts for this info. Whatever power tube your using go to Franks's tube amp data or some other source and look at the chart that has a graft with current, plate voltage, and grid voltage all intersecting at appropiate defined places. This way you will be able to determine how much grid voltage to expect for a specific tube and a specific plate voltage and plate current. For guitar amps the power transformers often have a voltage tap for fixed biased voltage supply. Perhaps some SET power transformer suppliers also manufacture transformers with this feature. If not you should be able to get a small low current  Hammond transformer for this purpose. Or you could just use your high voltage power transformer tap and on your way to your rectifier run a line to the fixed bias supply circuit. This should be ok as the fixed bias circuit draws very little current. To make this circuit you will need a diode to block positive ac and pass negitive ac, one or two filter caps to filter the ac ripple and a voltage divider circiut which includes a pot set up as a variable resister. This circuit should produce the dc equivalant of about 60% to 70% of the r.m.s. value of the ac source. If your voltage source is a lot higher then the voltage needed for this circuit then you will have to knock the voltage down with a voltage divider or by some other means before going to this circuit. You can find a sample of a fixed bias supply circuit at www.tedweber.com, under amp kits, the kit with a real good example of this supply is the java kit, there is both a schematic and layout. Of course this schematic is probally not just right fot your amp but the idea and principle should work with component value tweeks.       

[xcortes]

If you are asking how much current you need for a bias supply. The answer is very little DC current flows in the gird circuit. If you design for 10ma or so, you will be good

That's exactly what I needed to know (and what I suspected). Thanks John.

Thanks everyone else for your responses as well. I guess my question wasn't clear.