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In Reply to: RE: St70 bias resistor posted by Steve Franklin on December 13, 2014 at 05:29:50
You have to remember the original paired bias resistor value was calibrated to work with a 1.56V battery reading. So what a person did was to measure a battery and put a mark on the meter or where 1.56V is. So to bias a tube easily all you had to do was hook up a mater across the resistor and turn the bias pot until the needle of the meter was on the marked line. Again, this was all before digital volt/ohm meters. Also, if you were to bias the tubes according to the initial manual on today's wall voltages you would be running the tube hotter than previous because of the higher wall voltage of today.
Now since you are using individual bias pots you can use a metal film 1W resistor from pin 8 to ground. I don't get too crazed over what quality resistor to use at that small value as it has no effect on the sonics. You just want the resistor of choice to be as close to 10R as possible or just use 1% metal films or Mills. Mills are non-inductive. I personally never heard a difference over using one or the other.
With that said, my favorite way of biasing is through actual current draw. It is probably more accurate than using a 10R resistor but maybe not if the resistor is accurate.
To do it using the current draw is to set your VOM(two preferably) to amps and put the meter(s) in series with the cathode(pin 8) of each power tube. I use two VOM's so I don't have to keep taking the meter off one tube reconnecting the cathode, and putting it on another tube. Once you get the proper current draw(approx. a conservative value of 45MA-50MA)on both tubes then you shut the amp off and disconnect the meters from the circuit and reconnect with a straight wire. More work but for me I like it better that way.
Follow Ups:
Thanks ,I think I will try your method for current draw.
sf
Steve - Here's another tip I learned about the bias resistors. They are connected to Ground through the octal socket tabs. These tabs start to lose their Ground integrity after a while. I was fixing an amp whose bias would drift around a little but never stabilize. It drove me nuts until I measured the resistance of bias resistor Ground end to the main filter cap Ground. I found about three Ohms there. So, I hardwired the bias resistor Grounds back to the main cap and all was well. Don't depend on that old connection to chassis.
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