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Are these resistors, in 2watt suitable for the 10ohm bias resistors. They are 5%, but I have some that read almost exactly 10 ohm
I have a bunch in the drawer ,and would be able to arod another order from Mouser.
http://www.vishay.com/docs/28729/28729.pdf
sf
Follow Ups:
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.
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.
Do you know the accuracy spec for your meter in the range used to measure 10-ohm resistors? You'd want it to be 1% or better.Can you zero-out the meter before measuring a resistor?
When was the last time your meter was calibrated?
If there is any question as to whether the meter can accurately measure the ones you have to 1%, I'd say just order some that are made to 1% tolerance.
Edits: 12/13/14 12/13/14
I have several . All fluke inc an 87.
I also use this for better accuracy at work. ( marine electronics)
http://www.extech.com/instruments/resources/datasheets/380941_942_947.pdf
If I need wire wound ill get them.
sf
I think the original 1W 15.6Ω was wirewound, but I do not know why. Maybe it was a more convenient way to get a 1% value in those days. If we're talking about a slightly modified version of the original biasing scheme in the Stereo 70, then we're talking a 1VDC drop across a 10Ω resistor at idle:1VDC = I * 10Ω
I = 100mAP = 1VDC * 100mA
P = 0.1WMaybe the resistor needs to be able to dissipate more power under dynamic conditions, I don't know. But, to be able to set the bias accurately, you probably want to make sure the resistors measure within 1% of value, however you can get that accomplished.
Anyone, please feel free to correct my math if needed.
Edits: 12/13/14 12/13/14 12/13/14 12/13/14
Thanks.
It makes sense to me that your idea regarding the accuracy of wirewounds vs carbon comp resistors back then is fair.
I will be doing an individual tube bias ,using a second set of bias pots , so each tube will see even less power than the original setup.
I am confident your math is correct.
sf
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