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In Reply to: RE: St70 bias resistor posted by Steve Franklin on December 13, 2014 at 05:29:50
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 12/13/14Follow Ups:
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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