Bias test resistors

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StrummersOfThunder
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Bias test resistors

Post by StrummersOfThunder »

Regarding the 1ohm resistor from octal pin 8 to ground to facilitate easy calcualtion of bias current (1:1 mV/mA) in fixed adjustable negative bias set up.
Some call for 'current sensing' resistors ... where as lots of images just show regular metal oxide or metal foil 1% tolerance.
Also some say high wattage rating is needed but why would this be?

What resistors would the amp gurus favour here?
Can I safely just use plain jane metal foil 1/2-1 watt 1 ohm or do I ned to find something resembling a ochre coloured hot dog on the spit??

Thanks

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Re: Bias test resistors

Post by StrummersOfThunder »

In the mean time Ive just grabbed some 1% tolerance 2W metal film resistors.

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Re: Bias test resistors

Post by sizzlingbadger »

'current sensing' resistors is just the term used for a resistor put in place so that you can measure the voltage across it, nothing special.
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Re: Bias test resistors

Post by Jay »

It will be fine in 'normal' temperature range. You can try to find the temperature coefficient of that resistor if you can find its spec. Even 1W is ample assuming the valve's max cathode current won't exceed, say 100mA.

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Re: Bias test resistors

Post by MikeC »

StrummersOfThunder wrote: Sun Nov 03, 2024 6:59 am In the mean time Ive just grabbed some 1% tolerance 2W metal film resistors.
These should be perfect.
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Re: Bias test resistors

Post by StrummersOfThunder »

sizzlingbadger wrote: Sun Nov 03, 2024 7:16 am 'current sensing' resistors is just the term used for a resistor put in place so that you can measure the voltage across it, nothing special.
I love how stupid my questions look in retrospect.

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Re: Bias test resistors

Post by jeremyb »

StrummersOfThunder wrote: Sun Nov 03, 2024 7:58 am
sizzlingbadger wrote: Sun Nov 03, 2024 7:16 am 'current sensing' resistors is just the term used for a resistor put in place so that you can measure the voltage across it, nothing special.
I love how stupid my questions look in retrospect.
You're providing a wonderful service for all of us who have no idea about this kind of thing!
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Re: Bias test resistors

Post by RectifiedAmps »

The main concern is when the tubes fail and short out. Do you want resistors that will also burn out and need replacing or ones that will stand up to high current during failures? Some actually prefer to use low-wattage resistors here, to act like a rudimentary fuse but it’s probably a better idea to use 1W or more, unless you like replacing resistors every time you replace tubes.

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Re: Bias test resistors

Post by StrummersOfThunder »

jeremyb wrote: Sun Nov 03, 2024 8:09 am
StrummersOfThunder wrote: Sun Nov 03, 2024 7:58 am
sizzlingbadger wrote: Sun Nov 03, 2024 7:16 am 'current sensing' resistors is just the term used for a resistor put in place so that you can measure the voltage across it, nothing special.
I love how stupid my questions look in retrospect.
You're providing a wonderful service for all of us who have no idea about this kind of thing!
I ask the questions SO YOU DON'T HAVE TO!
Dial 1-800 - DIP-SHIT to listen to me curse after ordering yet more wrong parts!

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Re: Bias test resistors

Post by jeremyb »

StrummersOfThunder wrote: Sun Nov 03, 2024 8:25 am
jeremyb wrote: Sun Nov 03, 2024 8:09 am
StrummersOfThunder wrote: Sun Nov 03, 2024 7:58 am

I love how stupid my questions look in retrospect.
You're providing a wonderful service for all of us who have no idea about this kind of thing!
I ask the questions SO YOU DON'T HAVE TO!
Dial 1-800 - DIP-SHIT to listen to me curse after ordering yet more wrong parts!
There are no wrong parts, you're just opening up opportunities for other future projects!
Standing on top of the world

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Re: Bias test resistors

Post by StrummersOfThunder »

jeremyb wrote: Sun Nov 03, 2024 8:51 am
StrummersOfThunder wrote: Sun Nov 03, 2024 8:25 am
jeremyb wrote: Sun Nov 03, 2024 8:09 am

You're providing a wonderful service for all of us who have no idea about this kind of thing!
I ask the questions SO YOU DON'T HAVE TO!
Dial 1-800 - DIP-SHIT to listen to me curse after ordering yet more wrong parts!
There are no wrong parts, you're just opening up opportunities for other future projects!
:mental:
Yessssss

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