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Old 24th February 2012, 03:18 AM   #1
tim614 is offline tim614  United States
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Default how to install a bias meter in a cathode bias amp

Hi guys,
i wanted to know what is the correct way to install an analog bias meter to my amp, its cathode bias.
do i just put it in series with the cathode resister or parallel with the cathode resister?

thanks
tim
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Old 24th February 2012, 03:48 AM   #2
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Just measure voltage drops on cathode resistors, that's it.
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Old 24th February 2012, 03:58 AM   #3
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Quote:
Originally Posted by tim614 View Post
Hi guys,
i wanted to know what is the correct way to install an analog bias meter to my amp, its cathode bias.
do i just put it in series with the cathode resister or parallel with the cathode resister?

thanks
tim
If your meter measures a fixed scale like 0-200mA then yes put it in series. A lot of meters are scaled for 0-1mA and are meant to be placed across a sense resistor like Wavebourn suggested.
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Old 24th February 2012, 04:13 AM   #4
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Originally Posted by astouffer View Post
If your meter measures a fixed scale like 0-200mA then yes put it in series. A lot of meters are scaled for 0-1mA and are meant to be placed across a sense resistor like Wavebourn suggested.
If you want to measure voltage, you have to put a resistor in series with the 0-1 mA meter equal to 1K (minus the meter resistance) times the voltage you want to read, I.E. 10 volts, 10K. This is only suitable for measuring in low Ohms circuits as it causes significant loading and will give incorrect readings. A 50 uA meter would require 20K per volt.

Last edited by Bibliophile; 24th February 2012 at 04:36 AM.
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Old 24th February 2012, 09:24 AM   #5
DF96 is offline DF96  England
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Put an ammeter in series and measure current. Or put a voltmeter in parallel and measure voltage.
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Old 24th February 2012, 10:29 AM   #6
MikeR is offline MikeR  United States
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If the meter is permanently installed in the chassis, I'm curious if there are any audible artifacts with placing an ammeter in series with a cathode resistor? Is it better to use a voltmeter across a resistor for safety reasons (ie. failure of the meter causing an open circuit)?

Thanks
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Old 24th February 2012, 10:33 AM   #7
DF96 is offline DF96  England
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The cathode resistor is presumably bypassed, so the meter should have no audio effect. A voltmeter in parallel will be slightly safer.

I'm puzzled: what is the point of a meter when there is nothing to adjust? Just for show? Watch valve ageing?
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Old 24th February 2012, 10:46 AM   #8
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quote:
If you want to measure voltage, you have to put a resistor in series with the 0-1 mA meter equal to 1K (minus the meter resistance) times the voltage you want to read, I.E. 10 volts, 10K. This is only suitable for measuring in low Ohms circuits as it causes significant loading and will give incorrect readings. A 50 uA meter would require 20K per volt.


If one were wanting to purchase a meter to use for various mA scales say 0-1. 1-10, 10-50-100 mA what meter would you look for?
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Old 24th February 2012, 12:23 PM   #9
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Originally Posted by Original Burnedfingers View Post

If one were wanting to purchase a meter to use for various mA scales say 0-1. 1-10, 10-50-100 mA what meter would you look for?
Buy one from a reputable company. Cheap meters aren't very accurate and have a pretty wide tolerance. Stay away from the chinese ones on Ebay.
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Old 24th February 2012, 01:02 PM   #10
tim614 is offline tim614  United States
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thanks guys!
i was just wondering. they are cool looking i have to admit.

i am building my first PP fix bias amp with a meter in place, so i figure i experiment as i go.
since i have your attention i might as well ask some more question about BIAS.

my PTX doesn't have a bias tap so i ordered a separate TX (48-0-48) for neg bias, so with this TX should i just fullwave rect (diode |<-- backward of course) for -bias?
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