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Old 14th September 2010, 11:08 AM   #1
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Default Triple Checking Calcs: metering bias with mA meter

Okay, deal is as follows...

My PP EL84 amp wants output tubes biased in the ballpark of 35mA standing current.

For measurement purposes I have a 10R in the cathodes so there should be 350mV from the cathode to ground.

I have a panel meter which reads 1mA full scale deflection and has a 200R coil.

If I want my chosen bias point to be half of FSD, then I want 0.5mA flowing with 350mV applied.

This being the case, If I put 500R in series with my meter winding so the total is 700R I should read 50% FSD at 350mV. (and 100% FSD at 700mV or 70mA)

Correct? (I've done the sums about 5 times but I still don't trust myself)

I plan on having 4 momentary switches, one from each bias resistor so the meter is usually not in the cct at all, but a quick dab of the toggle enables me to read the draw on any of the output tubes.
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Old 14th September 2010, 12:42 PM   #2
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You’re pretty close with 500Ω. The actual series resistor needed is 490Ω, because the series resistor + meter resistance appears in parallel with the 10Ω sense resistor, and thus the voltage dropped by 35mA will be slightly less than 350mV.

However, bearing in mind the tolerances on all the resistance values given, the tolerance on the meter movement sensitivity, the accuracy within which the meter can be read, and the acceptable variation around your 35mA target current, I would say go ahead with 500Ω or any convenient value close to this.
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Old 14th September 2010, 01:12 PM   #3
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Old 14th September 2010, 01:27 PM   #4
TheGimp is offline TheGimp  United States
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The other issue is that you will be placing a 700 ohm resistance meter in parallel with your 10 ohm sense resistor.

You will add another 2% error.

Use a 470R resistor plus a 100R 10 turn trimpot and adjust the trimpot for the exact reading.

Calibrate it with a good high input impedance VOM.
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Old 14th September 2010, 03:55 PM   #5
DF96 is offline DF96  England
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A 1.5% error (700 vs. 10) is irrelevant for two reasons:
1. valve quiescent current is not that critical
2. moving coil meters are only a few % accurate anyway, and they can vary across the scale

Huntingdon 361 is correct.
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Old 14th September 2010, 10:41 PM   #6
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Thanks all, nice to know that I'm not having a total brain fade...
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