1 ohm resistor to measure plate current w cathode bias

[studioit]

Is there any reason this won't work? Assuming it's a 5-10 watt resistor.
The amp I've just finished building is basically a 5e3 but I've added a few things here and there - 1 being adj bias. 5watt wirewound 250 ohm pot in series with a 120 ohm 5 watt wrwnd resistor. Is there any reason why I can't mount a 1 ohm 10 watt resistor between 2 test points next to the pot in series
and measure milivolts as if it were a fixed bias amp?
Thanks
Ian

[Bruce / Mission Amps]

Fine but it doesn't need to be anything other then a 1/2 watt resistor.

[Enzo]

Or measure the voltage across the 150 ohm resistor and use a pocket calculator to divide 150 into the voltage.

[studioit]

Or measure the voltage across the 150 ohm resistor and use a pocket calculator to divide 150 into the voltage.

Right - but for the purpose of experimentation it would be alot easier to have testpoints and measure in real-time. 1/2 watt, right, I was confused about how to calculate that but now I get it.

Thanks guys.

Ian

[Bruce / Mission Amps]

If you decide to use 1 ohm... remember you are compounding the error factor by quite a bit.
Why?
Well, if your DVMM is not a very high quality one, it will have a hard time reading such low resistances as 1 ohm accurately and if you use a resistor with a 5% error factor... you have to add that too. Pick the most accurate one you can.
Also, your test leads in the DVMM will be a bigger part of the 1 ohm resistance you are measuring.
If you just can't do what Enzo suggested, (the obvious choice) I'd suggest you use a 10 ohm, 1 watt 1% precision resistor and mentally drop a zero off your mili-vdc reading.

[studioit]

Bruce, my meter is an old fluke 77 which is pretty acurate but I see what your saying. At 1 ohm it could be off as much as 20 % as the specs say +or-
.5% and 2 dgts of the last significant dgt. which would be 10ths or .2. or 20% of 1. So at 10 ohms the meter error would be much less relevant. I may not bother and just do it the way Enzo suggested as that seems pretty simple.
Thanks to both of you.
Ian

[Enzo]

If you use my way, measure the resistor so you know its actual resistance, Then calculate the voltage you want to see ahead of time from the target current, then just dial that number in with the pot.

[R ski]

The wire wound components sounds a little like an over kill for adjustable bias really. Most designs, the power rating isn't a factor. Series metering resistors are good for easy determination of power tube current flow. 1 ohm @ 25 mA yields 25 mV.

The problem is that the ripple is quite high in typical tube amps. Enzo mentioned
why the cancellation happens, in that configuration, some time ago.

Why complicate issues. Measure the out put tranny from center tap to each plate lead pin 3....power off...for a while.

Ohms law I=E/R...will work the best....now the meter will be reading in the volt range.....25 MA - 35 MA is desirable.