Where do I measure bias voltage?

[leadfootdriver]

Do I measure the negative voltage out from my bias supply? The voltage is different if I measure it right from the grid pin because I have a good size grid stopper.

[philo43]

The only bias reading that really counts is the one taken at the grid pin of the tube.

[leadfootdriver]

THanks!

So, the cathode current method says to install a 1R resistor between the cathode and ground. Can I leave this resistor in place permanently?

[philo43]

Sure. I put them in and leave them. At 1 ohm they have a minimal effect on plate dissipation. Put one in for each tube. Use a precision type or sort through a bunch of them to get as close to 1 ohm as possible because this will affect the current calculation. Remember to compensate for the meter leads as you sort and use an accurate meter. I use 2 watt resistors but a 1 watt will work in most cases. If it happens to burn open it will simply turn the tube off.

[oldhousescott]

Yes, you can leave the 1 ohm resistors in permanently.

When measuring, be aware that the cathode current contains both the plate and screen current.

[leadfootdriver]

THanks guys. I'm going to try this out then.

[Phil_S]

I think the information is not clear.

At the grid pin, you determine the negative bias voltage. This is used to control how much plate current the tube will draw (simplified explanation).

At the cathode, the voltage across the 1 Ohm resistor is the same as the current flowing through the tube only because Ohm's law works in your favor when 1 is the denominator.

The "bias" as we generally call it, is to determine the mA drawn by the plate. Depending on the tube, I generally just estimate screen current and deduct it. This isn't something that requires such exacting tolerance.

Many amps use a 1K Ohm screen stopper. In that case, you can get the voltage drop across the stopper and use Ohm's Law to determine the screen current so you won't have to estimate. Once again, the math gets very simple when your denominator is 1000 and makes for quick conversion from volts to amps.

Ohm's Law: V=I*R. Use it well.

[philo43]

Phil is correct. All you can measure at the tube grid is the bias voltage. What you want to know is the quiessant current. The voltage is really a variable that will change with the performance characteristics of the tube. Setting the bias voltage to "spec" may not work with all specimens of the same tube.

[Structo]

I was checking out the screen current on my ODS clone yesterday.
4 x 6L6

There appears to be about a 1v drop on the 500R screen resistor.

So that would make the screen current only about 2ma.

Does that sound right?