Re: Shunt resistor in DMM and baising Marshalls


"Mark" <ml@xxxxxxxx> wrote in message
news:zQK7l.41651$hr3.40841@xxxxxxxxxxxxxxx

Ok guys, since my thread on adding screen grid resistors helped me so
much,
I've decided that some folks here really do talk about amps, not just
political bickering. :)

Anyway, I've always biased my old Marshalls using the transformer shunt
method. Now I find out that many (most) meters have too high of a shunt
resistor internally to get an accurate reading due to Marshall OT having
very low internal resistance.

One article written by LV suggested that 10 ohms in the meter was too
much.
He suggested about 1 ohm.

Both my Fluke and my Protech meters read 11 ohms when set to 40 mA range.
My Protech meter reads 1.9 ohms in the 400 mA range and consistently tells
me I've biased 6-8 mA hotter than at the 40mA range.

What to I do here? I don't want to add the 1 ohm cathode resistors to
some
of my amps, and do not have a bias probe. Is there some math I can do
accounting for 11 ohms internal resistance in the meter at the lower
range?

Thanks again,

Mark



How about using, for lack of a better term, the indirect shunt method.

Unhook the primary leads and lift the B+ center tap on your output
transformer. Use the Fluke to get an accurate reading of the resistance of
the primary. Actually, if you pull the power tubes, you only need to lift
the center tap and you can measure from each tube pin (plate) to the center
tap. (Note, this isn't perfect because, as it heats up a bit, it will
change, but heck, it's pretty darn good.) Then, reattach all the leads to
their proper places.

Fire up the amp with the tubes in it and measure the voltage drop across the
primary coil. This is measured from the CT, where it picks up the B+
supply, to the plate pin on the tube. You just got the resistance
measurement, and you'll need that here. Ohm's Law is your friend: V=I*R.
You know V and R, solve for I. I=V/R. For example, R=50 ohms and the
voltage drop is 2V. 2v / 50 Ohms = 40mA.

It's a pretty sure bet that is the current being drawn by the tube. It gets
you out of the business of measuring amps, which always gives me cause to
pause. In the end, because this is a zero signal static reading, there is
probably limited value in achieving 100% accuracy. The idea here is to get
into a reasonable range for the tubes and the amp. As you run a signal
through the amp, these parameters are constantly changing. Don't sweat
precision beyond a certain point.

Your Fluke should be able to give an accurate measure on the Ohms of the
primary winding. This is likely to be a very small number, quite possibly
in the single digits.


.

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