Re: Shunt resistor in DMM and baising Marshalls

Thanks Phil,

I should probably start my posts with a disclaimer, something like:
"I am merely a hobbyist, and not a real tech or engineer, therefore I reserve the right to ask questions that I cannot possibly understand the answers to."

I *think* I get it. I'd rather not solder if possible (not that I suck at it). Could I pull the tubes and HT fuse and measure from the plate lead to the fuse?
Or, another thought, how close to in the ballpark could I be using the 400mA setting on my Protech meter if its internal resistance is 1.9 ohms?

Thanks again,
Mark


"Phil S." <psymonds@xxxxxxxxxxx> wrote in message news:gjo0od$plb$1@xxxxxxxxxxxxxxxxxxxxxx

"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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