Re: Shunt resistor in DMM and baising Marshalls


"Ant Moore" <nospam@xxxxxxxxxxxxxxxxxxxx> wrote in message
news:55mdnVkVrZJiGsLUnZ2dnUVZ8vOdnZ2d@xxxxxxxxxxxxxxxx
Hi Mark,



Mark wrote:

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


You could do the maths: and work out what proportion of current
is flowing through the meter and the transformer paths, and scale it
accordingly.


However: here's what I do:

Measure the DCR of each half of your OP transformer primary.
Then measure the DCV (not shunt current) across it. Use ohms law to
work out the current flowing. Note that the DCR is usually different
for each side. On one of my amps, I have DCR of 28.8 ohm and 32.5 ohm
respectively. The current is then V/R; in the order of 1.5V or so
for 50mA bias.


Caveat: both sides of the meter probes are at plate voltage (give
or take 1.5V). So clip the probes on well and keep your fingers
clear. But this is no different to using the shunt method....



Best of luck!

Cheers, and regards,


Ant. (Tamworth, UK)

--
"Make me one with everything." The Dalai Lama at a hotdog stand.

Hi Ant:
Nice to see you stopped by here. I feel obligated to offer my compliments
that you wrote essentially the same reply as I did, but you were so much
more eloquent and succinct. I must learn to measure my words better.
Regards,
Phil


.

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