The AM Forum

One circuit I am working on has a 22 MegOhm Grid Leak Resistor.
How does one check resistance values that high ? I know I can
buy a GR Meter, but spending a fortune to test a 50c part doesn't
sit right.

Even my ZM-11/B Bridge only goes up to 10 MegOhms.

My concern stems from the fact that the resistor in question is
a 50 year old Allen-Bradley carbon.

Something to ponder on a beautiful spring day.


Tnx es 73, Dick, W1KSZ

You replace it with known good metal film resistor(s).

Or, put two more 22m known good resistors in parallel and see how close you get to 7.33m

Or, buy a cheap megger off ebay or aliexpress.  They will read a lot higher.

FWIW, I've had this problem before.  I just replaced the resistor with new metal film.

--Shane
KD6VXI

There is an odd bit of humor in that question. If you're that concerned about a 50c part, why not just spend the 50c and replace it?

Shane #1 = KISS.

I have some on order to replace it, but then this begs the question ...

How do I know the replacement is any good ??

73, Dick, W1KSZ

Buy more than one ???

If you really want to measure, put 100 Volt on it and measure the current. will be close to a good measurable 5 uA Most DVM's can measure that

https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-resistor-color-code-4-band

You enter Red, Red, Blue, and then either Gold or Silver for the tolerance, then you'll be close.

If it is one that has been in the circuit, assume it has risen in value and out of tolerance and replace it with a new one.

and another method as suggested:


In the DX-60 the 22 mega-ohm resistor on pin 7 of the first stage of the 6DE7 has a stated voltage of  -1.0 across it. That means that a current of 45.45 nanoamps or approx. 0.05 microamps should be flowing through it, depending on tube gain.

What's the measured voltage?


Phil - AC0OB

Ok, so take the 22 meg and series it with some other megohm range resistor... 1 meg would
be fine, put 100volts or ANY OTHER DC VOLTAGE across it. Now you have a divider, do the ratio and look for the
result read out in volts after you multiply by the voltage...

The DVM might have its internal resistance low enough to load it a bit, so you could take the DVM and
measure across each resistor - the idea being that in parallel with the higher resistance you might expect
more of an effect, but if the voltage is the same proportion according to the ratio, then ur ok without any more
thought. IF it varies then when across the higher 22meg, the voltage ought to rise, compared with measured across
the lower resistance, where it would be a bit lower... well, rise when seen as a voltage across the resistor, the actual
value depends on which side the voltage source is...

but if the circuit works the resistor is good? (good enough?)

Hanging a DVM or VTVM across a 22 Meg Resistor gave me pause, then I
realized the answer was right in front of me !!

I have a Keithley 196 whose input impedance at low DC levels is >1 GOhm !!.

Now to fire up the DX-60 and see what I measure.

Thanks to all that replied.


73, Dick, W1KSZ

Fwiw, you'd be hanging the VTVM or DVM across a lower resistance, like one or two megs...
a divider...

Connect the 22meg resistor in series with a 1 meg resistor.

Connect the 2 resistors in series across a 12 volt battery.

The voltage drop across the 22 meg resistor should be approximately +11.4VDC

73
Mike W5RKL
www.w5rkl.com

.

Do you have a 10 Meg DVM or an 11 Meg VTVM?  Assuming the DC impedance of said meter is on specifications, you need never worry about large resistors again.

All you need is a well regulated DC supply.  For a 22 Meg resistor, a supply in the 40 to 50 volt range may do it.

Here is what to do.  See article by McMath. Have fun!

BTW, this technique is discussed on the HP-410B manual.  A 410B has a DC impedance of 122 Megohms, so you can measure a really large resistance!

Have fun, and be careful with HV DC.

https://www.americanradiohistory.com/Archive-Radio-News/50s/Radio-News-1956-01-R.pdf