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Finally real inductance of a grid resistor...

MGH

Member
2004-07-05 1:56 am
Texas
I bought some well regarded NOS 2.7 kOhm Shinkoh Tantalum resistor (2 watt) to use as grid stopper for my KT88 PP tube amp.

I measured the inductance at different frequencies (mH=milliH and uH=microH):

100 Hz -- 8 mH
1000 Hz -- 0.8 mH
10 kHz -- 65 uH
100 kHz -- 13 uH

8 mH and 0.8 mH seem very high. Can the high inductance defeat the purpose of using a grid stopper and actually cause oscillations?
 
I suspect your readings are not accurate.

Just consider what it would take to actually build an 8mh inductor @ 100hz in that sized package.

As an aside, it can be argued that you actually want inductance in your grid stoppers. Old timers would take a carbon comp grid stopper and use it as a former to wrap a few turns of wire around the body to add some inductance.

dave
 

MGH

Member
2004-07-05 1:56 am
Texas
Thank you. Both of you are saying my measurements are wrong, which lead me to believe I am measuring it incorrectly. I am using the Agilent U1733C LCR meter. I put it on Auto ranging mode, and the meter indicates I'm measuring series inductance. I will contact an Agilent tech and see what I'm doing wrong.

Dave, that's a very interesting aside. Most have indicated a grid stopper should have the least amount of inductance and recommended a carbon comp resistor. But what is the benefit of having some inductance in a grid stopper? It just seems you're increasing the chance of oscillations.
 
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A grid stopper simply has to add enough resistive damping to the grid circuit to avoid oscillation. A bit of inductance does no harm, it simply shifts the frequency of the potential oscillation. There is no need to use any special or expensive resistor type for a grid stopper. Any ordinary small carbon-film or metal-film resistor will do. Anything else is just a fashion statement.

The LCR meter may be measuring magnitude and phase of the impedance. If you do the sums you will see that at low frequencies the inductive reactance is so small when compared with the resistance that a small error in measuring angle gives a huge error in inductance. Even the high frequency measurement of 13uH is probably too high, unless this is essentially a wirewound resistor (i.e. a lossy inductor).
 

MGH

Member
2004-07-05 1:56 am
Texas
Ouch !!!!
9 Euro for each 2W resistor? :eek:
And what advantage is it supposed to provide?

Yea, that's what Shinkohs sell for, but I got them from a friend at fraction of the price so it wasn't expensive for me. He has used them as grid stoppers among others (carbon comps, carbon films and other metal films), and they are his favorite grid stoppers. He says they produce a very natural clear sound.
 

MGH

Member
2004-07-05 1:56 am
Texas
A grid stopper simply has to add enough resistive damping to the grid circuit to avoid oscillation. A bit of inductance does no harm, it simply shifts the frequency of the potential oscillation. There is no need to use any special or expensive resistor type for a grid stopper. Any ordinary small carbon-film or metal-film resistor will do. Anything else is just a fashion statement.

The LCR meter may be measuring magnitude and phase of the impedance. If you do the sums you will see that at low frequencies the inductive reactance is so small when compared with the resistance that a small error in measuring angle gives a huge error in inductance. Even the high frequency measurement of 13uH is probably too high, unless this is essentially a wirewound resistor (i.e. a lossy inductor).

Thanks for the explanation. Will look into this more.
 
He says they produce a very natural clear sound.
Mmmmmhhhh, and how should that phrase be understood?

a) the sound before was clear and natural, now it still is clear and natural, so they really did nothing special.

b) the sound before was muddy and artificial, now just by using them instead of standard resistors, it somehow became clear and natural.

Is it a) or b) ?
 
If grid stoppers are necessary, but omitted, then the resultant sound is likely to be quite muddy due to distortion caused by the stage oscillating. I guess in that case adding grid stoppers could be said to lead to natural clear sound. This will, of course, not depend in the slightest on the quality or cost of the stoppers, provided they are sufficiently resistive to stop the oscillation!
 

MGH

Member
2004-07-05 1:56 am
Texas
Mmmmmhhhh, and how should that phrase be understood?

a) the sound before was clear and natural, now it still is clear and natural, so they really did nothing special.

b) the sound before was muddy and artificial, now just by using them instead of standard resistors, it somehow became clear and natural.

Is it a) or b) ?

He was comparing it to the other grid stoppers he's tried. I don't know if the other grid stoppers sounded muddy and artificial to him, but he liked the Shinkohs the best. His taste is similar to mine, so I'll give the Shinkohs a try.
 
You should take this grid stopper nonsense as a warning that your friend's opinions on audio are unlikely to be based on reality. Be aware that if he tells you that certain components sound better then to you they will sound better even though the actual voltage waveform arriving at the loudspeaker is exactly the same. Human minds work like that, even when we know it is happening.