Finally real inductance of a grid resistor... - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 20th March 2013, 04:46 AM   #1
MGH is offline MGH  United States
diyAudio Member
 
Join Date: Jul 2004
Location: Texas
Default Finally real inductance of a grid resistor...

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?
  Reply With Quote
Old 20th March 2013, 06:29 AM   #2
godfrey is offline godfrey  South Africa
diyAudio Member
 
Join Date: Nov 2009
Location: Cape Town
All your measurements show a reactance between 5 and 10 Ohms. That's negligible compared to the resistor value. I suspect all the measurements are wrong.
  Reply With Quote
Old 20th March 2013, 06:30 AM   #3
diyAudio Member
 
Join Date: Mar 2003
Location: NYC
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
  Reply With Quote
Old 20th March 2013, 08:10 AM   #4
MGH is offline MGH  United States
diyAudio Member
 
Join Date: Jul 2004
Location: 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.

Last edited by MGH; 20th March 2013 at 08:12 AM.
  Reply With Quote
Old 20th March 2013, 11:31 AM   #5
JMFahey is offline JMFahey  Argentina
diyAudio Member
 
JMFahey's Avatar
 
Join Date: Mar 2009
Location: Buenos Aires - Argentina
Ouch !!!!
9 Euro for each 2W resistor?
And what advantage is it supposed to provide?
  Reply With Quote
Old 20th March 2013, 01:03 PM   #6
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
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).
  Reply With Quote
Old 21st March 2013, 03:05 AM   #7
MGH is offline MGH  United States
diyAudio Member
 
Join Date: Jul 2004
Location: Texas
Quote:
Originally Posted by JMFahey View Post
Ouch !!!!
9 Euro for each 2W resistor?
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.
  Reply With Quote
Old 21st March 2013, 03:07 AM   #8
MGH is offline MGH  United States
diyAudio Member
 
Join Date: Jul 2004
Location: Texas
Quote:
Originally Posted by DF96 View Post
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.
  Reply With Quote
Old 21st March 2013, 03:19 AM   #9
JMFahey is offline JMFahey  Argentina
diyAudio Member
 
JMFahey's Avatar
 
Join Date: Mar 2009
Location: Buenos Aires - Argentina
Quote:
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) ?
  Reply With Quote
Old 21st March 2013, 10:18 AM   #10
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
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!
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Resistor networks, low inductance? neazoi Parts 0 5th September 2009 12:02 AM
Resistor Inductance TubeHead Johnny Multi-Way 79 6th March 2008 10:59 PM
Finally going to give my TVC/active preamp a real home... darkmoebius Tubes / Valves 4 5th January 2007 11:56 AM
inductance of SMD 2W milliohm resistor zilog Parts 2 30th August 2006 11:10 PM
screen-plate resistor and grid leak resistor metebalci Tubes / Valves 4 26th February 2004 03:18 AM


New To Site? Need Help?

All times are GMT. The time now is 03:43 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2