Static Gm test - 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 26th February 2009, 11:06 AM   #1
diyAudio Member
 
Join Date: Feb 2002
Location: Western Sydney
Default Static Gm test

Will the attached work as a static Gm test?
If Ip1 = Vk1/Rk1 and Ip2 = Vk2/(Rk1+Rk2),
then gm = (Ip1-Ip2)/(Vk1-Vk2)...???
Attached Images
File Type: jpg gmtest.jpg (6.4 KB, 185 views)
__________________
Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
  Reply With Quote
Old 26th February 2009, 04:05 PM   #2
diyAudio Member
 
Brian Beck's Avatar
 
Join Date: Dec 2004
Location: South Florida, USA
Yes, this should give you gm. I would suggest making Rk2 a small percentage of Rk1 in your schematic. Set up Rk1 to give you the desired operating point for the tube - the point where you want to know its gm. That way the change in Vgk and Ip is a small percentage. Gm is a small signal (linear assumption) parameter, dIp/dVgk. With a typical DVM, accuracy is good enough to extract gm to a high degree of accuracy with very small changes in Vgk and Ip.
__________________
Brian
  Reply With Quote
Old 26th February 2009, 08:35 PM   #3
diyAudio Member
 
Join Date: Feb 2002
Location: Western Sydney
Thanks Brian, good to know I'm on the right track.

(I have 30 or 40 12ax7s (mostly Philips) I pulled out of an old church organ over 30 years ago, so I just want a simple way to test them)

Cheers,
Pete McK
__________________
Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
  Reply With Quote
Old 26th February 2009, 10:12 PM   #4
diyAudio Member
 
Brian Beck's Avatar
 
Join Date: Dec 2004
Location: South Florida, USA
To be even more precise, you'd really want to hold Vpk constant when measuring gm. In your case Vpk moves by the same amount as Vgk, but plate current changes due to Vpk are mu times less sensitive than plate current changes due to Vgk. For medium to high mu tubes, this technique will do well enough. For low mu tubes, you'll run into mu/(mu+1) errors. A better technique, perhaps not quite as simple, is to ground the cathode via a small sensing resistor, say 1 to 10 ohms, and set a HV power supply at the desired Vpk on the plate. Apply an adjustable low-voltage negative supply on the grid. You can step the grid voltage and monitor plate current changes on a DVM connected across the small current sensing resistor. You can also vary the plate voltages and recreate the entire set of tube curves! Just make sure you don't exceed rated tube voltage, current and power values.

By the way, there's nothing wrong with these "static" tests. The results will apply at virtually all audio frequencies. Tube curves were made this way, after all.
__________________
Brian
  Reply With Quote
Old 27th February 2009, 04:03 AM   #5
diyAudio Member
 
Join Date: Feb 2002
Location: Western Sydney
Thanks for the info Brian, although I don't really want to be that precise (but maybe one day... & I'm sure someone else will find it useful. It is useful in helping me re-learn stuff I'd forgotten from decades ago too).

I'm thinking guitar amp, so I'll probably use V+ 150-200V, Rp = 100K, Rg = 1M, Rk1 = 1K5, and maybe make Rk2 a low value preset, so I can play with it a bit...
cheers,
Pete McK
__________________
Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
  Reply With Quote
Old 27th February 2009, 02:31 PM   #6
diyAudio Member
 
Brian Beck's Avatar
 
Join Date: Dec 2004
Location: South Florida, USA
Oops, I see now that you show a plate resistor. I would eliminate it and replace it with a supply. Sorry, my arguments even for your approach with resistor switching in the cathode assumed no plate resistor. I just didn't pay enough attention to your first image. The plate will move down enough to significantly change Vpk and upset your reading. Vpk must be held nearly constant during gm tests. You may get by with your technique for doing a relative check of the health of your tubes. If you observe how a known-good tube checks, you can estimate whether the others are good or bad even if you can't arrive at a good gm number.
__________________
Brian
  Reply With Quote
Old 2nd March 2009, 07:05 PM   #7
diyAudio Member
 
Join Date: Feb 2002
Location: Western Sydney
OK Brian, a fixed supply should be easy enough, but then I suppose the issue becomes how to limit the current through the valve?
or alternatively, an adjustable supply with the Vp being made the same for both readings ?
__________________
Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
  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
Cracking/Static in Pre-Amp TheVille Solid State 2 17th April 2009 07:42 PM
static removal from lp dhole Analogue Source 3 8th April 2005 08:22 PM
Speaker Static Megahurt Car Audio 4 25th March 2005 07:40 PM
Crackling/Static on my GC. azira Chip Amps 2 25th May 2004 09:48 PM
static offroadbum Solid State 1 4th February 2004 12:50 AM


New To Site? Need Help?

All times are GMT. The time now is 01:02 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