DIY tube tester - 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 14th March 2013, 02:22 PM   #1
diyAudio Member
 
Join Date: Feb 2013
Location: Leuven
Send a message via Skype™ to Joe Lebowski
Default DIY tube tester

Hi folks,
I am currently (trying to) design a computer controlled tube tester. My first goals is to do a bunch of tests on double triode tubes like the 12A*7 family. I only recently started with this, and so far only have my transconductance test up and running. I can now read out the values for gm for both triodes.

Now I would like to implement an emission tester, but I am having trouble understanding how this test is best performed. In the case of the 12AX7 tube, I would have to connect the grid to the plate and connect the cathode to the ground to make the tube function as a diode. That I understand. So now I am wondering: 1) which voltage should be applied to the plate+grid? 2) When I then measure the plate current, which value do I use to compare it with?

Also, as I am programming the user interface in a computer program that is able to perform frequency analysis on the measured data, I was thinking of doing some sort of test for example to find relevant disturbances within the audio range, decreased gain response at certain frequencies etc Does anyone have experience with frequency related phenomena in old/bad tubes?

Please excuse me if these are stupid questions.. I am relatively new to the vacuum tube world

Any help, advice, reference to a good book, would be very much appreciated! If you would like to know more about my project, dont hesitate to ask me!

Lebowski
  Reply With Quote
Old 14th March 2013, 04:15 PM   #2
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
To test emission you compare anode current with full heater voltage against anode current with slightly reduced heater voltage.

Variations of response within the audio range are extremely unlikely, unless there is severe microphonics. Almost all of what people say about various valves affecting frequency response is either imagination, attributing to frequency what is actually due to distortion, or (in guitar amps) relates to 'fixed tone controls' created by huge grid stoppers and the Miller effect. Valves usually work fine from DC to about 20MHz - above that issues like grid current and transit time begin to have an effect.
  Reply With Quote
Old 14th March 2013, 04:19 PM   #3
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Might want to have a look at the u-tracer tube curve tracer here, it's very well thought out and can probably provide you with some pointers for your own project: The uTracer, a miniature Tube Curve Tracer / Tester.

I own one..
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 14th March 2013, 04:59 PM   #4
diyAudio Member
 
Join Date: Feb 2013
Location: Leuven
Send a message via Skype™ to Joe Lebowski
Thanks for your replies!

@ DF96: Do you know of a book/web page where this method is described in more detail, or can you tell me more about this? I can see that there will be a difference in anode current, but how do they give me a 'emission value' and how should this value be interpreted?

I now found a small table with values for voltages to be applied on the grid and anode, and a current value to compare your own current measurement with. For example for the 12AX7 they say anode voltage should be 25V, and the anode current value they provide is 50mA. They also say this measurement can only take about 3 second maximum (to not destroy the cathode). Is this a method that you are familiar with?

@kevinkr: I checked that page out! Found myself a nice read for this evening! Thanks a lot for that!
  Reply With Quote
Old 14th March 2013, 05:05 PM   #5
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
The emission test is really of limited usefulness, IMHO, and I would certainly not run a 12AX7 at 25V, doubt you could get 50mA through one under any circumstances without destroying it.

It may be useful to remeasure the transconductance at lower filament voltages as an indication of life remaining. You would make a determination as to whether the tube had useful life remaining based on whether or not the transconductance met reasonable minimums for the application intended. All very arbitrary really..
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 14th March 2013, 05:32 PM   #6
diyAudio Member
 
Join Date: Feb 2013
Location: Leuven
Send a message via Skype™ to Joe Lebowski
I find it extremely hard to 'set' these minimum gm values. Is there any sort of convention about at what percentage of nominal transconductance value this minimum should be set? Also, I did this lowered heater voltage test on a bunch of old, intensively used and new tubes, and the result was a very linear curve for all of them. (I recorded gm for 8 heater voltages between 4.8V and 6.9V) So no sudden drops in gm with lowered heater voltage. Was this a coincidence? Should tubes that are at the end of their lifetime show a 'sudden' drop in gm?
  Reply With Quote
Old 14th March 2013, 08:30 PM   #7
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
I don't have a reference, sorry.

A smooth decline in gm or current with heater voltage may indicate a good valve. A low emission valve is likely to show a bigger drop at some point.

In valve testing there is no hard limit for anything. It should be viewed as an initial test to see whether it is worth trying the valve in a particular circuit.
  Reply With Quote
Old 14th March 2013, 08:44 PM   #8
diyAudio Member
 
Join Date: Nov 2008
Location: Oakmont PA
http://frank.yueksel.org/other/RCA/R...ving-Tubes.pdf
  Reply With Quote
Old 31st March 2013, 05:52 PM   #9
diyAudio Member
 
Join Date: Feb 2013
Location: Leuven
Send a message via Skype™ to Joe Lebowski
@Simon: thank you for the link, it was extremely helpful!
  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
DIY Sofia tube tester? artm Tubes / Valves 4 3rd August 2011 03:23 AM
WTB: Tube IMP or VTV Tube Tester or? bmenary Swap Meet 8 24th July 2011 10:39 AM
Help finding a tube tester for Stereo tube amps. Fasterdamnit Tubes / Valves 8 19th July 2010 01:43 AM
DIY My tube tester sjh327 Tubes / Valves 0 30th November 2008 02:25 PM
DIY Tube Tester for 6336s poynton Tubes / Valves 1 16th August 2007 04:58 PM


New To Site? Need Help?

All times are GMT. The time now is 08:06 PM.


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