Mullard 12AU7 tests bad when running grid leakage 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 10th April 2011, 12:57 AM   #1
sbelyo is offline sbelyo  United States
diyAudio Member
 
Join Date: May 2004
Location: East Coast
Default Mullard 12AU7 tests bad when running grid leakage test?

Hello..

I have a Mullard 12AU7 that tests good for emissions but when I run a grid leakage test one side shows questionable, and the other side shows bad.

What will this tube do when I use it?
  Reply With Quote
Old 10th April 2011, 05:48 AM   #2
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
That excessive grid current will probably shift the Q-Point to a less linear portion of the plate characteristic and cause more distortion than you'd expect. It might also lead to poofage, depending on what the plate load is, and if it can allow that much current to flow.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 10th April 2011, 01:44 PM   #3
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
May be noisy, although whether this is noticeable depends on where it is used. Grid current may cause distortion if fed from a high impedance - this is a separate effect from the distortion caused by wrong bias.

Unless this is being used in a non-critical position (e.g. cathode follower) I would bin it. Alternatively, if this valve has been sitting in someone's cupboard unused for 40 years then it may improve with use so use it, then test it again in a few weeks time.
  Reply With Quote
Old 10th April 2011, 02:59 PM   #4
diyAudio Member
 
ruffrecords's Avatar
 
Join Date: Jun 2007
Location: Holt, Norfolk
Heater cathode leakage most often leads to higher levels of hum unless of course dc heater supplies are used.

Cheers

Ian
  Reply With Quote
Old 10th April 2011, 03:59 PM   #5
BZed is offline BZed  United States
diyAudio Member
 
Join Date: Jun 2009
Its a 12au7 that tests bad. Junk it and move on. All kinds of replacements are availble so I wouldn't risk the amp when it shorts or listen to the distortion it causes.
  Reply With Quote
Old 10th April 2011, 07:06 PM   #6
sbelyo is offline sbelyo  United States
diyAudio Member
 
Join Date: May 2004
Location: East Coast
thanks... I got it on ebay with a telefunken 12au7. paid 47 for the pair. The telefunken tests fine. I'll junk it and move on
  Reply With Quote
Old 10th April 2011, 09:13 PM   #7
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Quote:
Originally Posted by ruffrecords
Heater cathode leakage
I think it was grid leakage. Usual cause is gas, but an emissive coating on the grid can do it too when the valve gets hot.
  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
FS: Two Mullard GZ32, test as new. andyjevans Swap Meet 0 9th September 2010 09:40 AM
Testing Mullard 12AU7 on Hickok 600A ethanbriz Tubes / Valves 1 31st October 2009 08:48 PM
Test tones for listening tests loninappleton Full Range 11 23rd January 2008 11:36 PM
After test, is this bad? geminni Solid State 3 2nd May 2007 01:42 PM
pair of NOS Mullard CV4003's (high end 12AU7) Glowbug Swap Meet 0 28th November 2006 12:33 AM


New To Site? Need Help?

All times are GMT. The time now is 05:34 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