Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Instruments and Amps Everything that makes music, Especially including instrument amps.

Tube failure
Tube failure
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
Thread Tools Search this Thread
Old 30th December 2017, 04:05 PM   #1
ChurritoTierno is offline ChurritoTierno  Paraguay
diyAudio Member
Join Date: Jul 2016
Location: Paraguay
Default Tube failure

Hi guys.
A friend of mine brought a Marshall guitar amp for second hand. DSL40C model.
So, he brings the amp to me house to a revision... he says that he went playing and suddenly stop sounding.
I opened the amp and I found the HT fuse blowed.
I replaced that fuse with a new one of the same specs.
Playing for a 5 minutes the amp stop working again. The same fuse blowed again.
So. First suspect the power tubes, two EL34 in push pull.
I taked out those tubes of off te amp. Placed a new fuse and turned on the amp. It worked fine. Measured volt of all pins. Bias was -36V. Plate and screen about 470V.
So I haved to test those EL34.... I put a bias metter.
I messured 29mA and 34mA respectively for each EL34.
I left the amp on for about 2 hours and nothing wrong.
So suddenly I have the idea of testing for microphonic..
I took a screwdriver whith plastic handle. And did some gently knocks on the EL34s and suddenly when I hit one of them it made orage sparks and blowed again the HT fuse.
Never I have seen this.
The spark that I saw was between the plate and glass envelope.... but this makes no sense... looking the plate it has some squared holes. . So I guess that the spark was originated between plate and another inside electhrode and what I saw was part o it that escape through the squared hole of te plate.
So my question is if this type of tube failure is the classic "shorted tube".
  Reply With Quote
Old 30th December 2017, 04:23 PM   #2
kevinkr is offline kevinkr  United States
diyAudio Moderator
kevinkr's Avatar
Join Date: Sep 2004
Location: Boston, Massachusetts
Tube failure
Relocated to Instruments & Amps per forum policy..

The sort of failure you describe is not uncommon, I have run into the same problem. In an instrument amp given the handling it receives both in transport and use it seems to be a more common failure than in home use scenarios.

Replace the EL34 with a matched set and you should be good to go.
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
  Reply With Quote
Old 30th December 2017, 05:44 PM   #3
GeorgK is offline GeorgK  Austria
diyAudio Member
Join Date: Jan 2005
Location: Vienna
Unfortunately the quality of new tubes is not that good any more...
Especially Chinese EL34s, whatever the reason is.

When I see an amp with a blown HT fuse and there is no other obvious problem I usually replace the power tubes immediately.
  Reply With Quote
Old 30th December 2017, 09:40 PM   #4
TonyTecson is offline TonyTecson  Philippines
diyAudio Moderator
TonyTecson's Avatar
Join Date: May 2003
Location: Palatiw, Pasig City
Tube failure
some users deliberately do that, some even as crazy as using both the el34 and the 6l6gc in the same guitar amp, they say they like it that way....

for hifi, that is a big no no.....
planet10 needs your help: Let's help Ruth and Dave...http://www.diyaudio.com/forums/plane...ml#post5010547[B
  Reply With Quote
Old 30th December 2017, 10:00 PM   #5
Richidoo is offline Richidoo  United States
diyAudio Member
Join Date: Oct 2005
Location: Raleigh
Same happened to me even with the very high quality New Sensor Gold Lion KT88, and brand new from very reputable tube seller, who custom tested and matched octet. When the tube shorted it took out a resistor which is there for that reason. I noticed on removing the shorted tube from the amp that it rattled when shaken. I asked the tube seller to make sure the replacement tube does not rattle. I think this has something to do with internal tube shorts. In future tube purchases I will request no rattling tubes!
  Reply With Quote


Tube failureHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
300B Tube Failure ClefChef Tubes / Valves 30 24th January 2013 10:34 PM
HeathKit Euw-15 Tube power Supply failure. harkkam Tubes / Valves 3 6th January 2011 11:51 PM
Tube rolling causes failure... Zap Tubes / Valves 10 3rd August 2008 03:04 AM
Tube Failure jwatts Tubes / Valves 11 12th February 2005 06:54 AM
Tube failure Christopher Tubes / Valves 47 30th May 2003 03:35 AM

New To Site? Need Help?

All times are GMT. The time now is 08:55 AM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.79%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio