Blown power tube - a short story - 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 25th December 2011, 09:05 AM   #1
50AE is offline 50AE  Bulgaria
diyAudio Member
 
Join Date: Jul 2011
Location: Sofia, Bulgaria
Default Blown power tube - a short story

First of All, Merry Christmas!

As a beginner with tubes, I will share my first bad experience here.

It was the 23th December. A very kind Hi-fist promised me to give one of his valve amps to listen to for some time. While I didn't feel comfortable to take it for nothing, I offered him a set of fireworks for the NYE and we were even.

I took it. It was very nice looking, though the guy warned me it hasn't been powered on for a year. Never mind, I let it warm up from the cold for about a half an hour and then I turned the heaters on.

Everything seemed ok, but after a minute, what a surprise - the heaters of the left channel started to dim. They completely turned off. WTF, I called him and I told him what happened. "Sorry about it, try to open it and see what you can do", he said.

I did take the lid off and started to analyze the circuit and then - "Bingo!", I thought. I found a ripped soldered wire connecting the rectifier for the heaters. Then I soldered it and i thought - "This is it, it has to work now"!

I assembled it, put the valves back on, turned it on and for my surprise - the heaters weren't glowing again. I stood for about ten seconds looking at them and hoping they will turn and on and then..
BANG!!
Oh God, I was scared as hell, it came from the 6S4S power tube. "I screwed the amp now", I thought. "Now I have to buy him new tubes, Merry Christmas indeed"

I turned it off and I calmed down. I opened it up again to see that the same solder was ripped off again. Then I started thinking - "Something must be heating to dissolder the joint, bust what and why? Yeah, the diodes must be heating! If they do, then there must be a short circuit somewhere!"

But it wasn't from the tubes! Their heaters were ok. I followed the heater circuit completely and I found an electrolytic cap that was grounding the heaters with resistors in series.
I did measure the cap and guess what - it was shorting.

Now I don't really know what happened to that tube, but the parts inside look like moved and asymmetrical. There is something ringing inside when shaking the valve itself. I think it's screwed. I called the guy and he said he was sorry, I promised him I will buy a new pair.

Lessons learned
-Power tubes (and maybe preamp) can be screwed from a not glowing heater
-Always check old but important electrolytics.
  Reply With Quote
Old 25th December 2011, 10:17 AM   #2
diyAudio Member
 
KatieandDad's Avatar
 
Join Date: Nov 2011
Location: UK
A valve with no heater supply simply wont emit electrons unless very high voltages are applied.

There are of course cold cathode valves but they don't appear very often in audio circles to my knowledge.

It is possible that something in the valve has physically failed causing a short between the heater and some other part at higher voltage.

A common practice was to bias the heaters at some voltage above 0V. If this circuitry has failed then the heater may become an active part of the valve.
  Reply With Quote
Old 25th December 2011, 11:34 AM   #3
male42 is offline male42  Russian Federation
diyAudio Member
 
male42's Avatar
 
Join Date: Jun 2010
Location: Kemerovo Russia
removed after a short circuit, rectifier diodes, check for short circuit or a job. then find pinout for the lamp, and verify that all stresses: heat, grid and anode. if everything is normal. lamp during measurement remove. if all the voltage of approx. you can put the lamp in place. and listen to music.
  Reply With Quote
Old 25th December 2011, 05:35 PM   #4
M Gregg is offline M Gregg  United Kingdom
diyAudio Member
 
M Gregg's Avatar
 
Join Date: Jun 2010
Location: UK
Just for interest,


If the heaters fail on a tube amp:

The HT (B+) will stay up on the PSU caps if bleeders are not fitted.

The B+ will be higher with no load and the PSU caps could blow with over voltage..

The power tubes " could" flash over if the B+ is left at a high level.

So number one isolate and test all PSU supplies before you touch anything...

Disconnect the B+ AC supply to the rectifier and insulate it.

Then get the heaters going<<<remember that heater supplies can be up at 70V above ground so take care!

Only when the heaters are working reconnect the Dc B+ and run the amp..

Never run the amp with no load you may stuff the output Tx..

All just for fun

Regards
M. Gregg
__________________
What is the sound of one hand clapping?

Last edited by M Gregg; 25th December 2011 at 05:59 PM.
  Reply With Quote
Old 25th December 2011, 07:00 PM   #5
diyAudio Member
 
Join Date: Mar 2007
Location: Canandaigua, NY USA
Shorted caps are pretty common on equipment that hasn't been powered up in a while. The oxide film thins and the cap can't support the operating voltage without either heating up and failing, or just shorting out. Thus the advice to power up slowly using a Variac if the equipment has been sitting around. The oxide film rebuilds as the voltage is increased, but by going very slowly you don't get the excess current and internal heating in the cap, and the applied voltage doesn't get ahead of the caps ability to withstand it. This is a much larger issue with HV caps than LV.
__________________
I may be barking up the wrong tree, but at least I'm barking!
  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
Back From the Dead- A NOS Tube Story VictoriaGuy Tubes / Valves 7 18th July 2011 01:56 PM
Do tube curves tell most of the story? bigwill Tubes / Valves 8 18th November 2007 05:54 PM
My Tragic story about Emission Labs tube Michaellu5250 Tubes / Valves 2 31st May 2005 01:04 PM
Short list of DIY tube amps with PCBs available... wigginjs Tubes / Valves 8 5th November 2004 12:02 PM
Long and Short Power Cables chris ma Everything Else 1 22nd September 2003 03:10 AM


New To Site? Need Help?

All times are GMT. The time now is 02:40 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