300B tube going dead after a few months - 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 February 2008, 02:29 PM   #1
nl393a is offline nl393a  United States
diyAudio Member
 
Join Date: Jun 2004
Location: Florida
Unhappy 300B tube going dead after a few months

The 300B tube on (one of) my Cary 300B amplifiers goes dead after a few months' listening. There's no sign of burning (tube glows red), just eventually losing sound. It usually startsn when I notice what I think is a "phasing" problem from my speakers. Then one day, when I turn on my system, one side has no sound coming out of it (always on the same amp). Before I run out of money trying to replace 300B tubes on a regular basis (and the tube is not cheap), I thought I'd post it here to see if anyone out there is having the same problem, and if someone can offer some clue as to what is wrong with the amp. Thank you.

nl393a
  Reply With Quote
Old 25th February 2008, 02:49 PM   #2
diyAudio Member
 
ashok's Avatar
 
Join Date: Jun 2002
Location: 3RS
Did you check the dc bias ( quiescent ) current on that defective channel ? Maybe it's running too 'hot ' and burned out due to that.
You have a good channel to compare by but I'm sure someone on the forum can tell you what it's been generally set at ........75mA ?
Depends on plate voltage also I guess.
__________________
AM
  Reply With Quote
Old 25th February 2008, 03:01 PM   #3
robot is offline robot  Croatia
diyAudio Member
 
robot's Avatar
 
Join Date: Oct 2005
You didn t mentioned which brand of 300b is in the case, but witihout measuring it on tube tester it s hard to tel what is the source of the problem. If you changed the tubes from one channel to other, and same tube isn t working, the tube is to blame. I had problem with sovtek 2a3 which I bought from one english distributor, one tube had about 80% lower sound from another, but tube tester (old hickok) measured that tube is OK. After I shortly rised voltages of filament and anode tube started to work well. I suppose that "shocking" of tube probably burned some obstacles on filament, and tube started with full emission. But "shocking" of tube is dangerous for its health, and I suggest to do it when you have nothing to loose...
  Reply With Quote
Old 25th February 2008, 03:57 PM   #4
nl393a is offline nl393a  United States
diyAudio Member
 
Join Date: Jun 2004
Location: Florida
Thanks to Ashok and Robot for the quick responses. I regularly tune the bias (on both monoblocks), so I am relatively positive that its not the bias.

As for brands, I have used sovteks, svetlanas, JJs, and EHs, and they all behave the same - one amp's 300B is fine, the "bad" amp's 300B tube goes dead.

I have also read in some old electronic book in how to "revive" a tube by burning its filaments with high current. I may give it a try. Thank you.
  Reply With Quote
Old 25th February 2008, 04:32 PM   #5
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Quote:
Originally posted by nl393a
Thanks to Ashok and Robot for the quick responses. I regularly tune the bias (on both monoblocks), so I am relatively positive that its not the bias.

As for brands, I have used sovteks, svetlanas, JJs, and EHs, and they all behave the same - one amp's 300B is fine, the "bad" amp's 300B tube goes dead.

I have also read in some old electronic book in how to "revive" a tube by burning its filaments with high current. I may give it a try. Thank you.
Have you talked to Cary about this? Something is obviously wrong with that amplifier.. I've had the same pair of JJ 300B in my amplifier for several years now running at 80mA and no problems!
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 27th February 2008, 05:57 PM   #6
nl393a is offline nl393a  United States
diyAudio Member
 
Join Date: Jun 2004
Location: Florida
Hi Kevin,

Yes, I have talked to someone at Cary (I think his name is also Kevin), and he could not tell me what causes it (I have my JJs running at 82 mA, and the tube in the good amp is always fine. So I think something is causing the filaments to go dead - maybe from under heating - I don't know). Thank you.

BTW, I have looked at your 26 preamp and decided to make a 2A3 SE amp with the 26 as driver tube. What do you think?

David
  Reply With Quote
Old 27th February 2008, 06:52 PM   #7
работник
diyAudio Member
 
Rod Coleman's Avatar
 
Join Date: Jul 2004
Location: Warwickshire UK
had something like this with JJ 300Bs

worked OK two years, then anode current went dead.

Looked further & found the valve came back to life if the filament voltage was taken to 4.85V or above, below that, nothing.

If you're on ac-heat, as I was then, the line-voltage variation will take you too far below that required voltage with filaments that are like this. If the Cary is dc heated, measure the dc voltage across the two big pins of the 300B. It really needs to be as near to 5.0V as can be, and not less the 4.9V.

Keep those old 300Bs, though, since I solved that problem with some DIY:

New DHT heater

The 'failed' JJ has been working again for the last four years, with no problems...

and current source heating is SO much better than ac OR dc with voltage regulation.
  Reply With Quote
Old 27th February 2008, 08:45 PM   #8
nl393a is offline nl393a  United States
diyAudio Member
 
Join Date: Jun 2004
Location: Florida
Hi Rod,

It's nice to know that someone else has had the same problem (that I'm not losing my mind!). Thank you, Rod.

I will check the filament voltage and proceed with your method (good thing I saved all those "dead" tubes). Thank you.

And, once again, my sincere thanks to Ashok, Robot and Kevin.

David
  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
Blue glow in tube. Is the tube dead or not? Pentium100 Tubes / Valves 37 22nd October 2009 06:45 PM
Is my 300B tube dead? Evenharmonics Tubes / Valves 5 25th May 2009 04:51 PM
how does one tell when a tube is truely dead? alexmoose Tubes / Valves 2 29th April 2006 01:42 AM
Tube Cad Journal dead reinhard Tubes / Valves 5 21st March 2006 12:56 AM
Dead simple tube circuit mrjcfreak Tubes / Valves 1 29th June 2004 09:31 PM


New To Site? Need Help?

All times are GMT. The time now is 12:42 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