Hopefully a simple question. - 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 3rd September 2012, 04:25 PM   #1
diyAudio Member
 
Join Date: Apr 2011
Location: Belmont, MI
Default Hopefully a simple question.

This a hopefully a simple question. What damage, if any, does leaving a heater always on, on a tube such as a 12ax7, with B+ only applied when in use, do to the tube? The voltage for the heater would be DC.

I'm not thinking of using some rare Telefunken just the cheap everyday run of the mill Chinese 12ax7b. The heater supply would be a external 12volt switching supply with a old but very lightly used 6800uf computer filter capacitor, bigger than most output tubes.

This is a mod so it would be easier to keep it external. I've already done the mod and the circuit is now dead silent. The problem is wiring in the heater power supply so it turns off and on with the amp, would be be a slight pain.

Thanks James,
  Reply With Quote
Old 3rd September 2012, 04:37 PM   #2
diyAudio Member
 
Vinylsavor's Avatar
 
Join Date: Feb 2002
Location: Lindau
Hi!

Not recommended. The cathode can develop an interface layer which impacts the performance of the tube

Thomas
__________________
http://vinylsavor.blogspot.com/
  Reply With Quote
Old 3rd September 2012, 05:02 PM   #3
20to20 is offline 20to20  United States
diyAudio Member
 
20to20's Avatar
 
Join Date: Jun 2010
Location: W-S, NC
Quote:
Originally Posted by a95sonoma View Post
This a hopefully a simple question. What damage, if any, does leaving a heater always on, on a tube such as a 12ax7, with B+ only applied when in use, do to the tube? The voltage for the heater would be DC.

I'm not thinking of using some rare Telefunken just the cheap everyday run of the mill Chinese 12ax7b. The heater supply would be a external 12volt switching supply with a old but very lightly used 6800uf computer filter capacitor, bigger than most output tubes.

This is a mod so it would be easier to keep it external. I've already done the mod and the circuit is now dead silent. The problem is wiring in the heater power supply so it turns off and on with the amp, would be be a slight pain.

Thanks James,
In the last days of tube TV's the manufacturers sold "Instant On" sets that kept the filaments warm while the set was off. Not sure if the filament voltage was full or maybe 75% but it actually had the affect of reducing tube failure due to hot/cold cycles of the filament.
  Reply With Quote
Old 3rd September 2012, 05:09 PM   #4
diyAudio Member
 
Join Date: Oct 2011
Location: Tauberbischofsheim, Germany
Quote:
Originally Posted by 20to20 View Post
In the last days of tube TV's the manufacturers sold "Instant On" sets that kept the filaments warm while the set was off. Not sure if the filament voltage was full or maybe 75% but it actually had the affect of reducing tube failure due to hot/cold cycles of the filament.
Very often in the standby mode a diode was inserted, thus reducing the power consumption of the heater chain to somewhat more than 50 %.

Greez!
  Reply With Quote
Old 3rd September 2012, 07:26 PM   #5
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Yes, full heater power with no cathode current is likely to lead to an interface layer. However, this might take a few years.
  Reply With Quote
Old 3rd September 2012, 07:32 PM   #6
Merlinb is offline Merlinb  United Kingdom
diyAudio Member
 
Merlinb's Avatar
 
Join Date: May 2006
Location: Lancashire
Interface poisoning was largely cured by the 1960s, so it probably isn't a problem.
  Reply With Quote
Old 3rd September 2012, 08:55 PM   #7
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Interface was only cured for computer valves and certain other special quality types. The data sheet normally mentions this for those valves which have the cure. It is unlikely that other valves have the special nickel cathode sleeve material which avoids it. If this were the case, there would be no need for computer grade valves.
  Reply With Quote
Old 4th September 2012, 11:27 AM   #8
Merlinb is offline Merlinb  United Kingdom
diyAudio Member
 
Merlinb's Avatar
 
Join Date: May 2006
Location: Lancashire
Computer grade valves had double-super-extra-special quality cathodes, but ordinary receiving types still benefited from the nearly-as-good nickel and oxide used in the 1960s.
  Reply With Quote
Old 4th September 2012, 12:02 PM   #9
disco is offline disco  Netherlands
diyAudio Member
 
disco's Avatar
 
Join Date: Apr 2006
Location: Holland
Those computer types hardly turn up in audio designs (I know of) .. is the reason obvious?
__________________
jaap
  Reply With Quote
Old 4th September 2012, 03:10 PM   #10
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Some computer types were the same as other valves, but with a special cathode which can cope with periods of being hot but with no cathode current.

Others were specially designed, but as the aim was switching they didn't worry too much about linearity.
  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
very simple question mail2vasu2004 Power Supplies 5 25th September 2010 11:36 AM
A simple question from a simple man. inteificio Solid State 5 17th October 2005 04:00 PM
Simple C++ question annex666 Everything Else 8 31st May 2004 04:03 AM
Simple answer required for a simple question... rabbitz Multi-Way 4 31st August 2003 11:05 AM
A Simple Question OMNIFEX Multi-Way 3 19th July 2003 09:11 PM


New To Site? Need Help?

All times are GMT. The time now is 02:32 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