Leaving filaments on?

john dozier · 23rd May 2011, 07:47 PM

My tube amps have separate switches for filament and B+. (shades of my AM transmitter days) Am I better off leaving the filaments on or should I turn them off (after the B+ of course) Thanks for any experiences in this area. BTW they are powerful amps using 8 EL34s each.

Ar4 · 23rd May 2011, 07:51 PM

I think, it depends of how frequently you do turn it off. If very rare, you can turn filament off, but if frequently, just leave them on.

DF96 · 23rd May 2011, 07:58 PM

Don't run valves for long periods with full heater power but no anode current, unless they are special computer versions with cathodes designed for this, otherwise they may suffer from cathode interface - a resistive layer which builds up between the cathode metal and oxide layer.

Loren42 · 23rd May 2011, 08:15 PM

Quote:

Originally Posted by john dozier

My tube amps have separate switches for filament and B+. (shades of my AM transmitter days) Am I better off leaving the filaments on or should I turn them off (after the B+ of course) Thanks for any experiences in this area. BTW they are powerful amps using 8 EL34s each.

Unnecessary use of electricity and generation of heat in my opinion.

Heat will shorten the life of electrolytic caps and I am sure the life of the tubes if you run them continuously with filaments on.

tomchr · 23rd May 2011, 08:41 PM

If you're worried about filaments that burn out on turn-on, I suggest implementing a soft start. By ramping the filament voltage up gently, you can reduce much of the thermal stress associated with power-up.

~Tom

Miles Prower · 23rd May 2011, 09:30 PM

Quote:

Originally Posted by john dozier

My tube amps have separate switches for filament and B+. (shades of my AM transmitter days) Am I better off leaving the filaments on or should I turn them off (after the B+ of course) Thanks for any experiences in this area. BTW they are powerful amps using 8 EL34s each.

Yeah, me too. Turn the heaters off. Either running the heaters with no DC rail, or turning off the heaters with the DC rail still powered up will ruin most VTs. Heaters without DC ruins emission, and DC with no heaters poisons cathode coatings.

Bad news either way.

DF96 · 23rd May 2011, 09:45 PM

Quote:

DC with no heaters poisons cathode coatings.

Unlikely, unless anode voltage very high. Cathode poisoning can occur if the valve is run with warm heaters i.e. hot enough to emit, not hot enough to form a decent space charge to protect the cathode from ion bombardment.

SY · 23rd May 2011, 09:56 PM

Quote:

Originally Posted by tomchr

If you're worried about filaments that burn out on turn-on...

FWIW, in over 40 years of building tube equipment, this has happened to me exactly zero times.

john dozier · 23rd May 2011, 10:17 PM

Thanks folks, we never left the heaters on in xmitters either. I will turn them off. Kindest regards John Dozier

tomchr · 23rd May 2011, 10:36 PM

Quote:

Originally Posted by SY

FWIW, in over 40 years of building tube equipment, this has happened to me exactly zero times.

I've never seen old gear with soft start on the filaments either...

The tubes are obviously built to handle some amount of thermal chock.

I implement soft start on my filament supplies mostly because I can.

~Tom

23rd May 2011, 07:47 PM	#1
john dozier diyAudio Member Join Date: Dec 2010 Location: columbia sc	Leaving filaments on? My tube amps have separate switches for filament and B+. (shades of my AM transmitter days) Am I better off leaving the filaments on or should I turn them off (after the B+ of course) Thanks for any experiences in this area. BTW they are powerful amps using 8 EL34s each.

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23rd May 2011, 07:51 PM	#2
Ar4 diyAudio Member Join Date: Apr 2010 Location: Latvia	I think, it depends of how frequently you do turn it off. If very rare, you can turn filament off, but if frequently, just leave them on.

23rd May 2011, 07:58 PM	#3
DF96 diyAudio Member Join Date: May 2007	Don't run valves for long periods with full heater power but no anode current, unless they are special computer versions with cathodes designed for this, otherwise they may suffer from cathode interface - a resistive layer which builds up between the cathode metal and oxide layer.

23rd May 2011, 08:41 PM	#5
tomchr diyAudio Member Join Date: Feb 2009 Location: Greater Seattle Area	If you're worried about filaments that burn out on turn-on, I suggest implementing a soft start. By ramping the filament voltage up gently, you can reduce much of the thermal stress associated with power-up. ~Tom

23rd May 2011, 10:17 PM	#9
john dozier diyAudio Member Join Date: Dec 2010 Location: columbia sc	Thanks folks, we never left the heaters on in xmitters either. I will turn them off. Kindest regards John Dozier

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