Tubes flare on start up!!

Gobble · 19th July 2010, 02:28 PM

I recently obtained some 12AU7s for my preamp. Some Mullards from Blackburn and Funkwerks. The funny part is the FunkWerks tubes when the preamp is switched on - they flare up like a lit match for a second at two points on the base, before settling down.

Is something wrong with these tubes? Like slight leak in vacuum or something?

They sound superb though! Very musical with great top-end!!

Thanks for any advice.

Bone · 19th July 2010, 02:37 PM

These are the heaters. There is often a flare as their cold resistance is lower than when at running temperature so they will draw more current. This will reduce as they heat up. Nothing to worry about.

kevinkr · 19th July 2010, 04:11 PM

Quote:

Originally Posted by Bone

These are the heaters. There is often a flare as their cold resistance is lower than when at running temperature so they will draw more current. This will reduce as they heat up. Nothing to worry about.

Sadly I have had a couple of NOS Mullards (12AX7A/12AU7A) burn out in a relatively short time frame due to uncontrolled cold inrush current, this is a particular problem if the filament supply has a very low source impedance. Based on my experience I would recommend an NTC thermistor to limit the initial inrush current to a safe value.

Gobble · 06:55 PM

Quote:

Originally Posted by kevinkr

Sadly I have had a couple of NOS Mullards (12AX7A/12AU7A) burn out in a relatively short time frame due to uncontrolled cold inrush current, this is a particular problem if the filament supply has a very low source impedance. Based on my experience I would recommend an NTC thermistor to limit the initial inrush current to a safe value.

You scare me!

My Mullards, or BEL or Shugangs or JJ/Teslas dont flare up, only the funkwerks do.

For a few seconds actually. When I switch off and switch on immediately, it doesnt happen...

So I am thinking (or want to believe) like Bone said, "nothing to worry about" .

However perusing your line of thought, NTC thermistor - part number (edit: ok google was easy, but are there different parts/ratings for different circuits ) ? where in the circuit do I place them? I am only a solder specialist and a dud at electronics. Can I copy the circuit schematic so you can tell me where?

Anything that might potentially increase the life of my precious toobs

Cheers
Thanks.

Wavebourn · 19th July 2010, 06:59 PM

Quote:

Originally Posted by Gobble

Anything that might potentially increase the life of my precious toobs

NTC thermistor in series with primary will help.

Arnulf · 19th July 2010, 07:58 PM

Or even better, CCS heater supply.

Wavebourn · 19th July 2010, 09:32 PM

Quote:

Originally Posted by Arnulf

Or even better, CCS heater supply.

Or a voltage stabilizer with current limit.

kevinkr · 12:36 AM

Quote:

Originally Posted by Gobble

You scare me!

My Mullards, or BEL or Shugangs or JJ/Teslas dont flare up, only the funkwerks do.

For a few seconds actually. When I switch off and switch on immediately, it doesnt happen...

So I am thinking (or want to believe) like Bone said, "nothing to worry about" .

However perusing your line of thought, NTC thermistor - part number (edit: ok google was easy, but are there different parts/ratings for different circuits ) ? where in the circuit do I place them? I am only a solder specialist and a dud at electronics. Can I copy the circuit schematic so you can tell me where?

Anything that might potentially increase the life of my precious toobs

Cheers
Thanks.

How many tubes in the string, and what is the filament voltage ? Something on the order of 5 ohms cold and <0.5 ohm hot at an amp would do well for this directly in the filament circuit if it is a 6.3V filament circuit... (Particularly if the filament voltage is just a bit on the high side.)

Need a little more information, my preference is for DC heating with a CCS or current limited voltage regulator. (Limit current to about 10% greater than the required current.)

DualTriode · 20th July 2010, 02:51 AM

Hello All,
I use GE sensing CL-90’s from Digikey. These are used in series with the primary winding of the transformer.
IIRC this device has 120 ohms cold and 9 ohms when warmed up.
DT
All Just For Fun!

jjman · 20th July 2010, 11:56 PM

Is Funkwerks a manufacturer? I'm not familiar with that name. If they are not a manufacturer it is possible that the flashers are the ones that are "supposed to" flash, no?

I have a Zenith branded Blackburn Mullard 12ax7 that flashes, and it does so with pride.

19th July 2010, 02:28 PM	#1
Gobble diyAudio Member Join Date: Jan 2010	Tubes flare on start up!! I recently obtained some 12AU7s for my preamp. Some Mullards from Blackburn and Funkwerks. The funny part is the FunkWerks tubes when the preamp is switched on - they flare up like a lit match for a second at two points on the base, before settling down. Is something wrong with these tubes? Like slight leak in vacuum or something? They sound superb though! Very musical with great top-end!! Thanks for any advice.

19th July 2010, 07:58 PM	#6
Arnulf diyAudio Member Join Date: Feb 2009	Or even better, CCS heater supply. __________________ mod verb, transitive /mod/ to state that one is utterly clueless about the operation of device to be "modded" and into "fixing" things that are not broken; "My new amplifier sounds great so I want to mod it."

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19th July 2010, 02:37 PM	#2
Bone diyAudio Member Join Date: Jun 2004 Location: Dorset, UK	These are the heaters. There is often a flare as their cold resistance is lower than when at running temperature so they will draw more current. This will reduce as they heat up. Nothing to worry about.

20th July 2010, 02:51 AM	#9
DualTriode diyAudio Member Join Date: Aug 2009 Location: Sacramento	Hello All, I use GE sensing CL-90’s from Digikey. These are used in series with the primary winding of the transformer. IIRC this device has 120 ohms cold and 9 ohms when warmed up. DT All Just For Fun!

20th July 2010, 11:56 PM	#10
jjman diyAudio Member Join Date: Jan 2009	Is Funkwerks a manufacturer? I'm not familiar with that name. If they are not a manufacturer it is possible that the flashers are the ones that are "supposed to" flash, no? I have a Zenith branded Blackburn Mullard 12ax7 that flashes, and it does so with pride.

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