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Tube red plating the first 2 seconds only

G

Gabster2000

New Member
I got 8 nos cv4024 (12AT7)
When I start my amp MC275 they glow bright orange the first 2 seconds I think they are red plating After that they are fine and sound great is this normal ?
Same thing happens on my tube tester Hickok 6000 on all 8 of them. They test great.
Only happens when I power them and they are cold switching the amp off then on few seconds later is fine.
Anyone had experienced this or know much about it.
 
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It's normal, I have the same experience with 12au7 tubes made by RFT. Some Mullard made in England are also reported to do this. Someone with more technical knowledge might tell you why it happens. I think it has something to do with tubes resistance on cold startup due to the fact that something is different in the filament. It has nothing to do with red platting, it's the filament that glows, not the plate.
What brand are those tubes if I may know?
 
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Yup ... you got some "flashers". I got a few like that. Some of em just do that when first powered up. I'm told it depends on the heater design and materials used? Swap in some spares and it'll probably stop, but the load is the same, and it can't hertz anything ...

That said, if ALL your tubes do the same thing, you might have a bit of a problem. Again, not something usually to lose sleep over. Solution would be to slow down the filament voltage using a thermal time delay when first powering up. If you're old skool, you can get tube versions that basically suck up the initial surge and open slowly, or go with a solid state version readily available as a kit or pre-built. Either of these solutions require some head scratching as you tinker with the equipment.

Another trick that can work (does for me anyway) is to add a thermistor in line with the VAC power. Simple enough - I usually just solder one to a two position terminal board and solder the AC lead from one side of the fuse to that, making for a tidy install. A thermistor opens slowly as they heat and give you pretty much the same effect, soaking up the surge that causes the heater glow. Bonus - they never open completely, so using one can reduce the VAC a point or two, and most classic tube equipment prefer power lower than what comes out of the wall today. DO allow plenty of clearance around a thermistor, as they get HOT in use - that extra power has to go somewhere, and it's converted to heat.
 
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Stonegarden said:
It's normal, I have the same experience with 12au7 tubes made by RFT. Some Mullard made in England are also reported to do this. Someone with more technical knowledge might tell you why it happens. I think it has something to do with tubes resistance on cold startup due to the fact that something is different in the filament. It has nothing to do with red platting, it's the filament that glows, not the plate.
What brand are those tubes if I may know?
Click to expand...
  • MULLARD but not branded CV4024 made in England you are correct thanks for the info was confirmed to me by other members also some sort of a filament preheating a characteristic of this tube
  • Thank you
 
Was confirmed by other members that this type of tube has what is called controlled heater warmup.
a design in some older tubes and it is not red plating more the filament I am guessing it gets it warm faster
I just never experienced that during my newish tube experience. Thank you all trying to solve this.
 
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Red plating usually refers to output tubes where you would actually see the plate structure turn red. This is not the glowing center which is known as the heater in all tubes but the structure closest to the glass.

Look it up on youtube and you will see some catastrophic red plating failures. :yikes:
 
century tek said:
Red plating usually refers to output tubes where you would actually see the plate structure turn red. This is not the glowing center which is known as the heater in all tubes but the structure closest to the glass.

Look it up on youtube and you will see some catastrophic red plating failures. :yikes:
Click to expand...
i agree and thanks for your comment.i think people are getting confused with the white start up flash that happens with some tubes ( mullard etc..) which is normal but not red.
 
century tek said:
Look it up on youtube and you will see some catastrophic red plating failures. :yikes:
Click to expand...

This qualify as "red plating"? Had a rectifier blow on my custom build ... oh ... the humanity! Can't tell how the amp's doing - they tell me it'll be fifty years before I can retrieve it from the crater ... ;-}

nuclear-rectifier.jpg
 
sKiZo said:
This qualify as "red plating"? Had a rectifier blow on my custom build ... oh ... the humanity! Can't tell how the amp's doing - they tell me it'll be fifty years before I can retrieve it from the crater ... ;-}

nuclear-rectifier.jpg
Click to expand...
Chernobyl lol.
 
Dave451 said:
I expect this is not true red plating, just filament flash upon power up; correct?
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Were still trying to figure that out.start up flash on mullards is usallly white not red and true redplatingviscusally on output tubes and not preamp tubes and doesnt go away unless you adjust the bias or fix whateveris causing it.
Hopefully the original poster gets back to us.
 
radioactive said:
Were still trying to figure that out.start up flash on mullards as I have been informed by a few experts is usallly white not red and true redplatingviscusally on output tubes and not preamp tubes and doesnt go away unless you adjust the bias or fix whateveris causing it.
Hopefully the original poster gets back to us.
Click to expand...
It is the filament from what I can tell seems common to some old nullards it is actually orange just a brighter filament first time I saw it I thought it the filaments are going to burn may be over voltage but I got used to it now that I know it is a normal thing for these tubes
The good thing is that the sound on these tubes is amazing I have them in 2 MC275 as monoblocks
It is fun to watch them glow strong for 2 seconds
All 8 of them glow strong for 2 seconds
I still don’t understand how they can actually Glow strong for 2 seconds then normal after what is the mechanism I wonder if it is like a blinking Christmas light the heat moves a pin of some sort
 
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It's the result of the materials used. As the filament heats up the resistance rises and the current drops down. When cold is when you get the least resistance. I.e. More current when cold. Not sure why tubes from more manufacturers don't experience this behavior, but maybe Mullard used something different in their materials that makes this behavior more prominent.
 
Some filament wire changes its resistance as it's heated. Very much a European thing. At power up the low resistance causes a current spike that results in a very bright glow. Within a second the resistance comes up, the current falls back and the glow returns to normal.
 
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