Valve amp question

[Lawrence001]

I've bought a brand new integrated by Mistral. It has 4xEL34, 2xECC83 and 2xECC82. I noticed that when switching on one of the small valves glows very bright for a fraction of a second initially then goes back to normal. The other three are always normal. I thought this might be a function of the cheap Chinese valve but when I switched to NOS valves (all except the power valves) the new one does the same.

Does anyone know what the most likely cause is? If it was both of one type I would presume it's just part of the design but as it's only one of them (presumably in one channel) then it doesn't seem right. Should I be worried, at least about valve life if nothing more?

Thanks in advance for any advice.

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[bumpy]

I've found that many valve amps are a bit quirky in this way. I think its about the circuits coming to equilibrium when first switched on. Sometimes a valve will glow a bit momentarily and sometimes there will be an audible hum in the speakers for about a second. I think this is pretty normal and even occurs in some AudioNote kit.
Apparently there are ways to ameliorate these effects, but it involves additional circuitry that almost always has an inverse effect on sound quality.
Cheers Chris

[Haselsh1]

My amps have a switch on delay so that the valves only reach full capacity a minute or so after power up. Just a thought.

[Ali Tait]

This is quite normal and is nothing to worry about.

[Lawrence001]

Thanks all I'm less worried than I was, but still a little perturbed that only one of the two ECC83s does it, it makes me wonder if there is some imbalance between the pair in the circuit, although there is no audible channel imbalance.

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[Mikeandvan]

Once you've had a listen let us know what its like. I'm after an EL34 valve amp meself.

[Lawrence001]

I've been listening for a week now it's bloody lovely! I need to decide which one of my EL34 amps to sell as I want to keep them both!!

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[Slawts]

My VTL pre does that on one of the ecc82's. None of the others do that and it has no effect on sound or running.

[Lawrence001]

My VTL pre does that on one of the ecc82's. None of the others do that and it has no effect on sound or running.That's good to know. I wonder if a slight difference in resistance or whatever in one channel means the switch on surge all flows through the path of least resistance.

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[Barry]

The heaters of the EF86s in my Radford STA15 III intially glow brightly on switch-on, then immediately 'die down' to a normal glow. So nothing to worry about! :)

[Lawrence001]

Yes but that's both not one!

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[bumpy]

It's counter intuitive, but sometimes the same valves can be employed in different parts of the circuit. My 211 amp has two E182cc valves that serve different functions in the circuit.

[anthonyTD]

Is it always the valve that's in the same position ?
I've bought a brand new integrated by Mistral. It has 4xEL34, 2xECC83 and 2xECC82. I noticed that when switching on one of the small valves glows very bright for a fraction of a second initially then goes back to normal. The other three are always normal. I thought this might be a function of the cheap Chinese valve but when I switched to NOS valves (all except the power valves) the new one does the same.

Does anyone know what the most likely cause is? If it was both of one type I would presume it's just part of the design but as it's only one of them (presumably in one channel) then it doesn't seem right. Should I be worried, at least about valve life if nothing more?

Thanks in advance for any advice.

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[Lawrence001]

It's counter intuitive, but sometimes the same valves can be employed in different parts of the circuit. My 211 amp has two E182cc valves that serve different functions in the circuit.Yes I thought that, they're double triodes so rather than each valve being left or right, one valve could do one task for both L/R and the other a different one. They could just be placed left and right to look symmetric. (Rather than suggest it myself I thought I'd see if anyone else would first.)

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[Lawrence001]

Is it always the valve that's in the same position ?Yes with a sample size of two pairs of valves, one pair stock Chinese and one pair NOS. I could try another pair.

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[anthonyTD]

What i meant was; does the initial brightness always happen with every valve you put in that position ?
A...
Yes with a sample size of two pairs of valves, one pair stock Chinese and one pair NOS. I could try another pair.

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[Lawrence001]

What i meant was; does the initial brightness always happen with every valve you put in that position ?
A...I meant yes, with the 2 valves I have tried so far.

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[anthonyTD]

Ok,
If this only happens in that one position with any valve you put in there, then it is not the usual heater filament diffrences you get in diffrent types of valves that's causing the valve to come on bright, its posibly a heater circuit issue, especialy if this amp has DC heaters on the first valve, which many amps do!
I am not saying for certain that your amp does indeed have an issue, but i just find it odd that it only seems to do this in one position. A look at the schematic for this amp would be a useful start.
A...
I meant yes, with the 2 valves I have tried so far.

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[Lawrence001]

Ok,
If this only happens in that one position with any valve you put in there, then it is not the usual heater filament diffrences you get in diffrent types of valves that's causing the valve to come on bright, its posibly a heater circuit issue, especialy if this amp has DC heaters on the first valve, which many amps do!
I am not saying for certain that your amp does indeed have an issue, but i just find it odd that it only seems to do this in one position. A look at the schematic for this amp would be a useful start.
A...Thanks I'll see if I can find out more about the amp.

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[da2222]

For several valve types it's not a quirk or an anomaly but a characteristic of certain tube/ brand types- completely within the norm. In your case it depends: when you swap the tube to the other position does it glow there too?

[Lawrence001]

I had assumed that it was unlikely to be the valves given that the one in the same location did the same thing when I changed the pair to NOS. But I will make doubly sure and swap them around next time I use the amp. At the moment I'm using valve pre/SS power as my new Celestions seem to prefer that.

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[Lawrence001]

Ok,
If this only happens in that one position with any valve you put in there, then it is not the usual heater filament diffrences you get in diffrent types of valves that's causing the valve to come on bright, its posibly a heater circuit issue, especialy if this amp has DC heaters on the first valve, which many amps do!
I am not saying for certain that your amp does indeed have an issue, but i just find it odd that it only seems to do this in one position. A look at the schematic for this amp would be a useful start.
A...Ok I've changed back over to the new valve amp today and swapped the 12AX7s around before I switched on. The bright flare up at switch on followed the valve so it looks like it's not the amp at all!

My assumption that I was highly unlikely to have two completely different valves that did the same thing, and put them in the same location, has been proved to be false! (Will strictly it hasn't, I may have just been very unlucky with a tail event but that's another question!).

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[anthonyTD]

Sounds Good!
Always good to re-check things, and in this case it looks like it is indeed just the character of the valve heater.
Anyway, Good news.
A...
Ok I've changed back over to the new valve amp today and swapped the 12AX7s around before I switched on. The bright flare up at switch on followed the valve so it looks like it's not the amp at all!

My assumption that I was highly unlikely to have two completely different valves that did the same thing, and put them in the same location, has been proved to be false! (Will strictly it hasn't, I may have just been very unlucky with a tail event but that's another question!).

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[Lawrence001]

Thanks for your help sir, and everyone else.

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[Lawrence001]

I've been thinking about this some more as something bothers me intellectually. I can't believe that the variation in heater resistance/electrical characteristics in two otherwise good valves could be so different that the voltages across then would vary so much to cause the big difference in brightness on start up. So I've got a theory!

I surmise that the behaviour of transient current may not behave in the same way during a brief voltage surge than in a steady state. So the flow of current will take "the path of least resistance" rather than distribute more evenly according to the relative resistances of the heaters.

An analogy may be a sudden surge in flood waters down a river if, say, a dam burst. Suppose the river split and one part was straight ahead while the other turned off (say it went round an eyot). The momentum of the moving mass of water might take the vast majority down the straight path rather than distribute the flood water between the two paths as would normally happen.

What do people think? Does anyone care?? It might have implications for circuit design and safety/longevity so I imagine it is of interest.

[Firebottle]

I can't believe that the variation in heater resistance/electrical characteristics in two otherwise good valves could be so different that the voltages across then would vary so much to cause the big difference in brightness on start up.

I think you have a wrong assumption there, I don't believe the voltage varies across the two valves. The heater supply will be fairly stiff as it will be rated at an amp or more (particularly if shared with the power valves).

I think the phenomenon is due to thermal mass, the 'tail' of the heater wire not wound together or withing the heater/cathode sleeve will have a lower thermal mass and will heat up more quickly. Once the bulk of the heater heats up and the hot resistance increases the heater voltage is distributed evenly across the whole heater.

[Lawrence001]

I think you have a wrong assumption there, I don't believe the voltage varies across the two valves. The heater supply will be fairly stiff as it will be rated at an amp or more (particularly if shared with the power valves).

I think the phenomenon is due to thermal mass, the 'tail' of the heater wire not wound together or withing the heater/cathode sleeve will have a lower thermal mass and will heat up more quickly. Once the bulk of the heater heats up and the hot resistance increases the heater voltage is distributed evenly across the whole heater.Sorry you're right I was thinking of them in parallel so the same voltage but power dissipation varies ie. I squared R as the current flows mainly through the slightly lower resistance heater.

Still doesn't agree with your idea though.