Power Tubes = Light Bulb??

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Power Tubes = Light Bulb??

Postby JMPGuitars » Thu 05/23/13 5:50 pm

Here's the situation:

When first turning on a brand new set of tubes one of the tubes will glow BRIGHT like a light bulb, then fade down to normal and then match the other tube as it warms up.

I've had 6 or 7 pairs of tubes do this, all brand new, all in different amps, and it doesn't matter which socket the bright tube goes in to, it's always the same tube no matter what socket or amp it goes in to.

This only happens with "cold" tubes. The first time it is powered the brightness happens, then not again until the tube is cooled off. If the amp is shut off and then turned back on before it is fully cooled, it will either have less of an effect or none at all.

I should also mention that the tube voltages are good, right about where they should be in all cases.

Can anybody explain this? Is it defective tubes or do they just need to be broken in?
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Re: Power Tubes = Light Bulb??

Postby JMPGuitars » Fri 05/24/13 3:15 pm

Here's an example of what I'm talking about: http://youtu.be/21G6TkukwY8" onclick="window.open(this.href);return false;
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Re: Power Tubes = Light Bulb??

Postby CurtissRobin » Fri 05/24/13 4:31 pm

I've seen that before, very long ago. i.e. The late 60s. It must not have been a problem back then because I recall that the amp went on for several years without any hint of a problem in the sound. Are you hearing anything unseemly when you play it?

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Re: Power Tubes = Light Bulb??

Postby JMPGuitars » Fri 05/24/13 7:01 pm

No, the amps sound great. It concerns me though because I'm going to be selling most of the amps I build(t), and if that's a sign that the tube isn't stable, I don't want it in any amp with my name on it.

It's not always as drastic as that video, but a few are, and a couple others were more subtle. I have one set that I'm not AS worried about, they both light up equally bright, but not so bright like that one, before it fades to normal. I put that pair in an amp I made for myself, and I'm only keeping that pair because I'm curious to see what happens.
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Re: Power Tubes = Light Bulb??

Postby tooloud » Sat 05/25/13 9:08 am

some mullards do that I have one that does the same thing... I asked the same question some years ago and got this answer..
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Re: Power Tubes = Light Bulb??

Postby Daviedawg » Mon 05/27/13 1:32 am

That happened with two new Sovtek EL84s which were the first I fitted in my 18 watt at the first power up. That is the only time I have seen it happen and as you can imagine for a moment I was seriously concerned that I had made a mess of the amp build.
Those valves are in the cupboard at the moment. But I do not believe that the flash was ever repeated after the first two times I switched on the 18watt.
I did some research to see what causes the flash. I am sure I saw some reference in this forum (among others) but never came to any conclusion other than getting "it happens" type of responses.

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Re: Power Tubes = Light Bulb??

Postby Freddy » Fri 05/31/13 10:03 pm

Tooloud is right about the Mullard tubes doing that. Mullard 12AX7s can flash pretty bright. Kind of scares you the first time you see it. I haven't seen any other tube do this other than Mullards.
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Re: Power Tubes = Light Bulb??

Postby JMPGuitars » Fri 05/31/13 11:19 pm

Freddy wrote:Tooloud is right about the Mullard tubes doing that. Mullard 12AX7s can flash pretty bright. Kind of scares you the first time you see it. I haven't seen any other tube do this other than Mullards.
5 brand new sets of JJs did this to me. Some worse than others, but you're right, it definitely scares me.
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Re: Power Tubes = Light Bulb??

Postby dotfret » Sat 07/27/13 7:16 pm

Does I has to say it again?

It's a 60s thing. Back in the 60s a TV set would take several minutes to warm up and get going, so the manufacturers talked to the valve makers and asked for fast warm-up valves, so the TV set would come on in under a minute. The makers worked on valves with a fast warm up time, in Europe the Philips group came up with the heater design that "flashes", and all the European makers copied it. In the US RCA and GE came up with a different way to do it, so US valves don't flash but European ones do, and a lot of Russki valves sulk for a while before they wake up.

No harm, it is normal behaviour for valves. You're just too young to have peered into the back of a valve TV when it was waking up ...
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Re: Power Tubes = Light Bulb??

Postby JMPGuitars » Sat 07/27/13 10:28 pm

The tubes in question are new production JJ EL84, not quite the same thing as tv tubes from the 60s. On modern tubes it's definitely not normal behavior, but I'm hoping you're right about it not being a problem.
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Re: Power Tubes = Light Bulb??

Postby dotfret » Sun 07/28/13 5:25 am

JMPGuitars wrote:The tubes in question are new production JJ EL84, not quite the same thing as tv tubes from the 60s. On modern tubes it's definitely not normal behavior, but I'm hoping you're right about it not being a problem.
On the contrary, it is "normal behavior", and it is not a problem.

Once the manufacturers had developed heaters that warmed up quickly, they used them in every valve because that was what people wanted. They didn't stop using them when TV went solid state.

TV was the reason behind the demand for a fast warm-up, but people still want the fast warm-up time whatever the valves are used in. That is why they still make heaters that way.

Nobody wants an amp where you go on stage, wait a couple of minutes, and then you can play.

If you have a really old radio, you turn it on and go make a cup of coffee while it warms up. In my workshop I have an old radio and a modern one-cup coffee machine. The coffee machine wins every time - but it can't receive short wave!
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Re: Power Tubes = Light Bulb??

Postby JMPGuitars » Sun 07/28/13 8:10 am

I hear what you're saying, but it's absolutely not normal behavior on modern tubes. I've had 3 tubes out of 100 that do that. 3 tubes in 3 different matched sets, only one tube in each set does that.

15 to 30 second warmup time is normal, but the light bulb effect is not.
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Re: Power Tubes = Light Bulb??

Postby CurtissRobin » Mon 07/29/13 12:26 am

"Nobody wants an amp where you go on stage, wait a couple of minutes, and then you can play."

That's why the manufacturers built (& build) amps with standby switches in the B+ line. In the standby position the heaters are on but B+ is not and the amp is quiet; throw the switch to the play position and you're ready to play without delay. If you have no standby switch, as in many practice amps like the old Champ and Princeton, you'll either need to turn it on and wait or turn it down let it simmer while you're not playing so you can turn it back up and go right to work. I know of no gig-worthy amps without a standby switch.

I found a mention of flashing preamp tubes on the Upscale Audio website so the corksniffers have seen the issue as well. (http://www.upscaleaudio.com/tube-faq/" onclick="window.open(this.href);return false;)
"My tubes flash at start-up. Is that normal?
Many European tubes in the 12AU7, 12AT7, and 12AX7 families may flash brightly when you first power up your gear. This flash is normal. It is also normal for the intensity of the flash to vary from tube to tube, and the flash intensity may also vary or even go away as the tubes age.
What is NOT normal is a flash from power tubes. If this happens at any time, turn off your amp immediately!"


You should probably mention it in the paperwork you give your customers. If the flash doesn't show up when you deliver the amp it may still happen when they change tubes. I'd at least go through the motions of telling them about the possibility. That way, when they call you with this particular "emergency" you can refer them to the instruction sheet they received but didn't read.

HTH
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Re: Power Tubes = Light Bulb??

Postby JMPGuitars » Mon 07/29/13 6:48 am

I'm just going to return the tubes in question after reading that. It's the power tubes flashing, so I don't need to take any risks with amps I'm selling.
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Re: Power Tubes = Light Bulb??

Postby lesgoldy » Mon 07/29/13 11:26 am

CurtissRobin wrote: I know of no gig-worthy amps without a standby switch.
Suhr Badger 18, more than gig-worthy!
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