Power On

About the Emission Labs tubes and how to use them.

Power On

Postby Solari » Wed Nov 26, 2014 9:12 pm


Is it possible to damage the Heaters/Filaments if HighVoltage is applied to the cold tube?
Will the Tubes KT88 and EML300B last for longer time if the HighVoltage is applied at a later time, compared to the heating Power?
thank you
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Re: Power On

Postby RodColeman » Fri Nov 28, 2014 9:20 am

Yes, I would like to confirm the official recommendation for starting an amplifier with high quality power tubes.

In amplifiers with solid-state rectifiers, I have seen DHTs with white powder loose inside the valve. Is the cathode coating, blasted away by High Voltage on the half-warm cathode surface?

I have never had white powder in my 300Bs - but I use very slow-warm-up 6CJ3 rectifiers (delay start AND slow rise-time).
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Re: Power On

Postby Admin19 » Fri Dec 19, 2014 11:07 pm

Rod Coleman is absolutely right :)

pocket-Scope-55Euro-small.jpg (47.72 KiB) Viewed 3214 times

When you find white powder in the tube, you have been blasting off cathode coating. Needless to say you have been doing something dead wrong. White powder seems to blast off mainly from rectifiers, as these are the most mistreated tubes of course. These have to digest full start up current of the whole amplifier AND the extra current to charge all the empty capacitors in less than a second. Which by itself are often choosen at or even above the rated maximum for the rectifier tube. So that really hits the rectifier tube. The amplifier tubes get hit less hard, unless you have a DC coupled stage.

However, no white powder, doesn't mean you don't need to check anything at all. White powder just means, it gets visible that way. Without white powder, mistakes can take place silently, and harm the tubes just as much. Reason being simply, most people don't BOTHER to check start up behaviour. However this is in very many cases the #1 wear out cause for your tubes. I mean, would you start your car at a cold winter day, and then drive off immediatley at full engine power, at the maximum rpm? Probably not, because you know one start like this, wears out your engine more than driving 1000km with it. Tubes are definitely things that do not like full power when they are cold. (let's better say half-warm). They hate over load in general, but they really hate that when they are cold.

Excessive start-up conditions are often present. This can only be evaluated conveniently with an oscilloscope. A pocket scope costs 55 Euro on Ebay. When checking the start up current, all you need to do, remove the decoupling capacitor that is across the cathode resistor, and check the voltage ramp up across that resistor. Many times you will see an excessive over shoot. So let's say with 80mA 300B stage, you can see it overshoot at 120mA shortly, and then it quickly stabilizes at 80mA. That two seconds wear out the tube more than the 10 hours afterwards. You do need to disconnect the capacitor, otherwise things much look better as they are, and you are fooling yourself.

Well, unfortunately I repeat this over and over, and best way people understand there is "something" to be aware of. Which does NOT mean they become active, and check their amplifiers or self made designs this way. Rather surf the internet for information, find mostly useless babble of people that do not know either, and choose to believe the most convenient explanation.

So really, my advise is invest 55 Euro if you don't have a scope yet. Make your own opinion. This will open up your eyes. And if you already have one.... well then... USE it :)

Ideal is a two channel scope, so you can observe plate voltage and plate current in one screen, but also a low cost one channel pocket scope can do this, you just need to make two separate pictures, and relate the one to the other. So when you see a plate current over shoot, while the plate voltage is not stabilized yet, you can be SURE you are doing this in cold condition, meaning you wear out the tube by this.

This is really a very simple measurement. And yes, I know.... it is regarded a waste of time for people that have a scope, and a waste of money for those who don't have one. This sounds a bit crazy, and that's probably what it is ;)

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Re: Power On

Postby J. Leven » Sat Dec 20, 2014 11:21 am

I have zero problems with white powder, or Spark
with my tubes, neither 300B, 520BV3, from 2003
and I turn a lot of in and out,
all run with diode rectifier, :shock:
the problem he will probably design of the circuit and are not diode rectifier :idea:
J. Leven
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Re: Power On

Postby zoobia » Mon Feb 02, 2015 8:37 am

Taking into consideration, that eml20am works at lower dissipation and usualy at lower current it has a higher Rp, so bass at the same plate load should be weeker and less defined.

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