2a3 mesh or 2a3-S ?

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2a3 mesh or 2a3-S ?

Postby nnerto » Tue Oct 17, 2017 9:43 pm

HI , I am Stefano from Italy and new in the forum.
In my DIY single ended amplifier, the 2A3 work with 17,5 watt of anode dissipiation. The Ia is 60 mA and Va is around 290 volt. The bias is fixed. I would buy the EML 2A3 Mesh, but I am troubled that the anode dissipation of 17,5 watt is too strong for the mesh plate. The data sheet of the 2a3 mesh EML recommended a 17 watt maximum continum plate dissipiation. What do you recommended? :(
I prefer the mesh version because I am interested to the sound of the mesh. I never heard a mesh plate, and I hope that the sound is wonderful like I heard in the web.
The driver stage of the 2A3 is transformer coupled with a high transconductance triode that drive the 2A3 in the A2 region, until 4,5 Watt RMS .
Thank you for the advice.
nnerto
 
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Re: 2a3 mesh or 2a3-S ?

Postby Jac » Thu Oct 19, 2017 10:06 am

Hello Stefano,

first, let me tell you, this is possible to do as you want it with 2A3-Mesh. I notice you approach this with care, and that is what we need.

Please let me explain the idea behind our datasheet maximum specifications. If you use a silicon diode, it will work for ever, as long as you don't exceed maximum limits. So with silicon, just check it and then you can forget about it.

No so with electron tubes. When you use them, they will stop working some day. You have to realise it.

1) The harder you use the tubes, the shorter the lifetime.

Another thing is reliability.

With electron tubes, they get hot by default. There is no other way. But by definition, heat triggers failure mechanisms of resistors, transistors, and also in a tube. So either a tube will fail because of end of life, or otherwise by mechanical failure caused by heat.

2) The harder you use the tube, the higher the failure rate. However failure rate with 2A3-mesh begins not at 17 Watt. It begins at much higher power.

Actually 2A3-Mesh from EML has zero failure rate just because failures would begin only a a lot higher dissipation, which by itself is not allowed, in order to get good lifetime.

All in all, this is very difficult, because users do not like to think a lot about it! They want a few quick numbers, so they can forget about it. Same as with silicon.

A hot anode, gives lifetime shortening, but this does not begin at 17 Watt. The real number at which lifetime shortening begins is sure higher, but we do not write this in the data sheet. Otherwise people staying exactly 0.1% below that, and do not understand why lifetime is not at full maximum. So in the datasheet is written a value, you can simply use, and lifetime is at maximum.

So when 17 Watt (as in the data sheet) is totally safe, this does not mean 17.5 Watt is totally unsafe. There is some difference in lifetime, but only theoretical.
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