Push-Pull 845 Tube Amplifier

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Re: Push-Pull 845 Tube Amplifier

Postby Admin19 » Sun Feb 03, 2013 11:45 am

I was amazed about the most interesting ideas on Roger's homepage. Some of those really kept me busy puzzling :)

Please let me add this information to the discussion:

Heater Voltage:

Correct heater voltage is elementary for best lifetime. Though just a "wire" it is the hardest to make part of a tube, full of company secrets. It is where nice tube curves (low distortion) and good lifetime are coming from.

For best lifetime, there are two processes in balance, the heater is a Pyhsical-Chemical device.

A) Slow evaporation of the emissive layer.
B) New emissive material gets regenerated from within the wire, called the depot. When the depot is used up, emission will go down.

There is this one-and-only voltage where the processes A and B balance up for the maximum lifetime. This is the datasheet heater voltage, and this is why you often see no heater tolerance in the datasheet. Indeed, the closer you are to 10V for an 845, the longer it will live.

At 8V process A will be slower, which is good. However at 8V, process B will be slower too, and it slows down faster than process A. The result is, not enough regeneration, and emission will fade away, as evaporation of emissive material continues. If this happens, the tube comes in a damaging operating condition, making things worse very fast.

At 12V, Process A will be faster. Also process B will be faster, so that covers up for a while, and you will observe normal operation. It is clear that in total, things go faster now, and lifetime shortens a lot. After the internal depot is used up, the tube will fade away. Additional, heater crystallisation will go faster. So the tube may also die of a broken heater before it's time.

With AC heating, there is no such thing as zero tolerance, and the most commonly used tolerance was 5%. Still at 10.5 V or 9.5V lifetime is not as good as at 10.0V. Some tubes were specified at 10%, but most of the tubes could not take that, and were 5%. If you take a 10% tube, and go to 10% indeed, you will have a lot less lifetime.

Maximum frequency:

The maximum frequency of an 845 amplifier is mainly limited by the transformer. Though PP transformers require less intersectioning than SE transformers, and are simply easier to make, I think still for an 845 you will have a hard time getting 50kHz at low loss.

Circuit design:

For such complex and dangerous circuits as 845PP, expensive to build also, I think good simulation before actually building it, it the best way. So you need some theoretical proof of what the circuit can do or not do. It's a good idea to show it to others first.
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