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Nov 03 2014

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Mercury_Vapor || Silicon Diode(s)

P1120102 b

I just finished an experiment. With a complete success.

An experiment comprised of two diodes, connected in paralel.

One diode is a mercury tube, followed by a capacitor filter, with NO front-loading choke. The bank of capacitors boasts a basillion microfarads, which is a bucketful of orders of magnitude more than that lousy little capacitor that they specify in the mercury vapor rectifier tube specs.

The second diode, is a plain stupid, high voltage silicon diode, which bypasses in a paralel connection the mercury vapor diode during the first one or two minutes after power-up.

After the lapse of such a time constant, this silicon cheater diode is disconnected from the circuit by a simple relay.

By this time (i.e. after say 120 seconds):

a). The filament of the mercury vapor rectifier heats up and the mercury vapor within the tube reaches an ample (and hence safe for HV operations) level of pressure / density, and

b). The basillion microfarads worth of capacitor bank will charge up to the final, normal level of operating voltage, mostly by means of a current surge / inrush, taking place via the silicon diode. An current surge which is, actually, initially dampened out / muted / limited by a soft-start circuit, this inserted before the mains toroid transformer.

As long as the silicon diode (with about 0.7V worth of voltage drop in the forward direction) is connected in parallel with the mercury rectifier, the mercury rectifier is utterly safe.

Even if the fillament of the rectifier tube is completely cold or only just starting to heat up, or even if it is already heated up, but still with an insufficient pressure / density / amount of mercury vapor around it.

I have checked this by means of an experiment. Literally, connected the full HV voltage to the two diodes and filter.  Switched on the fillament voltage and the HV Anode voltages – both AT THE SAME TIME. With no delay.  Essentially – nothing bad happened.  The silicon diode was in charge. It took the current surge upon its chest. It routed the current surge to the filter capacitor bank and loaded up the filter capacitors to their full operating voltage (actually, by 11,3 Volts more than needed). The whole device started normal operations with no delay. Yummi.

But what was going on at the mercury rectifier side of the paralel connection?

Obviously, the mercury rectifier is perfectly capable of withstanding a high reverse voltage – even with a totally cold filament. Provided however, that the mercury droplets are where they SHOULD be, i.e. at the BOTTOM of the vertically standing rectifier tube.

On the other hand … in the Forward direction….

The mercury vapor rectifier does not reach the ionization voltage, or the ignition voltage, as the ionization voltage is about 12 full volts,  or the whereabouts, in stark contrast to the forward bias voltage of a conducting silicon diode, which is a mere 0,7V.

Therefore, cold filament, luke warm, or hot, vapor present or not quite yet, … virtually no significant forward current flows through the rectifier tube.

Well, actually, a very small bit does indeed flow, but this is good. This actually facilitates the creation of an ample space charge around the cathode. This space charge provides additional protection from any possible ion bombardment, that potentially could follow (see below).

Now, after 120 seconds have passed, we disconnect the silicon diode.

Disconnecting the silicon diode from the tandem, i.e. elimination of the bypass-paralel path results that the mercury vapor rectifier gracefully starts to perform its work. Without any “arc-back” effects. No sparks. No explosions inside the tube.

{{BTW…. such “arc-back” explosions are extremely impressive, but after a few seconds, and after a few such attempts, when you fool around with such cold tube fireworks, you are bound to inevitably disintegrate the filament and hence end the life of your tube. And yes – a very spectacular end indeed }}.

When the mercury vapor rectifier diode takes over operations, the voltage on the capacitor bank actually sags a bit, by the aforementioned 11,3 Volts, which is a result of the difference between the forward voltage drop of the mercury vapor rectifier tube ( 12 Volts )  minus the 0,7 Volts of forward voltage drop that was the case with the silicon diode. So, even at the moment when the tube is activated, initially … it has “nothing to do” because the voltage on the filter capacitors is some 11,3Volts higher “than it should be” – at least from the tubes perspective.

In essence, the mercury vapor rectifier has a slow-start period, and current gradually increases, as the charge / voltage on the filter bank is depleted by the connected load.

The disconnection of the bypass silicon diode is controlled by a relay, timed by a time constant But there is a catch here …

This time constant must be immediately reset upon the slightest, even momentary power outage. If you do not do this, the filter caps will drain out under the load of the amplifier, and then, …. the subsequent “power return” will arc back within your mercury vapor rectifier and disintegrate the cathode of the tube, as it tries to replenish the drained reservoir of the filter capacitors.

And now …

Back to the colors of the rainbow …

I have a way of achieving the color of orange-red (this would be the glow of the anodes in the power tubes),

I have a way of achieving the color of violet-blue-mercury (mercury vapor), ….

So now the question is: Which gas glows GREEN ????

Argon?
Neon?
Xylen?

Mixtures of these?

Please advise.

Cheers,

Z.

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