5R4 input overload

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5R4 input overload

Postby rogerk8 » Thu Apr 19, 2012 10:38 pm

I wonder what limits the data for rectifying tubes. Maximum current, I think, reflects the maximum power the anodes can withstand whithout melting or otherwise degrading. This figure seems to be in the order of 10Watts (which is reasonable with comparison to the mechanical dimensions of the anodes in tubes like 6550). The maximum peak output current seems like it must depend on the maximum thermal emission of the electrones at the cathode and is therefore not a destructive parameter as long as it is short-lived. With maximum inverse voltage I however really start to wonder. In a perfect vacuum the dielectric strength is in principal infinity (the only time it isn't infinity is when the electrical intensity, E, is in the order of 1GV/m or 1MV/mm when thunnel-emission occurs) so why this restriction? The distance between anode and cathode should in a f.i 5R4 be around 1mm which in fact means that it should withstand 1MV. Why is this not true? Is the vacuum that bad? It gets even more rediculous when you look at the maximum rms input voltage which for 5R4 is somehow limited to 750V. Why? In my opinion 5R4 should be able to withstand several thousands of volts (both forvard and reverse). Can anyone explain why I'm wrong? I want to use cossors because then there's a smooth onsetting of voltage when I turn my newly designed push-pull 845-amplifier on.
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Re: 5R4 input overload

Postby Admin19 » Fri Apr 20, 2012 2:20 pm

You damage the tube, when you exceed ANY of the maximum limits, not just the fatal one. They're all fatal if you exceed them. It gets worce when you have one or two at it's maximum, and exceed another one just a little. Each maximum has it's own explanation. Often Voltage is limited, since the very few gas molecules that are always there, will ionize once hit, and the positive ion will hit the cathode, setting of remarkable local energy since these are often attracted by local spots. From here some white layering will be blasted away. The impact energy must be limited in order to prevent cathode damage. This limits the voltage, the peak current, and the output power.

For rectifiers, users may exceed the maximum current density at the cathode surface. Since this value can not be specified by itself to the end user, it needs to be broken down to the other "unpopular" limits. Since it is at pulsed operation. So you get presented limits such as peak current, maximum first capacitor, and some others you will not like. Ignoring this, willl lead to cathode stripping, the white powder particles that you can see inside tubes that were used in bad designed equipment. Please read the EML 5U4G datasheet if you're interested, we have written a lot of information in there. Also read the "white spark" links from the EML website. (Goto the main menu of emissionlabs.com and choose "Articles"
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Re: 5R4 input overload

Postby rogerk8 » Mon Apr 23, 2012 12:06 am

I thank you for your reply! I have now read your recommendations regarding power supplies. Firstly, I don't agree with you regarding the necessity of well regulating a push-pull supply. I don't even agree with you of the necessity for chokes in that case. This is because when you are designing a push-pull amplifier, which is the only type I like, you actually don't need a well regulated supply. I have designed and built some 10's of Williamson amplifiers and have come to the conclusion that if you select the supply RC-links carefully you can achieve virtually no hum at output and this is partly because of the two input stages being the inversion of eachother and partly because of push-pull. As you probably know, push-pull means that no CM-voltages (read power supply ripple!) can propagate to the loudspeaker. And if ju tune the input stabilization filters correctly, you will hear no hum regardless how bad the supply ripple is! This is in the OP-world called CMRR.
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Re: 5R4 input overload

Postby rogerk8 » Mon Apr 23, 2012 12:12 am

Is there any full-wave rectifying tube that can withstand 1200V? Or do I have to use some serial connected silicon 1N4007 diodes?
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Re: 5R4 input overload

Postby J. Leven » Sat Sep 20, 2014 2:15 pm

reeta wrote:Thank you for that interesting and quick reply!

:?:
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