G9 Cathode stripping

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caps

Well-known member
Joined
Jun 7, 2004
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Location
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Just wondering if anyoe has had any issues with cathode stripping, seeing B+ seems to be applied from the get go, on power up, and switched off straight away on power down...

As a side note, my G9 makes a loud click on power up and down, maybe realys I dont know, anyone else have this ?
 
> had any issues with cathode stripping, seeing B+ seems to be applied from the get go

This is one big difference between Receiving tubes and Transmiting/Industrial tubes: you do not have to be nice to home-radio tubes. They are MADE to be hot-started.

The trade-off is that we have to feed plenty of heater power, and can't work at very high voltage, on receiving tubes. It is obscene that a 12AX7 eats a whole Watt of heater power for a mA or two of plate current. And if we put 1,000V on a 12AX7's plate, the electron storm will tear the plate and cathode apart. But a lot of tube systems can work this way. And this way, you don't need any special training to work a tube radio. My grandmother did it.

Yes, when we get to BIG stuff, bigger even than 6L6GC or 6550 (both of which will stand hot-starting when used within rated plate voltage), then we have to cut things a little closer. You can kill a kilowatt transmitter tube in seconds if you don't know how to start it. We could over-build it to survive hot-starting, but it already costs more than the techs who watch over it; make them start it gently.

Tubes are a lot tougher than you young ones seem to think. You are really more likely to blow-up a transistor than a tube. I've killed many times more junctions than bottles. Not just because there have been more transistors in my life, but because transistors DO die rather easy. If designed right, chip or bottle, they normally don't die. If, in rare cases, they have to be designed to the edge of the abyss, that should be in the instructions. But only highly trained well-paid techs read instructions, so most gear is designed for use by total idiots (this includes those trained techs when in panic-mode).

Fender guitar amps an an important Special Case. To maximize loudness per tube, Leo ran his biggest amps far past the tube maker's ratings. The loss of life from 10,000 hours to a few hundred hours is acceptable if you are getting paid to play. Using the Standby to pre-heat the tubes may reduce the strain, though I think the real reason is so you can go on break leaving the amp dead silent, yet come back and play in an instant.
 
Dont like to bring up a old thread, but I have just been thinking...

By using a standby and pre heating the tubes prior to applying B+, dosent this give the getter a chance to react with any gases in the tube from last use and remove them?

For the sake of the question, lets think transmitting high power tubes and what not with big b+. As pointed out it dosent seem much of a issue for tubes running on lower B+.

It seems the barium compound, that is the getter ,only reacts with gases when hot.

Ok, so, under normal operating conditions, wouldnt the getter be removing any residual gases (from small vacuum leaks, and gases that emerge from the components as they heat up etc), as the tube operates?

Whats the difference between the getter working before B+ is applied, to it absorbing gases anyway when operating, as of course, the getter will continue to be hot?
 
[quote author="caps"]It seems the barium compound, that is the getter ,only reacts with gases when hot. [/quote]
If a tube breaks the getter flash turns white - so some part of the getter at least is working when the tube is cold.

Best regards,

Mikkel C. Simonsen
 
That is another difference between Receiving tubes and Transmitting toobes. Transmitting tubes use getters that only work when HOT. Indeed, if a tube has been idle a while, the process is to bring the heater up to heat, for hours (not seconds), to warm the getter and suck up stray gas. But receiving tube getters are formulated to work "cool". A disadvantage of receiving tube getters is that if they ever do get too hot, they fart out all the gas they've absorbed; however this temperature seems to be about the melting point of the glasses used in receiving tubes, so if you get that hot you have lots of trouble.
 

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