[Amps] grid fuses

[Will Matney]

Lon,

That's exactly the point I was trying to get at. Some protection is better than none at all. If a tube arcs internally, it's gassy as an arc can't form in a vacuum. It needs to be replaced no matter what. However, if we was to have an arc between the anode and control grid, which kept happening for any amount of time, it could surely damage the B+ supply. I seen a comment in another post saying that the grid would not be a series circuit in the B+ supply if an arc occurred between the control grid and anode, but it is. The current would be drawn from ground (which the B+ supplies - is tied to), through the grid, and arc to the anode then back to the B+ supply. Since the B+ supply uses the same ground, it has a path that makes a series circuit which would allow the B+ to arc across that gap. If it wasn't in a series circuit, there shouldn't be an arc, the path has to make a complete circuit and the gas acts as the switch closing the circuit so to speak by allowing the arc. The resistor/fuse won't benefit the tube any, but may save the B+ supply. In a gassy tube, even an arc to the cathode could form, which I've never once argued it couldn't. It's gassy, take it out and throw it in file 13. I'd much rather to change a tube, than to do it and have to repair the B+ supply with it.

As far as a control grid charging up to some high positive potential, it can't connected to ground, it's at ground potential. If it's open, the most it might do would be to attract a few electrons being negative like was mentioned by Bill. After this, the triode would act as a diode. The electrons being negative, the same as the control grid can not make it positive. Electrons have a negative polarity unless something has changed that I don't know about. The definition of a charge is a substance receiving electrons and storing them which makes it more negative, not positive. The only way to make it positive would be to apply a positive potential to it like applying one to a screen grid. In either case, I don't see this happening in a tube that is in operating condition, or in one that the control grid is open from ground, both with a good vacuum.

There was also a mention of a control grid forming a cathode. How can it be a cathode unless it's heated red-white hot to where it would emit electrons? I highly doubt it ever getting much warmer than the internal temperature of the tube. The only way it could would be to short to another element allowing a current to pass through it to ground heating it up. Then it would emit electrons towards the anode. It would also have a source to draw the electrons from. Every electron that is pulled from the cathode by the anode has to be replaced with one from the negative rail since current only flows in one direction.

The original question had to do with a 3-500Z if I recall, and some tried to throw other tubes in the mix. I've tried to stick with discussing the 3-500Z triode and safety measures for it.

It's also been stated on here, and by several off the list to me that the amp was ok after a resistor blew that was placed in the grid circuit. The only reason I think the whole discussion was started was to try and dis-prove and belittle another member on this list.

Best,

Will

*********** REPLY SEPARATOR  ***********

On 7/21/06 at 4:48 PM Lon W. Cottingham wrote:

>Greetings to all,
>
>            This discussion has deteriorated  to that of electing officers 
>at a 7th grade 4H Club Meeting.  The original point has long been
>forgotten. 
>Who cares what you call it; grid protection or grid fusing.  The results
>is 
>what we should be after, not all this, mostly unrelated, speculation and 
>theory.
>
>            One thread that I have taken from this bilious discussion is 
>that some of you do not realize that most of the commercially built
>Amateur 
>triode linear amplifiers throughout the 70's, 80's, and 90's have floating 
>grids, of one sort, or another.  It is time to pull the heads out of the 
>sand and realize this.
>
>             More often than not, when the "big bang" occurs, the tube/s
>are 
>lost/shorted.  Anyone who has been repairing amps and uses "grid fusing" 
>knows that the loss of tubes is reduced by approximately 50% with grid 
>fusing when the "big bang" occurs (at least that has been my findings over 
>the past 25 to 30 years repairing 3 or 4 linears each and every month.  I
>do 
>not care why it works or what you call it, it works.  In my opinion, grid 
>protection, of some sort, should be a standard upgrade, along with anode 
>surge resistors, in any amplifier repair or restoration.  Inexpensive 1/4
>or 
>1/2 watt resistors are the most cost effective way to do this that I have 
>found.  This idea of using anode surge suppressing resistors alone, as has 
>been suggested by some of us,  to protect the tube/s is like starting to 
>brush your teeth after they all have cavities.
>
>         I would like to see a thread start on trying to salvage shorted 
>tubes.  My success ratio over the years has been about 1 in five.  Richard 
>has mentioned a technique of spinning the shorted tube at a high speed in 
>some sort of a centrifuge.  I have heard other techniques such as 
>applying 
>voltage (both AC and DC) across the short.  Some have suggested
>discharging 
>large capacitors across the short.  One Ham I know simply attaches an AC 
>line cord across the elements and plugs it into the mains.  Apparently
>this 
>works, some times.  I have always wanted to try Richard's method but
>simply 
>have not done it.  I would like to hear success rates from the various 
>methods.
>
>73 de K5JV
>
> Lon W. Cottingham
>1110 Golden Bear Ln.
>Kingwood, TX 77339
>
>281-358-4207
>281-358-4234 FAX
>281-795-1335 CELL 
>
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