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RE: tube arcing, fuse ratings and some tips.

Posted by Ralph (M) on December 26, 2011 at 09:58:12

In Reply to: But why? posted by Lew on December 24, 2011 at 17:53:19:

So far in 35 years the only source of arcing in tubes has been due to the tubes themselves.

The way the setup works is that any of our amps (since the beginning of the MkII era in 1996) have three fuses by the power cord. The first one is for the B+ for the power tubes and can blow if a tube arcs. The value is chosen so that if the output at the time of the arc exceeds the amp's normal power output, the fuse will blow.

The other two fuses are for the power tube filaments (2nd fuse) and the driver circuit power supply (3rd fuse). These rarely get blown, in fact we know of no cases where the 2nd fuse has ever blown.

This approach is very simple and proven effective to protect both the speaker and the amp in the event of a tube shorting.

The value of the 1st fuse is listed in each manual, but is

5A slow for S-30 (3AG style)
3A slow for M-60 "
5A slow for MA-1 "
8A slow for MA-2 (FNM style, the 'bottom' fuse)

Once a fuse has blown, you have to see if the guilty tube is still up to its tricks. I usually just install the fuse, warm up the amps and then turn on the B+ (red light). Sometimes the tube will misbehave right away, sometimes it won't. I knock on the tops of the tubes with my knuckles, something that any technician working with tubes learns to do fairly early on. Usually the tube will react and this way lets me know which one it is.

If I don't have a replacement on hand I simply operate the amp with a reduced complement by pulling a tube from the opposite tube bank as well as the defective one. This will get you by if the tube arcs at the most likely time, 5 minutes to 5:PM on a Friday :) However back in the old days it was a fairly common practice to have a few spares on hand for just such a situation, and its still a good idea today.

This post is made possible by the generous support of people like you and our sponsors:

Analog Engineering Associates

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