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In Reply to: Re: not to worry.... posted by kolbay@axelero.hu on September 1, 2005 at 00:17:50:
There is no outright protection like a relay circuit or the like.The 6AS7G has what amounts to an internal fuse for each section. see
http://www.atma-sphere.com/papers/visual.html
Beyond that, the B+ fuse is set to a value that will blow if the amp is run beyond its means. Over the years this technique has been very effective.
Assuming an 8 ohm speaker your fuse will blow at 12 watts. Is that what you want?
Follow Ups:
Thank you for the information.
I was surprised of the "internal fuse" of the 6AS7G tubes.I use the roboust 7241/7242 as output tubes, with very thick internal "wire". They lost a lot of dynamic when using the external 1.2A fuse compering it to the fuse bypass case.
But I think this is not so critical in case of the 6AS7G, where the fuse's dynamic limit is already built in.
I'm intend to replace the fuse to a relay, keeping some of the speaker protection, and getting extra dynamic.
If I were you, I would consider a seperate fuse for each tube. The fuse should be 1.0 to 1.5 amps. They run a common plate so you can place the fuse link there. The tube has three cathodes and you need a resistance in series for each of them of at least 1 ohm, proababily 7-10 watts each.I think you will find that protection relays have audible effects too. My recommendation since this tube can have some rather spectacular failure modes is to set up a voltage sensing circuit on the output that operates a crowbar circuit on the power supply. This would bypass the need for the relay and would act faster too.
Ralph,
thank you for the practical advices.
What do you mean " rather spectacular failure modes " of 7241 tubes ?
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