[Amps] one more time - Ameritron fuse question

[Ian White, G3SEK]

Dan wrote:
>Well you guys are batting 100% so far i now have one more Ameritron 
>811H question.  I have replaced the grid resitor and replaced all four 
>tubes (one was shorted causing the resitor to blow). All is well works 
>great never better...switched to 240 from 120 and even better. Great 
>output and amp seems happy sounds and works very well.....
>
>now the problem. To keep the fuses to spec Ameritron states a 8 amp 
>fuse on both sides at 240 volt versus 12 amp at 120 volts. It was 
>running fine with the original 12 amp ceramiic 3ag 250 volt fuses but 
>thought i better get them down to the rated 8 amp. Put in a set of C3ag 
>8 amp 250 v ceramic fast blo fuses and switched on the amp.. nothing. a 
>little jump of the needles bit nothing. checked the fuses both blown.. 
>tried again same thing. replace with 10 amp fuses all ok.... voltage on 
>the 240 voly line is near 240 so it is where it should be
>
>should I try/use slow-blow fuses at 8 amps. would that be better then 
>the 10 or 12 amp fast blo fuses?
>
>I do have a step start (slow start) device coming.. This might correct 
>the problem with fast blo at 8 amps.. BUT you guys are the experts.. 
>thanks for any and all opinions...
>
The step start will limit the peak switch-on current, and then you'll 
almost certainly find that the 8A fuses are OK.

The manufacturer has a very good reason for specifying fast-blow fuses: 
the 811H is a simple amp that doesn't have much other protection. So 
don't change to slow-blow - it will reduce the level of protection, and 
in a fault situation it may allow damage that a fast-blow fuse would 
prevent.

The beauty of the step-start is that it allows you to keep the whole amp 
well protected by a low-rated, FAST-blow mains fuse (or in your case, 
one on each side of the 240V mains).


By the way, folks, what is the correct formal description of US mains 
voltages now? Is it 110/220V, 115/230V or 120/240V? We've seen all of 
the above in various publications, but how does the *current* issue of 
your NEC describe it?


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek