[Amps] one more time - Ameritron fuse question - correction

[Will Matney <craxd1@ezwv.com>]

Dan,

I need to re-state this. I forgot you mentioning running 240 Vac at 8 
amperes or 120 vac with 12 amperes. 120 vac would give a max of 1,440 
watts input at 12 amperes being drawn. That would be somewhere around 
684-685 watts output with 60% efficiency not counting power supply and 
transformer losses to be added to it. The problem is the huge surge 
current from the transformer if it doesn't have a inrush limiter. Fast 
blows I doubt would hold if they are close to the operating current due 
to this as the surge happens over several cycles, the highest during the 
first 2-6. It takes somewhere around 23-26 cycles, I've seen on some 
tests, for the current to level out giving the transformer time to 
settle down where the normal current is drawn when it's first powered up.

Will




Dan,

Slow blow would be best in my opinion using 8 ampere fuses which in my
opinion is still a little to small. I dont think running 10 or 12 ampere
ones would hurt. When a short occurs, there's a huge amount of current
drawn and each of the three sizes would blow quickly. Matter of fact,
just the transformers surge current is whats blowing the 8 ampere fast
blows. You can figure that 8 amperes at 120 Vac is 960 watts input which
is not very much. At 50% efficiency let's say, would only be 480 watts
output, and about 576 watts at 60%, this being only the amplifier class
efficiency. There are other losses too which will add to this number
(more torwards 400 watts output for 960 watts input, or power supply and
transformer losses).

Will


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...

Dan
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