On Tue, 17 Dec 2024, 19:02 jim.thom jim.thom@telus.net, <jim.thom@telus.net>
wrote:
>
> ## 300 volts P-P ripple on a 3000 vdc supply (loaded to 1 amp) = 3.5%
> ripple or -29 db,
> You will easily see 3.5% ripple on any RF monitor Scope when a dead cxr is
> applied. Dunno if folks will hear the ripple (6F or 300 hz) or not. IMO,
> 3.5% ripple is way too much.
>
What is acceptable? Of course I know "as good as you can make it", but
let's say we have a tradeoff to make. What is the most ripple we can
tolerate and still consider the amp output good?
> ## Ripple is proportional to capacitance. With 16 uf, your ripple will
> drop to just .4375%.....or 37.5 V P-P or -47db. Much, much better.
>
> ## to get around the arc suppression problem, here is what I would do. I
> install a FAST HV (1 amp rated) fuse in series with the B+, followed by a
> 50 ohm, 50 watt resistor. Under a fault condition, it's then just 3000 vdc
> / 50 ohms = 60 amps of fault current.
>
> ## 60 amps of fault current will open off the HV fuse in < 1 msec
> everytime...event over.
>
That's what I use in my DIY amp, but it replaces the original protection
with an inferior option.
You can no longer press a button to reset, you have to switch off, wait for
it to cool down, replace the fuse (maybe you have no fuses left?) and so
on. So I'd prefer to keep the original with some modification if possible.
I wonder, 2uF at 3kV can store roughly 9 joules of energy. It doesn't seem
much. At 16uF we're up to 70J range. I don't think a disc MOV exists that
could take such punishment repeatedly, but there are small GDT that are
rated at 40kA for a standard 8/20uS impulse. Calculating the energy of such
impulse at 400V we get 160J. So a GDT combined with a MOV may make the
original protection usable.
Using a 50 ohm resistor is probably prudent in any case, but then finding a
resistor that is not in reality a resistor shaped fuse is quite difficult.
In my DIY amp I only had an arc over few times. Every single time the plate
supply resistors were fried. Jim, do you also use "resistor shaped fuses"?
Or did you find one that can take a short at 3kV and not break?
73,
Łukasz - SP4IT
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