How about a resistor in series with fuse.
---- Paul Whatton <email@example.com> wrote:
> Hi Carl
> Carl wrote:
> > The bigger the better up to the point of no improvement.
> No disagreement with that.
> > However the purpose of the resistor is not to absorb 100 % of the
> > energy BUT to limit the current to a managable level until the primary
> > breaker trips.
> If I understand what's going on during a flashover correctly the first
> job is to hold the current down to a level that doesn't damage the tube
> or any other components in the amplifier until the breakers trip. The
> first problemm is the energy discharged from the capacitors. After the
> glitch resistor has dealt with that it needs to hang on in there until
> the breaker, or fuses, operate to protect the rectifiers and transformer.
> I guess it's a question of amp design philosophy. Do we want the glitch
> resistor to survive a flashover intact? If we don't mind it failing and
> choose, or have, to use a component that isn't rated for the job, how
> reliable is the failure mode? I don't want a glitch resistor that arcs
> or goes short circuit, so the volts the resistor can survive under
> overload are also important. Does the time needed to get the amp back on
> the air matter? It does to me in a contest. I really ought to use a
> G3SEK control board but being a cheapskate I trade off protection & cost
> against recovery time.
> > In a SB-220 for instance there is no room for a 50W or larger enamel
> > resistor and a 25W will survive. The fact that it works is that the
> > original PS diodes have survived.
> The comments I made about the size of a glitch resistor were about
> building from scratch rather than retrofitting. I'd stick by my comment
> that a 50W+ wirewound is far better than any thick-film. I have just
> the same problem of limited size with my AEA LA30 which I want to fit a
> glitch resistor in, I'd love to use a 50W WW but there isn't enough
> room. So I'll use the biggest, best, vitreous wirewound I can fit and
> pray a bit. But in home-made amps there isn't an excuse and we can
> design in the space for a decent well-rated sized wirewound.
> Guess we might all agree on the following? Use a vitreous ceramic
> wirewound. If at all possible use one that can sink the energy and HT
> volts under a flashover within its ratings. If not use the highest
> power, highest voltage, ceramic wirewound that will fit in the box.
> Thick-film resistors, even big ones, are pants in this application.
> Wirewound resistors, even big ones, are a lot cheaper than tubes or HT
> 73 Paul G4DCV
> ----- Original Message ----- From: "Paul Whatton" <firstname.lastname@example.org>
> Cc: <email@example.com>
> Sent: Monday, September 08, 2008 2:08 PM
> Subject: Re: [Amps] 3-500Z glitch resistor
> > For Vic who originally asked this question, I've seen the figure of
> > about 40A suggested as the current limit so for a 3-500Z with 3kV, 75R
> > should do. It isn't the normal power rating of the glitch resistor that
> > is the problem but as David points out, it's the resistor's ability to
> > survive the energy pulse from the power supply during flashover.
> > In my 2m amplifier I use a modest 2kV supply with about 50uF. When I
> > built the amp I used a 47R 250W thick-film as the glitch resistor (5
> > times bigger than the 50W RCH series). It "felt" and certainly looked
> > like it should be more than big enough. But it failed, fortunately open
> > circuit, on a flashover. Hindsight is a wonderful thing and checking the
> > datasheet I'd exceeded the overload ratings. BTW such a thick-film
> > resistor costs about GBP45 new from RS in the UK, that's an expensive
> > fuse. Fortunately I'd only paid a couple of quid at a hamfest.
> > IMHO a 50W+ ceramic wirewound is a much better and cheaper bet than the
> > biggest thick-film in this application.
> > 73 Paul G4DCV
> > David G4FTC wrote:
> >>>> Everyone seems to agree that a glitch resistor between the plate choke
> >>>> and the HV supply is a good idea, to limit huge current surges due to
> >>>> arcs caused by gas, etc. But recommendations for the value seem to
> >>>> vary
> >>>> all over the map.
> >>> I like the look of Vishay RCH series or similar (Tyco do an
> >>> equivalent). The 50W version is rated at 5.5W without heatsink and
> >>> is specified to handle 2500Vrms (that's 3500V peak) short term
> >>> overload. If you want to bolt it to a panel for heatsinking, the
> >>> insulation is rated for 3500Vrms (5kV peak).
> >> Checking page 4 of the datasheet for the RCH50
> >> http://www.vishay.com/docs/50006/rch.pdf
> >> shows a maximum overload capability of about 50 Joules.
> >> A capacitor bank for a linear of 50uF charged to 3kV will deliver
> >> about 225 Joules and
> >> under glitch conditions the resistor will be absorbing most of this
> >> power, more if
> >> the power supply doesn't have glitch detection/protection and remains
> >> connected
> >> to the mains.
> >> In order words to be safe you'll need about five RCH50 resistors in
> >> series.
> >> I don't know what energy the tube can absorb under flash over
> >> conditions without
> >> damage and this will also influence the selection of the resistor.
> >> Regards
> >> David G4FTC
> >> __
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