First I'm a firm believer in using a ramp up for both plate and filament
voltage and particularly in amps.
The specs will show that most indirectly heated cathodes do not *need* a
step start and from that wording they are correct...mostly. There is a
big difference between needed the soft start and benefiting from the
soft start. Tubes like the 8877 and 8874 compared to the GS35 do not
suffer from a massive inrush current at start up, or at least not much,
but to me every little bit helps. So, as you see in many amps, the
choice may have been made to save a bit of money as the tube may have a
small, low current filament and at 50 watts the filament is certainly
small compared to the thoriated tungsten ones and it does take it three
minutes to heat the cathode. Still, almost all filaments go through a
rather stressful times with the initial inrush current, resultant
heating, and dimensional changes. It may be small compared to many
tubes, but It should still increase the tube's life if eliminated. How
much and is it financially expedient? I can only answer with a fairly
firm, "I'm fairly certain" it is. It certainly makes me feel better
that I've done all I can to prolong the tubes life, particularly with my
sometimes heavy handed operation.
I'm also a firm believe in a "soft start" for the power supply and
particularly to protect the stuff (switches and fuses) that can cause
real problems when they fail. Also using a good soft start which is
preferably a ramp up rather than step up to eliminates the load on both
switches and fuses makes life easier on all of the components. This
then gives those relatively cheap switches on the mains at least a
fighting chance to do what they are supposed to. Also, even though it's
a nuisance, any time the power drops out even momentarily it should go
through the ramp up cycle again, but that's only 3 to 5 seconds and less
with a step start.
More below...
Jim Thomson wrote:
> Date: Tue, 23 Feb 2010 22:42:35 -0500
> From: "herzog@frontiernet.net@frontiernet.net"
> <herzog@frontiernet.net>
> Subject: [Amps] Step Start circuit - needed or not?
>
> We got around to discovering the why;
> so subsequent commercial transmitters had the step start added.
>
> The inrush current varies. depending upon the magnet memory of the core
> when it was shut off, and the voltage of the sine wave upon
> contactor/relay closure.
>
I'm not sure I'd phrase it quite that way but we're probably going to
end up at the same spot.
When the power comes on there are two items of concern. One is where in
the AC cycle the contacts on the switch close. When it's near that 90 or
270 degree point, the transformer usually lets out a resounding
thump...whmmmmm sound. That's a lot of inrush current. To some extent
is also depends on where the transformer was on the BH curve when it
shut down. However this "residual magnetism" *should* not amount to much
change in the inrush current in even a Kalifornia Kilowatt ...although
there are some rigs out there with big iron in them.
> A par 30 amp inrush could be 900 amps every hundredth turn-on.
> This manifested itself in several fronts:
> Fuses in the mains quit. misc-unexplained HV rectifier diode failures.
> AND MOST IMPORTANTLY: welding of a hv contactor/relay, causing
> intermittent hv to come on with vibrations closing the other side of the
> stuck-on relay; fortunately caught before someone had to die!!!!
>
>
It surprises me to see slide switches used in power circuits even if
they are spring loaded. In industry we started using contractors at
relatively low loads. For the typical ham amp that switches the plate
transformer directly, the step start isn't often *needed*, but a good
contractor switched via the front panel would probably eliminate most of
the problems. For those not familiar with "contactors" they are more of
a specialized switch that is usually spring loaded and "self quenching",
meaning they close and open *fast* and use arch chutes sometimes along
with magnetic fields to prevent arcs from becoming established. They are
more expensive and rugged than open frame relays. They also tend to be
a bit on the noisy side (relatively speaking). A soft start properly
done can completely eliminate this hard life the switch is subjected to
and can make fuses last much longer.
*Normally* I'd not expect the magnetizing and switching on of the
transformer to cause any problems down stream with caps or diodes even
when switched at the worst possible time because the transformer's iron
and windings tend to swallow these disturbances. OTOH those flimsy
little switches in the primary of the transformer, or fuses blowing open
under load and maybe even developing a plasma inside due to the voltage
developed by the arching fuse can develop tremendous voltages within the
transformer primary. I think you find these high voltages that take out
caps and diodes are due to the failure of switches and fuses, not the
timing of the on current or the magnetizing currents. HOWEVER although
it's for a different reason than expected, if a soft start were used we
could get by with those cheap switches and there should be no current
pulse to blow the fuses. Still, a fuse that lets go under load is likely
to develop some pretty high voltages.
> ## agreed. My buddy has fried a total of 4 x on/off switch's on his
> Drake L4B. It's just a 10A slide switch [rocker activated].
I assume that switch is in the primary and not subjected to 2650 VDC as
the text would indicate. <:-))
> 2650 vdc
> no load.. and has 25 uf filter cap. We added external step start,
> [25 ohm ww in one leg of 240 line] which also step starts the fil.
> ZERO problems after that.
That's a good way to kill two birds with one stone so to speak.<:-))
In industry we used those big contactors, with arc chutes. They sounded
like someone taking two boards and slamming them together with a "BANG!"
and these were on 20 ton plus transformers with SCRs in the circuit
using 3 or 4 short runs of 500 MCM locomotive cable to each SCR. OTOH
the SCRs were biased off any time the systems underwent *intentional*
switching. On top of that those contactors were BIG and a two man job to
remove.
73
Roger (K8RI)
> I fried 2 x on/off switch's on my own
> drake amps. Added step start to all of em.
>
> ## Back in 1976, I had a 4-1000 amp, with 4800 vdc no load supply,
> and 35 uf oil filter cap. NO step start, and 1N5408's. I used a P+B
> PRD-11ago series DPST 30A relay to apply the 240 vac. Also had
> 2 x 80 lb Hammond [identical] plate xfmrs, with sec's in Parallel, and
> 8 ga wire back to main panel !
>
> ## welded the contacts 25% of the time. A tiny jab with the eraser
> end of a pencil would free it easily enough. It got so bad, that after
> 6 x months there wasn't enough contact material left, for contacts to
> even make contact! Contacts were blowing themselves apart on each
> turn on. In one ssb SS contest,the B+ would not even come on. Quick
> fix was to jam some folded AL foil between contacts, turn on breaker,
> then turn on the relay ! Worked, with the usual blue flash. Relay was
> replaced, nothing left at all of the contacts, just like you ground em
> down to the armature. Then added a 2nd relay, and step start resistor.
> Step start relay was manually activated after XXX sec's. At the
> VE7ZZZ MM contest station, in desparatation, we used a breaker to
> manually shunt the step start R, that works too. At the telco I just
> retired from, everything now has 'soft start'. No more slamming on
> 6 x -52 vdc @ 800A rectifiers.
>
> later... Jim VE7RF
>
>
>
>
>
>
>
> A step-start relay with fusible resistors, instead of the huge relays
> collinbs used, saved all these BG problems, and I damn near learned too
> late about the why. Wil Herzog K2LB
> =========================RE:
> To: <HeathKit_HFAmps@yahoogroups.com>, <amps@contesting.com>
>
> It is not necessary but if it makes you feel good then do it. You will
> s also help the economy.........
>
> Many ignore the fact that the power and filament transformers or
> windings when combined, offer more than sufficient inrush protection.
> Instead they ignore those items in the calculations.
>
> The SB-1000 requirement was established years ago when the designer
> stated that inrush was not necessary and only included in later
> Ameritron amps to satisfy the Nervous Nellies that believe every thing
> that is published in ham rags by uninformed people. The SB-1000 is an
> AL-80 clone for the most part and was sold to Heath by Ameritron.
>
> Ameritron also offers parts support even today.
>
> Carl
> KM1H
>
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