On Aug 13, 2005, at 4:58 AM, Harold B. Mandel wrote:
> Gents,
>
> In building step-start units to ease the trauma of powering up
> various amplifiers a number of issues were addressed:
>
> The filter capacitors present an almost dead short when
> first being charged. (Anode Supply). In the latest case,
> there's 50uF at 5KV WVDC needing to be charged,
> and I was worried that my rectifier bank would be
> unduly stressed if I didn't limit the inrush current.
The rectifiers are typically not the weak part, it's the power
contactor's contacts (or switch in smaller amplifiers) that connects
the mains to the HV xfmr. For example, the first component to
fail-shorted in a TL-922 or a SB-220 that does not have the step-start
mod is the On/Off switch. The cause is peak current at turn on,
>
> As an example, the Alpha 77 series with dual voltage
> composite transformer uses two resistors, one on each leg,
> shorted by a 24 volt contactor, K2, after stepping. These
> resistors were protected by a 3AG type fuse in series with them.
>
> Just like the poster who talked about the chicken with no legs
> in the oven, when designing the stepper for the 3 ampere
> outboard supply mentioned above, a resistor was inserted
> in series with each leg of the new xfmr by a 40 amp relay driven
> by a Crouzet timer. Unlike the Alpha that had only one fuse
> in series, this supply has a fuse in series with both legs. My
> thinking was to reduce the total trauma, even though the
> electrical ramifications only impose one resistance.
>
> However, is it wise or necessary to limit the step-start to
> one second or less?
The wisest move is probably to let the charge on the HV filter C
determine when to pull the relay in. This is accomplished by having an
18V-rms or so winding on the anode PS xfmr that powers the DC supply
that furnishes DC to the step-start relay's 24vdc coil. By adjusting a
R in series with the relay coil, the pull-in point can be set for c.
2/3 of normal HV. Typical pull-in time is c. 1-second +/– a bit --
unless of course the filter C is housed in a railroad boxcar on a
siding next to the shack.
> Would not the oil filled caps need
> a few seconds to charge up?
>
Stepping at c. 2/3 of full charge is about right, and it won't take out
the lights.
> What damage would occur if the step-start period were too long,
The contest would be over? Visiting guests who are amplifier-savvy
might raise the eyebrows and chortle?
> considering that the stepping timer and drive were run by a full
> voltage supply and didn't chatter?
Use a DC relay and move on to the next design decision.
cheerz, Hal
>
> Lastly, when switching the output of the transformer from 2.5KV to 4KV,
> well after stepping, (going from "CW" to "SSB"), the xfmr still makes
> an
> audible thump. Is there any protection needed at that point?
Probably not. The high-hurdle is starting from zero V.
>
> Respectfully,
>
> Hal/W4HBM
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>
>
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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