[Amps] Step Start on a 3KA Amp

[Bob Maser]

Please remember that this thread started by me asking about a step start in 
a 30+ year old amplifier from Henry, the 3KA.  There are a lot of things 
that could have been done differently in this amp but it is a rock solid amp 
that will put out the legal limit and then some all day long.  Try that with 
some of the modern day Kleenex amps.

Bob W6TR
----- Original Message ----- 
From: "Ian White GM3SEK" <gm3sek at ifwtech.co.uk>
To: <amps at contesting.com>
Sent: Tuesday, September 19, 2006 2:48 PM
Subject: Re: [Amps] Step Start on a 3KA Amp


> Phil Clements wrote:
>>
>>
> Will had written:
>>> The first 1 to 5 cycles is the worst part of the surge, but from tests
>>> that was run by a ham on this mailer, the transformer didn't settle down
>>> until 30 to 40 cycles. This was captured on a storage scope and I have 
>>> the
>>> results here somewhere saved as a jpeg photo. Anyhow, a 1/2 second delay
>>> is about all that's needed on the step start. I'd say they used this
>>> heater cone over its greater power rating than would be a 25 watt or so
>>> resistor.
>>
>>The reason for the cone heater instead of a 25-50 watt resistor is that 
>>this
>>is a "manual" step-start circuit on the Henrys. It only has three parts; a
>>cone heater, a switch, and the operator's hand. As the switch is rotated
>>from off to on, it allows the mains current to flow through the cone 
>>heater,
>>step-starting the power supply. Since it is possible for the operator to
>>screw up and leave the switch in a position that the cone heater is still 
>>in
>>the circuit, it required a robust resistance in order to stand the
>>continuous duty in a situation like this. Some folks who don't read the
>>Henry manual think this is a "hi-lo" or "SSB-CW" power switch instead of a
>>step-start, and try to operate the amp in the intermediate switch 
>>position.
>>Even the robust cone heater has its limitations (300 watts-1000 watts,
>>depending on the model) and will fail open after this abuse goes on for a
>>period of time. A savvy operator, when turning on the amp, will pause in 
>>the
>>intermediate switch position until the HV meter gets up to 3/4 of its 
>>final
>>normal value. This will usually take less than one second. At any rate, 
>>when
>>the HV meter has come to a stop during turn-on, the "step-start" function 
>>is
>>complete, and the switch should be moved to the "on" position immediately 
>>to
>>prevent over-heating of the coil on the cone heater.
>>
>
> Sounds a bit primitive - no wonder it needs that big resistor! With so
> many well-known ways to make a completely automatic step-start, it seems
> a really strange decision to use a manual switch and leave the whole
> thing wide open to operator error.
>
> Replying to Will's comment also, a step-start is really only about the
> first few big cycles of inrush current. There is no point in waiting
> until the current has "settled down" to the last few percent before
> switching the surge resistor out. All it does is heat up the resistor,
> with no real benefit to the power supply components.
>
>
>
> -- 
> 73 from Ian GM3SEK
>
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