[R-390] Current Inrush

[chuck.rippel at cox.net]

Drew outlines exactly my approach; have done this on a couple Henry 
amplifiers.  Ended up using a 47 ohm, 5W cement resistor and a 120V 
relay from Radio Shack all enclosed in a small, plastic "project box" 
from same.  On the bottom of the shelf where the tubes mount on power 
supply, there is a terminal block.  The terminals are labled 1-6 or 
something linke that.  If the A/C feed for the radio is properly wired, 
the outboard screw on terminal #1 has a white wire with black tracer 
which is the feed point for the switched A/C feed.  I disconnected that 
wire and lug from the terminal and removed the lug.  Connected an 
insulated pair, one of the wires had a white tracer and ran those 
between the power transformer to the relatively open area in front of 
the transformer.  Wire with the white traced was spliced to the white 
wire with black tracer and on the other wire, I installed a spade lug 
and connected it to terminal #1 on the transformer, where the stock wire 
had been removed from.

The wire with the white tracer goes to the resistor which is connected 
to one side of the relay coil.  A 3rd wire goes from the relay coil, 
routed next wiring harness to the left of the PTO, around the back, 
inside of the radio to the A/C neutral connection on the mains filter.

The relay is configured such that when it pulls in, the 47 ohm resistor 
is shorted.

You will need to drill a couple holes in the plastic "Project Box" for 
the wires to pass; I affixed the box to the vertical chassis between the 
MC change shaft and seperator between the power transformer compartment 
and PTO.

Turn the radio on cold, the relay takes approx 1/4 to 1/2 second to pull 
in, shorting the resistor.  It becomes >slightly< warm to the touch. 
There is a bit of relay chatter @ 60 CPS as the relay pulls in but not 
much.  The choice of employing a 47 ohm resistor was a balance between 
pull in time, heat and relay chatter.

This approach is not as elegant as perhaps a 10-30 second timer relay 
and larger, say... 100 or 150 ohm, 20W resistor to dampen the inrush but 
it certainly takes the "edge" off.  Unlike the thermistor, (which in 
light of failing power transformers, is admittidly better than nothing 
if sized properly 3A X 120V or 360W) there is no resistive load in 
series with the A/C to heat up and wonder about.  If for some reason the 
"Heater" switch is accidentally turned on and the current load 
subsequently increased, (no one here is in an arctic climate, yes?) the 
relay/resistor approach is ambivilent to the increased load.


---
No  trees were killed to deliver this message.  However, a large number 
of electrons were terribly inconvenienced.

Chuck Rippel

>
> Both concerns, inrush and undesirable series resistance may be 
> addressed with a simple, classic circuit.  The NTC thermistor (or 
> fixed resistor) is inserted in series with the transformer primary 
> supply (hot lead). Then, relay's coil (appropriately voltage rated) is 
> connected across the primary.  The relay's normally open contacts are 
> strapped across the resistor, that's it. Upon application of power, 
> the resistor limits the surge, and the primary voltage stays low until 
> the surge has largely passed.  With reduced current comes less voltage 
> drop across the resistor, the primary (and relay coil) voltage rises, 
> and the relay pulls in, bypassing the resistor and applying full 
> voltage. Some experimentation with the resistor and with relay pull-in 
> characteristics is in order.  A resistor can be inserted in series 
> with the relay's coil and/or the armature spring tension can be 
> varied.
>
> Drew