[R-390] R-390A Power Transformer, going SS and turn on surge (was R390A Audio Solution)

[Ken Harpur]

Hi Bernie,

Hmm...some food for thought there. Yes as mentioned the series dropping resistor that's in the transformer primary is in circuit at switch-on, so the transformer sees a reduced line voltage initially then after a few seconds (or long enough that the initial surge has dissipated) the relay shorts out the resistor allowing full line voltage to the primary. My understanding is that this delay doesn't need to be very long, maybe 2 to 5 seconds perhaps...

Admittedly the ramp-up to full line voltage isn't as smooth as with a CL-80. With voltage going up in two steps as opposed to a gradual increase. 

Next time I place an order for components I'll get a couple of the CL-80s and do some experiments. This whole idea came about with me trying to get some heat away from the transformer...then it evolved into reducing the turn-on surge. If I keep thinking about this it will further evolve into me having the radio suspended in the wind-tunnel things skydivers use for training...now THAT is forced air cooling! hihi...

Regards,
Ken


On 4/01/2013, at 10:22 PM, Bernie Doran <qedconsultants at embarqmail.com> wrote:

> Hi Ken: The only thing wrong with this idea is that the transformer is still hit with the current inrush when first turned on. IE before the relay pulls in.   I still am amazed that the current inrush limiters are met with any objection. They are simply the best solution, Based on performance, cost, and size.   Perhaps that is the problem, many may think that any design must have esoteric qualities and great cost to be effective.  The only place where time delay circuits  become necessary are those that are turned on and off frequently( the inrush limiters must cool for a minute or so)   and other special circuitry that probably already has time delays built in.
> 
> Just try the stupid things and see for your self.  Watch the voltage start low and rise to the applied voltage and I bet you will say, why did I not do this earlier!  Usually takes a second or two depending on the selected limiter.   Properly selected they will stabilize at a voltage drop of about a volt or so in the typical 100 to 200 watt equipment range,  less in high power stuff.   They do get hot, and have to be, to be functional, the cold resistance is many times the hot resistance.  They do not get as hot as many vacuum tubes. However, they should mounted as one would mount any resistor that dissipates a modest amount of power.  Just position them away from the chassis enough  to allow movement of air around them.  The concept that they cause poor voltage regulation based on ambient room temperature also baffles me.  Properly selected as indicated their normal device temperature in the range of a few hundred degrees, enclosed inside a box that does not reach a stable temperature for hours, you are going to tell me that a small change in room temperature is going to have an discernable impact?
> 
> Now if you select an inrush limiter designed for a several ampere load and set up a test feeding a load of say 250  millamperes and stand there with a fan turned on and off, the load voltage will jump all over the chart!! Guess what, you just discovered a thermister!  This is a case where "safety factor"  does not apply, they must operate at their design range of current flow.   For most equipment one can simply use the rated watts and voltage to calculate the amperes, for those that are fussy use a power factor meter or guess at a power factor of .8 to .9.   For a transmitter that uses a common transformer for all power there will be a change in line current between standby and transmit.  Likely not much more that two to one, should not be a problem, just select the inrush limiter that has a maximum rating for the current drawn on transmit.   They are designed for a small range of current that and selection should not be too difficult.  If there is a separate plate transformer then use one for the plate transformer primary and another in the B+/ filament transformer.
> 
> These discussions rather remind me of the concerns about all sorts of exotic replacements for the non off the shelf current regulator tube for the two oscillators.  Just remove the regulator, save it, place a jumper in the socket, and replace both oscillator tubes with their twelve volt equivalent. Then when you sell it, change it back and extol the virtue of this fantastic design that even regulated the filament power!  Never mention that it was designed that way to operate from sources of power with poor regulation.
> 
> How do these changes impact performance? My 390A  has  all of these modest changes plus (horrors) even a series ten watt dropping resistor to operate the tubes at their minimum rated filament voltage,and a MOV following the dropping resistor. I have turned on my gear in the garage on days that are far too cold for me to stay there very long, Does it drift? certainly it drifts, usable within a minute,and for my purposes fully stable enough for me to operate in the time it takes for the transmitter warm up and look around the band/bands.   I do keep a light on in the room during cold weather when there is a chance of condensation under warming conditions. Now suppose you say, well I demand  that it is totally stable immediately and able to read the frequency to one cycle because I operate only SSB  on one exact channel for my round table, also my line current jumps from ten ampers to seventy ampers when I yell at the parallel 3CX5000s.  Then I would say you might have several issues, and perhaps stick with Jap stuff, and hopefully on the high end of 80 and away from me.
> 
> This is way too long and I apologize for boring any one to death.  I will not do it again.
> 
> I have never done this before, but I will say that this is based on my opinion as an amateur with nearly 60 years of experience (1954) and also my Professional opinion.    Bernie Doran    P.E.   W8RPW
> 
> ----- Original Message ----- From: "Ken Harpur" <igloo99nz at yahoo.co.nz>
> To: "R-390 at mailman.qth.net" <r-390 at mailman.qth.net>
> Sent: Thursday, January 03, 2013 10:00 PM
> Subject: [R-390] R-390A Power Transformer,going SS and turn on surge (was R390A Audio Solution)