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

[Bernie Doran]

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)