[R-390] R390 Posts: Excess line input voltage Noise Figures

[face at netunltd.com.au]

First, thanks to Bill,Barry and Cecil for their replies and advice..
Its nice to feel wanted.  Aint the net great!!
 
Aplogies too, for inadvertently replying to Barry alone  and not via the
administrator.  In my inexperience 
I simply repied to Barries email, thinking it would also go to mailman.
Wrong again, John!
 
So I shall reply to you all by re typing and re submitting  this
condensed version and sending it correctly ..... serves me right !
 
 
EXCESS LINE VOLTAGE TO R390A
Barry replied by suggesting I use a variac in front and twiddle the knob
to maintain line constancy. 
(tongue in cheek, maybe?).
I replied that I had solved this problem by (first) using a servo
feedback motorised variac with a front panel screwdriver adj.
When this broke down on me I used a constant voltage, ferroresonant
transformer, (CVT) with sine wave output option
(an Advance Model CVN230A)
This gave a very well regulated, transient free  230v AC constant output
to run my R390A and allowed me to remove the line bucking transformer
previously installed..
 
Barrys  reply was that he uses a similar CVT, but that it 'ran hot'  and
was noisy, which I take to be lamination buzz. 
 
REPLY:  Barry, if your CVT is not actually faulty, thats usually caused
by not running the CVT at, or close to, its rated output current.
These things are designed to run under a load close to the rated maximum
output and literally shake themselves to bits when they arn't. !  Not
only do the darn things then run hot, they add a whacking big amount on
your electricity bill !!!! 
 
Mine has a max rated output of 230W with a power factor of 1.0. 
The R390A is rated at 225W which matches it pretty well.  My CVT then
runs quiet and cool . You wouldnt know it was there. 
One of the big advantages of running a CVT in front of the R390 is its
ability to suck out damaging line transients.
I made up a box adding silicon carbide voltage supressors to go across
inputs and outputs  when I first used this thing, but removed them as
they wern't needed, even in the electrically noisy environment I was in
before moving  house (was on a main industrial line feed 50 ft behind a
pole mounted line boost transformer).
Also, they were only 275 v rated and was scared they would smoke out on
the frequent high line peaks I had then (275v RMS measured when quiet
!!!).
 
Barry also replied that servo controlled variacs often fail because of
brush wear tracking carbon dust over a small segment of the winding due
to motor hunt when stabilizing.     A valid and sensible point , Barry!
I will check mine out next time I find it. (Its buried neath a near ton
of gear forced into a garden shed.  Moving from a five bedroom house
with three large rear sheds to a 2 bedroom plus garden shed forced the
loss of a pile of good gear..but i'm sure thats still around.
Methinks a servo'd Variac in front of my CVT would make a 'cool'
combination for my R390A.
 
Not everyone out there's got that sort of gear to hand, though.
Nor does it solve the over high HT when changing to Si diodes.
=============================================
Bill suggested that high HT does not impact on R390 performance much.
 
REPLY: That may well be so. Probably because of the R390 remote cut of
valves used in the RF/IF  stages, necessary for AGC.
             I remember, as a student, being given the task of designing
a VTVM, though transistors were plentiful enough then, but   expensive.
  If my memory is reliable on this, I found that the sharp cut off twin
triode (12AX7) initially used, DC drifted a lot, but a wider grid base
12AU7 fixed the probem. (same pinout)  Seems that sharp cut off types
had a 'u' more dependant on small changes of anode current than the
others.   Could be that pentodes have the same characteristics and the
remote cut off types used in the RF/IF stages of the R390 don't worry
too much about stability of HT supplies.  Bill could be quite right
there.
 
BUT: ....they will run hotter !!! (as will all the dropping resistors in
the HT chain)
We should be kind to our bottles and keep dissapation down.  Hence I
think keeping HT at the recommended level by Collins is 
probably a neat thing to do.  Less stress on the filter caps too.
 
Bill suggested that my suggested mod of placing a resistor in the CT
return of the HT transformer would increase stress  on the (winding)
insulation.
REPLY: The voltage across this resistor to chassis is  going to be
around 20 to 50 volt, at a guess.  (depends on line voltage and R390
settings and condition).  The transformer insulation is factory tested
to some 2,000v, usually for one minute.  An increase of 50v is not going
to cause a surgeon generals health warning here.
But,  the measured voltage between the end of the resistor (chassis) and
the outer end of each secondary winding is still the same.
There's no algabraically added voltage between transformer winding and
core.
All we have done is add a few ohms exrtra to the secondary winding
resistance.
The ac voltage drop across it isnt added to the AC secondary voltage, it
becomes part of it.  There is still the same voltage as before across
the secondary windings and between them and core.
The stored charge on the capacitive input filter used on the R390 takes
care of worsened transformer regulation due to increased resistive
losses in the xformer winding.
 
Its the relatively low voltage across this proposed dropping resistor
which seems so attractive to me for regulating the HT voltage to a fixed
setting, independant of AC line and R390 load current (within reason,
anyway).
If the needed voltage drop acroos this 'R' is more than , say . an
LM317/337 max rating, we just add a couple of rated  Zeners in series to
drop it to a safer value to use
 
Further, with good, working vacuum bottles getting scarcer to get
(certainly the case in Aussie)   it does seem that preserving tube
working life would be enhanced by ensuring operation inside specs.
 
So far as my R390 is concerened, all this is armchair theorising on my
part.  I havn't had to do a thing to it since buying it.
So until I get my hands dirty and actually do the above mods, I will be
taking all your advice thus far  to heart and thank you all for
contributing.
 
I end with a question.
 
Does any of you know if Collins specified a noise figure test on the
R390 series ?  Ie recommended test circuit.
 
I was going to use a carbon resistor across the co ax input (known noise
voltage). to determine overall receiver noise floor.
(when I get my hands on a replacement set of good bottles)
 
But I have seen 'dummy antenna' used to evaluate more realistic noise
figures when connected to a 'real' antenna.
Example:  LCR network used to evelaute noise performance of LF
receivers... such as seen in Termans and Langsfords Smiths books) 
We used such LCR networks across antenna  terminals years ago for
lw/shop evaluating probable S/N floor under actual operations in remote,
desert  areas when using a resonant long wire.(2 harmonically related
'ops' frequencies)
 
I dont own an active, calibrated noise source and wondered whether the
LCR dummy load would give a more realistic figure than just the resistor
(The LCR circuit can be tuned to the resonace of the 'real' antenna, or
to its receiver entry  impedance at a working frequency and thus give a
more realistic result than the broad banded carbon resistor approach.)
 
 
 
Regards to all , particularly the guy that got all this running on the
net!!!
 
 
John Byers