[R-390] R390 Posts HT Dropper, HT AC regulator

[face at netunltd.com.au]

Re 390 Posts:
 
A better way to reduce HT on the 390A after Si diode replacement, an
idea for a simple  HT voltage regulator 
and an introduction ....
 
"Fresh Grist to the Mill" from Perth, Western Australia
 
Hi.  I am new to the R390 chat  scene and as this is my 1st post to
anyone on the i/net... please bear with me if I don't seem very
polished. 
 
I bought my R390A around 1974 (Capehart S/N 1881)  Although I have
maintained and used many professional brands of radio gear since the
days of valves, my R390A still amazes me with its depth of mechanical
and radio engineering know-how as well as its fine performance   
My R390A came, I believe, from our NW Cape, U.S. submarine comms station
ALONG WITH A BOX OF UNUSED, BRAND NEW SPARES , including all the ovened
bits !   sadly, though the deal was ALL the spares, the set of boxed
valves (tubes), spare XTALS and mech filters were missing after I paid
my A$400 for it.. I am still waiting for them !!
 
Only other thing amiss was the dial lock mechanism and knob.  My R390A
had been in a rack with others and had stayed on one fixed frequency
throughout its working life.  The guy that sold it to me claimed it had
frozen in position and he had to remove it to free the dial !
 
The unit had both the top and bottom covers missing, probably to keep it
cooler in the rack with its hot brothers stashed top and bottom of it.
My unit is still in its original condition, but hasnt been used since I
changed location some 2 years ago.
 I Have obtained a 50 ft pole so hope to have another LW + Vertical
antenna strung up soon and be back to marvelling at its performance.
 
I will take advantage of all the excellent knowledge you fine fellows
have passed on and shall replace problem  caps first as you suggest.
 
Now to the more interesting bits:
 
So far all the mods I have seen to reduce too high an HT after replacing
valve rectifier with Si diodes are as follows.
 
1. Put a buck transformer in series with the line
2. Put a drop resistor in the HT line, after the filter caps.
 
1. (bucking xformer) is the neatest approach, it needs no mods to the
R390 proper. 
    This is an excellent approach when the line voltage is too high (I
used this in my own R390 as our line voltage in
    WA goes up  to 275 Volt in places !!!).
    BUT for simply reducing the effects of too high HT due to diode
replacement, (when line  voltage normal) it will also            REDUCE
THE FILAMENT VOLTAGES in the same ratio !  Not too 
    good for maintaining valve life as it could wind up poisoning the
filaments.  It reduces
    the performance of the set by reducing tube gm as well. 
 
2.  (Series res in HT line.)
    Ok, but if placed after the HT filter cap, it adds to the internal
resistance of the HT line  and can affect performance on strong signals
or high audio levels due to HT fluctuating with  signal level.
    Putting it before the filter cap is better and protects cap from
excess surge current at switch on  as well.
    Even better, put resistors, one each in series with diode anodes to
HT wire ends of  HT  transformer, halfing power dissipation of each
resistor. (saves money,too)
 
    BUT you have to be careful installing the resistor(s) in this above
situation as they are at  full HT potential to chassis and add to the
hazards of servicing.
 
===================================================================
A better way is to lift the centre tap (CT) of HT pwr transformer
secondary winding from chassis, and place a power resistor between this
CT wire and chassis.
The resistor now operates AT A LOW POTENTIAL to chassis and can be
safely clamped down with an Al bracket (a square pwr resistor is handy
for this) , helping to heat sink the resistor and keep the under chassis
temperature down a bit.
 
You can pick off a useful dc voltage from the ungrounded end of the
resistor using another diode.. useful for activating low voltage,
current stuff such as solid state additions, provided you dont exceed
the total HT transformer VA specs.
And if your finger accidently brushes against the exposed active end of
the resistor, you wont notice it as its only at a few volts above ground
! (by the amount by which you want to reduce the HT).  The value of
resistance will vary according to the excess Ht voltage you wish to
reduce, which itself  depends on your local line voltage.   Simple Ohms
law applies here.
 
I have used this approach in West Aussie with complete success many
times in the distant past.
 
I really have not seen this posted before anywhere, at any site, but if
it has been I would not be surpised. The technique was quite common over
here when we had our own radio industry in the vacuum days, and I am
merely passing on  what used to be common place.
If it saves a shock or two, then maybe its worth passing on.
 
REGULATING THE HT LINE:
 
Since we are now operating, with the above mod,  at a low voltage to
chassis, its quite feasable to put an AC voltage regulator (possibly
using a couple of Lm317 V regs) to keep the Ht constant between light
and heavier Ht loadings.  I have actually done this in the days of Ge
transistors using a long tailed pair of transistors as the
reference/control inputs, then driving a pair of cross inverted power
transistors with isolating diodes. All this in series with the HT
winding CT to ground. 
(Used in some tube fitted nucleonics counting gear... again to
compensate for our high local West Aussie line voltages.. up to 270V AC
here, remember!)
 
If there is any interest in the HT ac regulator, I might knock one up
and fit it to my R390 when I get a chance, then pass on the circuit to
others.
 
Hope this note has been of some interest to other R390 admirers.
 

Addendum: 
Been observing feedback comment on running tubes on DC.
Read an article in an old Wireless World once, (by 'Free Grid'), that
valve radios 
which had been running prior on dc line supplies had a high failure rate
(open filaments), when changeover to AC line occurred. (early 30's ??)
It seemed that running heaters on 
Dc for a period crystalised the heater material and when put onto AC
they didnt like it.  
 

John R. Byers ( face at netunltd.com.au <mailto:face at netunltd.com.au> )