[R-390] Ballastubes (was inrush current limiters)

[Drew Papanek]

Hello Gentlemen,

I see that the old BallasTube Thread is alive and well.  If you want to read 
the lively and animated discussion of this topic from several years ago, go 
to r-390a.net. Click on References, Pearls of Wisdom, Ballast Tube.

There is a problem with substituting a diode for the ballastube.

>On Sun, 22 Dec 2002 20:38:24 -0500 (Eastern Standard Time), Helmut
>Usbeck wrote:
>
>
> >Put a diode (1N4007 will do)  between pins 2 and 7.  No muss, no fuss, no
> >heat.  YOu now have 12 vac for the two 6 volt tubes to fight over.


>Oh, I get it now.  Half-wave rectify the filament voltage to accomplish
>a reduction in the average voltage.  Average value of a half-wave
>output is .318 of peak, according to my reference.  Doing the math, 25
>VAC RMS into the rectifier would give 11.21 volts.  Close enough.

RMS voltage and current are what define heating power in a waveform (DC 
"waveform" included). Since what we are doing here is heating cathodes, peak 
or average current and voltage values do not apply (when peaks are within 
reason and waveform's period is much less than cathode's thermal time 
constant). RMS is what counts.

One of my references lists RMS value of a half wave rectified sinewave to be 
half the PEAK value.  Peak voltage of the 25.2 VRMS winding powering the 
series ballast, PTO, and BFO tubes : ( 25.2)(1.414)=35.6v peak.   The half 
wave rectified RMS value:  ( 35.6)(.5)=17.8 VRMS.  Hence,  with diode in 
place of ballastube, each 6BA6 tube heater receives 8.9 volts instead of the 
6.3 volts it was designed for.   The single diode modification will work, 
but the life of the PTO and BFO tubes will be reduced.  I read of one List 
Member using this modification for 20 years!  The amount of abuse these 
tubes will take is amazing.

(Helm's comments regarding adding jumpers to 3TF7 socket so that 12BH7 can 
be substituted...)

> > And as I mentioned, it's a useless cludge, extra work and some people
> > actually think that a 12BH7 has to be used.  Any 12 volt tube will do.
> > While one rewiring the socket, put in a 12au7 and set it up as an 
>internal
> > product detector (one of those things I,m going to try when I get a 
>round
> > tuit).  At least it does some thing besides producing heat and a rewired
> > socket.  It's attribute as having a controlled warm up filament is lost
> > since the other two tubes are not.
>>
> > My conclusion is that the best thing to put in the 3TF7 socket is a 
>3TF7.
> > I think some people should also check the voltage drop across there 
>tubes.
> > Yeah, I know they all work.  But at the expense of other problems.
> > Unfortunately my 390a seems to go though one every 2months or so and 
>it's
> > a bit expensive.  One can buy alot of good German lager for $45.00.
> >
> > -Helm.

A number of tubes can be used instead of 12BH7....the 12BY7 comes to mind.  
The tube needs to have a 12V heater at 300 mA.  A 12AU7 will not work as it 
draws 150 mA when configured for 12 volts.  The 12BH7 could also be used for 
a product detector.

Yes, if it's current regulation you're after, that 3TF7 is hard to beat.  In 
the aforementioned "Pearls of Wisdom" reference Nolan Lee quoted information 
from Amperite, the 3TF7's manufacturer, showing +-1% current regulation over 
a fairly wide voltage range.

MMMMM....German Lager!!  The major brand American "beers" cannot hold a 
candle to what Germany has to offer.
Some American microbrews are pretty good though.

The BallasTube can also be replaced by a power resistor.  Under IF deck 
mounting would be a poor choice because of heat buildup, but above deck 
mounting looks ugly (to some).  The calculated value is 43 ohms.  A 47 ohm, 
5 or 10 watt unit works well.

On Sun, 22 Dec 2002 16:03:01 -0800: Bill Smith  wrote:


>A very simple modification is to simply jumper the ballast pins 2 and 7 
>with
>a simple plug-in wire jumper and replace the 6-volt tubes with their 
>12-volt
>equivalents.  The receiver remains very stable even when the filaments are
>not current regulated by the ballast tube.

I like this ballast solution.  It involves no rewiring and heat generation 
in the R-390A is reduced by about 4 watts (not much, but eva' li'l bit 
he'ps). 12BA6's are cheap and easy to find.  Use a piece of paperclip for 
jumpering.


On Sun, 22 Dec 2002 22:17:28 EST ToddRoberts2001 wrote:
>
>..... I wonder if anyone else has tried the modification
>for the R-390A where the filament wiring for each of the BFO and PTO tubes 
>is
>re-routed in parallel to the 6.3VAC filament buss used in the rest of the 
>IF
>strip? It is not complicated to do and only involves unsoldering, 
>re-routing
>and re-soldering a few leads in the underside of the IF strip.  No extra
>wiring leads are needed. I have done this mod. on an R-390A to eliminate 
>the
>need for the 3TF7. The 3TF7 socket is left empty and is non-functional 
>after
>this mod. The receiver works fine and this insures the exact same 6.3VAC
>across the BFO and PTO tubes and retains the original 6BA6's for both. I
>confirmed the receiver stability by varying the line voltage + and - 
>several
>volts with a variac. When tuned in to an AM broadcast station with the BFO
>turned on I could not detect any change in pitch when varying the line
>voltage several volts up and down to simulate line-voltage fluctuations. 73
>Todd Roberts WD4NGG.

I have read positive things about this modification.  It will also reduce 
heat generation.  Todd's good experience with frequency stability vs line 
voltage fluctuation reinforces the contention of some that current 
regulation provided by the ballastube is not really necessary.  Moving those 
few wires around (in my opinion an insignificantly minor mod) may make some 
of the purists cringe.

Replacing the BallasTube with solid state current regulator has also been 
discussed.   A while back Dr. Jerry designed an AC current regulator using a 
full wave bridge wrapped around an LM317 configured as a DC current 
regulator, providing a clipped sine wave.  I did a computer analysis of his 
circuit showing a +1.5%, -2.5% variation in its 300 mA RMS current over a +- 
15% line variation.

As far as regulator-induced noise is concerned, filtering would help but I 
am not sure that this is necessary.  First in line after the 3TF7 is the BFO 
tube.  The regulator's noise contribution here would probably be small 
compared to the fairly high signal level.  Second in line is the PTO tube 
and the already present brute force filtering in PTO tube heater supply line 
would eliminate any noise at this point.  Chuck Rippel's regulator-based 
ballast repacement module's noise filtering certainly doesn't hurt though.  
Chuck is a very thorough sort who likes to have all the bases covered (his 
excellent R-390A restoration workmanship reflects this).

Another solid state regulator approach is to half wave rectify the 25.2VAC, 
filter and apply to a 3 terminal regulator configured as a 300 mA DC 
regulator.  PTO and BFO tube heaters then operate on DC.   Dr. Jerry was not 
in favor of this method as it places an unbalanced load on power transformer 
and in his opinion increases core saturation and heating.

I believe that the imbalance is small compared to the total transformer load 
and probably wouldn't make much difference.  The high current peaks (which 
cause core saturation) caused by charging the filter cap on each positive 
peak could be reduced by adding a resistor in series with the rectifier 
diode.  This also would reduce filter cap voltage and dissipation in the 
regulator.  By rough calculation I figure somewhere between 6 and 10 ohms 
with 1000 uF for a filter.

A few weeks back Francesco from Italy posted a message about his 
non-operational R-390A.  I corresponded privately with him and he found the 
problem to be the BallasTube.  I mentioned various options to him and this 
set me to thinking about these aforementioned aspects.  For me, resurrection 
of this intriguing (previously) "dead horse" thread could not have come at a 
better time!

Have a happy holiday and may Santa bring each of you a sleighload of 3TF7's!

Drew

>

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