[R-390] Ballist Tubes

[Roy Morgan]

At 03:02 PM 12/3/2004, you wrote:
>I recently acquired a Boonton 250A RX meter.

David,

Good for you.  It's a neat gadget. (I have at least one of them here.)  You 
can see for yourself now whether or not carbon film resistors are inductive 
enough to matter. (Please let us know what you decide.. maybe you'll start 
another long, long thread on the topic.)

BAMA has a manual for that thing. It is in dejavue format.  See:
<http://bama.edebris.com/manuals/boonton/250a>

>   Boonton used a 6H-6 ballast tube

Make sure it really is a 6H-6 ballast tube (Glass with fine wire filament 
strung between the mica wafers), not a 6H6 duo diode tube (likely metal and 
short).

>  The spec calls for 6.3 +/- 0.3 volts.  I am measuring 8.3 V

Make sure BOTH of the tubes being regulated are lighted up. With one or 
more not present or filament open, the rest will get too much current.

>Can ballast tube fail in such a manner that it will still pass current but
>fail to regulate?

Probably.  There are some things you can do:

1) Note that R-407 and R-401 are in parallel with the load. I strongly 
suspect these have risen high in value and are no longer doing their 
job.  They get warm at lease, hot likely, were too small to begin with, and 
were carbon composition types most likely.  They are in parallel with the 
regulated filaments to help the situation when one of the tubes is removed 
or opens, making sure that the ballast tube is dropping the amount of 
voltage within it's range, or at least not too far out of it's range.

2) If the ballast is operating past it's normal range, for example at too 
high a voltage drop, then add a resistor in series with it to bring the 
voltage drop down to the lower end of the range.  You will note that there 
already is R-508 (0.55 ohms?) in series with the thing.. do check the value 
on that one.. Feel free to raise that value as high as you need to in order 
to get the regulated tube voltage correct.

3) Try different tubes in the regulated spots - some industrial tubes draw 
more or less than the normal equivalents.  The tubes specified are two 
5718's.  I was going to suggest that someone substituted normal tubes in 
their place, but these are subminiature ones and I was not able to locate 
easily any substitutes other than a CV number.  It's not likely that you 
have a box of spares..  Make sure both tubes are being heated up.  (If the 
meter is working, then both oscillators are working.)

4) Put a 6SK7 (300 ma) in the ballast socket and see what 
happens.  Actually, a duodiode 6H6 would do the same thing. Measure the 
actual transformer winding output.. the schematic says 13.5 volts.  Here IS 
a good use for a variac.  Lower the line voltage till you do get 13.5 at 
the transformer winding/input to the ballast and see what is going on.

5) Put a big resistor in there instead of the ballast.  (13.5-6.3)/.300 = 
24 ohms.  Measure the actual transformer output to see what resistor you 
need.  It will dissipate 2.4 watts or more, so use a 5 watt unit or 
bigger.  If your instrument measurements wander due to changing line 
voltage (that's why they used a ballast), get a voltage regulating transformer.

6) Get another ballast tube and try it out.  This is listed last because 
you may have trouble finding spares.  But ... Playthings of the Past has 
them for about $10
http://www.oldradioparts.com/2a2fl.txt

See http://www.amperite.com/Uploads/Ballasts.pdf for a description on how 
they work. Do not be encouraged when you find 6H-6 in the list of 
still-available tubes.. they want over a hundred dollars each for other 
types that are also listed (the 3TF7, for instance, according to R-390 list 
postings of the past.)

Happy RX measuring.

Roy

- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Cell 301-928-7794
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