[R-390] R390-A Power On B+ Voltage and Choke Filter

[Larry H]

Charles,  Sorry for not responding sooner - vacation, etc.

Thanks for catching my math error on the 0A2 current.

On 10-4-14 you wrote:

"To reach the 75mA strike current, there must be at least a short 
excursion of the C606A voltage to ~420v (75mA through 3.6k produces a 
drop of ~270v, plus the regulator voltage = 420).  In practice, most 
0A2s strike at less than 75mA, so the overhead is typically less than 
270v -- but the total voltage required with 3.6k of dropping 
resistance is still well above 300v.  Even a strike current of only 
50mA requires a C606A voltage excursion to ~330v.

25mA *is* enough (just barely) to hold the C606A voltage below 300v, 
once the 0A2 is conducting."

As to the operation of an 0A2 and most other gas regulators, I 
was aware of a small increase in current during the transition 
from 'Plateau' to 'glow discharge' known as the 'breakdown' or 
'strike', but not aware of a large current requirement or any 
voltage spike at all.  As a result, I did a little research to 
confirm what I thought was the operation - and it does confirm 
what I thought.

There's lots of reliable info written on the subject.  I've read 
the following and more:

Neets - Naval Electrical Engineering Training Series, module 6
Introduction To Electron Tubes and power supplies.
http://electriciantraining.tpub.com/14178/

Gas Discharge Tubes - Introduction, D. W. Knight, 2013.
http://www.g3ynh.info/disch_tube/intro.html

Engineering Electronics, G E Happell, W M Hesselberth. McGraw-Hill 
1953.
http://www.tubebooks.org/Books/Happell_engineering.pdf

Electron Tube Design, RCA 1962. 'Gas Tube Design' by H H Wittenberg'.
http://www.g3ynh.info/disch_tube/Wittenberg_gas_tubes.pdf


Let's look at the very commonly used 'voltage/current' graph 
referenced by most of the documents I read.  This is the one in 
D. W. Knight's document 'Gas Discharge Tubes - Introduction':

http://s28.postimg.org/i0cqyy399/IMG_6339s.jpg

What this shows is the voltage applied to the anode through a 
resistor is increased until the 'strike' point is obtained and 
then the regulator transitions into the 'glow discharge' area.
This does not require much more current and this is where we want 
it to stay for proper regulation operation.  For an 0A2, the 
current range is 5ma to 30ma (this is the 'constant voltage' area).
You can refer to the RCA spec sheet here:

http://frank.pocnet.net/sheets/049/0/0A2.pdf
http://frank.pocnet.net/sheets/191/0/0A2.pdf

Below 5ma it drops out of ionization and more than 30ma and it 
leaves the linear regulation area headed for impaired operation of
the tube and possible damage.  The resistor feeding the voltage to
the regulator must be selected to keep it in the preferred area.

There is an interesting spec listed in the 0A2 sheet 'Average 
Starting Current ........... 75ma Max'.  The brief explanation 
could lead one to belief that 75ma was required to start the tube 
for proper regulation.  This is NOT true, 15ma is all that is needed.

I tested this on 3 0A2's (because I don't have any more), and they 
all showed me the same result on my scope - a smooth increase to the 
transition 'strike' zone of 155V to 163V and then dropping quickly to
the 'constant voltage' area of 150V when applying 200V, 225V, or 250V 
using a 3.6K feed resistor (as the R390-A uses).  There was no spike 
in current or voltage.  This was substantiated by all that I have read


on this.  

Now let's get back to the 'Average Starting Current ........ 75ma Max'
spec.  If we look closely at the 3nd paragraph of the above 0A2 spec
sheets under 'Operating Considerations', it makes it clear that any 
current over the 'Maximum DC Cathode Current' (30ma for an 0A2) will 
impair it's operation for up to 20 minutes.  Now, we know this does 
not happen in our R390-A's and I see no reason that it does or should.  
This spec is just a statement to designers that you can exceed the 
30ma limit up to 75ma without permanent damage to the tube, but if 
you do, the regulation will be impaired for up to 20 minutes.  This 
does not happen in an R390-A.  The 0A2 feed resistance is 3.6K in 
standby.  That's about 25ma with no load at power on.  Since my 'power
on' scope trace at C606A shows no voltage excursion above 255V, more 
current than that will not be drawn.  Link to my trace picture:  

http://s29.postimg.org/a2yqc2tyv/IMG_6239s.jpg

Now back to the reason the '12H choke input' prevents a high voltage 
excursion at power on in an R390-A.  Look at the picture of the 
associated choke input graph from Norman's book. This is the same 
graph used in many articles: 

http://s9.postimg.org/usu46ncgv/IMG_6241s.jpg

The formula for figuring the knee point (where the voltage is down to 
the RMS value) is: max supply ma / choke H = 180ma / 12 = 15ma.  This 
means that when L601 is drawing more than 15ma, the output voltage will 
be approximately = to the RMS value at C606A.  This will happen at about 
185V.  This is the sum of the current from capacitors charging (180 mfd), 
0A2 current, and circuit resistance. 

I powered on my R390-A's using my variac to produce 200V at C606A with
3 different 0A2's and they regulated correctly.  I scoped the voltage 
and again saw no voltage above 200V during warm up.  

Regards, Larry J. Haney