[Boatanchors] OA2 CURRENT

[Rodger]

Hi Bill,

The best way to check 0A2 current is to lift the pin(s) connected to 
ground and insert your current measuring instrument between that pin and 
ground so as to measure actual 0A2 current.  It sounds like the 0A2's 
are in series since it would be unusual to run them in parallel (you 
would need to provide current equalization between the tubes since they 
are not going to be matched).

The most likely reason for excessive 0A2 current is too much voltage to 
the VR tube and with all tubes out of the circuit the normal current 
draw of the components receiving regulated voltage is not present AND 
the voltage feed to the 0A2 dropping resistor is also going to be 
greater than design intent which could easily cause this situation.  Do 
not let the tubes run at this level because they will quickly die since 
the maximum rated current flow through an 0A2 (or a string of 0A2 tubes 
in series) is 30 mils.  If you want to test this part of the circuit 
without the other tubes in place you will have to estimate the normal 
current draw of the circuit receiving regulated voltage and simulate 
that draw with a resistor to ground in parallel with the VR tubes.  If 
the source voltage to the dropping resistor for the VR tube also 
supplies power to other circuits you would also need to add an 
additional resistor ahead of the series dropping resistor (also to 
ground) to simulate the normal voltage feed.  Practically, it is better 
to install all of the tubes in order to test the circuit instead of 
trying to calculate and simulate normal current consumption through the 
use of load resistors.

Basically, the dropping resistor feeding the 0A2 tube (and providing the 
regulated voltage downstream) should be chosen so that with normal 
minimum circuit load the 0A2 is drawing not over 30 mils of current and 
at maximum circuit load the 0A2 is drawing at least 5 mils to keep the 
gas inside ionized.  The VR tube operates effectively as a variable 
resistor by decreasing its resistance and increasing current draw when 
the source voltage tries to increase and increasing its resistance (thus 
decreasing current draw) when the source voltage drops.  So the value of 
the series dropping resistor is critical and the VR circuit cannot 
operate normally when normal circuit loads in the regulated circuit (and 
other normal loads on the voltage source feeding the VR tube) are not 
present.

Rodger WQ9E


Magoo wrote:
> I'm trying to troubleshoot what appears to be excessive current draw in a 
> P/S employing a pair of OA2s.  With all tubes out of the chassis, 
> approzimately 10ma is being drawn through the choke (in the ground lead) by 
> the bias supply.  When both OA2s are plugged in (with no other tubes 
> installed)  that current rises to about 90ma, and that seems excessive to 
> me.  Both tubes test OK.....but are they?
>
> B+ at source is 315 and each tube is fed through 4K; with no load I 
> calculate 7 ma for the idling current on each tube.  Is my logic flawed, or 
> am I dealing with a pair of tubes which test good but are actually bad?
>
> Bill VE3NH 
>
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