Schematic Question

[W7SOE]

Another Junker amp question!

http://www.w7ekb.com/glowbugs/projects/JunkerAmp.pdf


I am trying to understand the plate current metering circuit.  The meter is a 200uA version.  The plate current is sampled at R14, which is 3.3 ohms and I think the ground return path.

If there is 200uA through the meter then there is 0.94V at the junction R13 and R16.  This means 285 mA though R14.

This does not appear to be a high enough current range.  My analysis must be wrong...


Rich

[KO6YB]

Hello Rich,

Your calculations seem spot on. He has the meter set to 285 ma full scale.
In the tune up procedure for the 813, he stated 200ma plate current. So
the scale is large enough, but agreed, 285 ma is a bit odd for full scale.

Stan.

[w8khk]

Rich, I believe your calculations are accurate.  With the resistor values in the schematic, the full-scale grid current will be 100 ma, plate current will be 285 ma, and plate voltage will be 2000.

The article indicates three different tube types may be substituted.  For a single 803 or 813, the tune up paragraph suggests a maximum plate current of 200 ma, which correlates to the resistor values in the schematic.  For a pair of 811As, they suggest running 350 ma, obviously this is in excess of the meter range with the specified resistors.

My assumption is that the author neglected to address using different value shunt or series multiplier resistors to cover the different tube requirements.  

It is interesting to note that when using a series resistor to read voltage across the shunt resistor, the actual meter resistance is so small in comparison that it can be ignored in the calculation.  In your case, you could easily use anywhere between a 100 ua and a 1 ma meter and calculate the series and shunt resistors to get the desired grid and plate current, and plate voltage readings.  

Your project looks neat, I look forward to seeing it's completion.

73, Rick

EDIT: KO6YB and I apparently posted at the same time.

[W7SOE]

Well there you go!  Thanks gents.

I always assume I am wrong, occasionally I am not.  ;-)

 I'll have to pick resistor values to rescale the meter.

Rich