Tube Specifications - Keep me honest!

[Wil_Bloodworth]

I'm working on trying to understand the Effective Load Resistance, THD, and Power Output of a power output tube as well as how it relates to an output transformer... stay with me.

The screenshot below is taken from the schematics for a 50L6-GT tube.  I'm simply trying to see if what I *think* I understand (i.e. what the schematic is trying to tell me) is correct.

"In general", the bottom portion is saying the [effective] Load Resistance should be 2000 Ohms when the plate voltage equals 110V and the resistance should be 4000 Ohms when the plate voltage equals 200V.  The total harmonic distortion should be 10% approximately. And, finally, the power output should be 2.1 and 3.8 watts respectively.

Now, if we look at the top graph portion, the purple/pink horizontal line denotes 10% THD. The yellow section highlights 2000 to 4000 Ohms load resistance. The circles (1) and (2) are the points where the measured distortion intersects with 2000 ohms and 4000 ohms.  The two red squares are where the power output intersects with 2000/4000 ohms.  The wavy green line is simply highlighting the measured distortion.  The large black arrow is a ballpark estimate of where the distortion is at its lowest point.

Based on that, I *think* the graph is saying that the measured power output and distortion at 2000 ohms is roughly 3.4W and 12.75% AND at 4000 ohms, those values are 4.35W and 8.25% roughly.

Now, IF I am reading all of this correctly, it appears that the optimal power output (4.2W) with the lowest THD (~8%) should be around 3600 ohms load resistance.  So... am I reading all of this correctly?



Thank you,

- Wil

[rsjsouza]

Wil, from where I see you seem to be correct. Just be mindful that the graph has a plate voltage fixed at 200V and a grid voltage of 110V, which is a setpoint that is not reflected at all by the table: the 2000 ohms sets plate and grid voltages to 110V and the 4000 ohms is set to 200/125V. Not much clear.

[Wil_Bloodworth]

Wil, from where I see you seem to be correct. Just be mindful that the graph has a plate voltage fixed at 200V and a grid voltage of 110V, which is a setpoint that is not reflected at all by the table: the 2000 ohms sets plate and grid voltages to 110V and the 4000 ohms is set to 200/125V. Not much clear.

Excellent point!  Mentally, I was trying to wrap my head around the clear differences between the "Typical Operating Conditions" specifications and the measured values... because the graph shows very different values than the table at the same input values.

- Wil