PG2DTA: Choosing a Transformer for the 300B

In my last post of the Practical Guide to Designing Tube Amplifiers I described how to choose an output transformer for a triode-connected 6V6. If you recall, a reasonable load for a triode is twice the Plate Resistance:

RL = 2 * ra

In summary, you calculate ra using the following steps:

• Locate the tube data sheet
• Determine the maximum plate dissipation (Pa-max)
  
• Determine the maximum plate voltage (Va-max)
• Plot Pa-max and Va-max on the Average Plate Characteristics graph
• Draw a line tangent to the grid curve closest to where Pa-max and Va-max intersect
  
• The slope of the tangent is equal to ra

This time I will go over the steps again for a 300B, but with a little less detail.

I located a copy of the 300B data sheet (135K PDF) at the Western Electric website. I figure they are probably a pretty reliable source. The Pa-max and Va-max are reported in the Maximum Ratings section.

The following is the grid curves graph with the Pa-max, Va-max and ra lines plotted.

Examining the tangent line you can see that it rises around 180mA over 125V. We determine the plate resistance using the following formula:

ra = V / mA
ra = 125 / 0.180
ra = 694.444

By doubling the value of ra we have determined that a transformer with a primary of around 1400 Ohms would be appropriate. Suggested values I have seen range from 1250 to 5000 Ohms so again I think the calculations are correct.