# RF Power Amplifier Tube Performance Computer

 This article describes a design tool for calculating anode performance of a vacuum tube RF power amplifier based on the method used in CPI/Eimac's "Care and Feeding of Power Grid Tubes".

# Introduction

CPI/Eimac publishes an excellent text entitled "Care and Feeding of Power Grid Tubes" (CFPGT) on design and operation of high power RF amplifiers using vacuum tubes. The publication can be downloaded from their website, look in the library section.

CFPGT Chapter 3 deals with electrical design considerations, and introduces a tool for analysis of anode current waveform, the tool is the "Tube Performance Computer". This article describes a spreadsheet based calculator tool for analysis using the same underlying method, but using more accurate interpolation of transfer characteristics and Fourier analysis that is specific to the tube's transfer characteristic.

# Usage

## Characteristic

The first step is to enter the tube's anode current / grid cathode voltage characteristic on the "Characteristic" worksheet.

Choose an appropriate opearting line as described in CFPT Chapter 3, and draw the operating line on the anode characteristics chart. Draw the opearting line all the way from one axis to the other.

Then choose six Vgk, Ia pairs along the operating line. Choose points that:

• include cut-off;
• are reasonably evenly spaced; and
• are on charted lines rather than interpolating by eye;
• include the non-linear region near saturation.

It is vital that the first point is the Vgk for cut-off, even if it means interpolating / extrapolation from the chart. Note that the operating line MUST pass through the idle Ia / Vgk point.

These data points are used for a cubic spline interpolation to predict anode current at arbitrary Vgk values. Inspect the graph on the "Characteristic" worksheet to ensure that the interpolation appears sane.

Fig 1 is the constant current characteristic for the 4CX20000A / 8990  used in the example in CFPT Chapter 3. Characteristics plotted for constant grid voltage can also be used.

Fig 2 shows the data points chosen from Fig 1 for the model.

Fig 3 shows the model anode / grid characteristic of the 4CX20000A / 8990 for the operating line used in the example in CFPT Chapter 3 as constructed from the data in Fig 2.

## Anode

The second step is to enter values in the yellow cells on the "Anode" worksheet.

The anode current waveform will be calculated from 64 samples of sinusoidal drive voltage over a full cycle using the interpolated transfer characteristic. A Fourier analysis (FFT) is performed on the sample set to calculate the magnitude of the DC and fundamental component of anode current waveform. The anode current waveform, DC component and fundamental component are plotted.

Fig 1 shows the key part of the Anode worksheet. Input values are in the yellow cells, and the green cells are calculated results.

Different models can be created by copying the "Anode" worksheet and changing the input data.

The spreadsheet does not attempt to perform calculations for a harmonic multiplier. To be accurate, the anode characteristic for a harmonic multiplier would need to be used as a basis and the accuracy of scaling that from a graph is poor. Eimac tool qualifies their harmonic factors with "Use only for tetrodes or pentodes - Approximate Only".

# Prerequisites

The RFPATPC spreadsheet was developed in Excel 2000 and uses Excel macros. The macros are necessary to the calculation.

In Excel prior to 2007 the spreadsheet depends on the Analysis Toolpack Add-In which can be enabled from Excel menu Tools/Add-Ins. The installed from the Microsoft Office distribution disks if not installed, but it is usually installed, just not enabled.

If the spreadsheet doesn't recalculate correctly when opened, hit the F9 key to force recalculation. If it still has errors, see the previous paragraph. Recalculation is a moderately intensive process as it will often involve recalculation of a 64 sample FFT, so some delay in response may be noticeable on slower computers.

For Excel 2007 and later, the functions in the earlier Analysis Toolpack are integrated, and no add in is required.