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Designing with the GS31B

I have been asked for hints on how to use the RF Power Amplifier Tube Performance Computer for design of a GS31B based amplifier.

Limits

Limits on operation of a GS31B in AB2 include:

  • anode dissipation 1000W max;
  • cathode current 1.4A RMS max;
  • grid dissipation 22W max;
  • anode voltage 3kV DC;
  • anode voltage peak 6kV;
  • various component temperature limits.

The anode dissipation might suggest that the valve may be capable of up to 2000W continuous or so, but the other limits are more constraining.

The specification of maximum cathode current in RMS terms reflects the original application of this valve for pulse operation, and is rather difficult to assess from published anode characteristics because it means predicting instantaneous anode current and grid current over the operating cycle, and calculating the RMS value. The extent of grid current contribution to RMS cathode current depends on how low the anode is driven during a cycle.

Initial load line

Many users claim 1500W output from amplifiers based on this valve. Taking that as a design objective, and limiting the DC anode - cathode voltage to 3kV, an initial load line can be calculated using Calculate initial load line of valve RF amplifier. Taking a guess that the anode can be driven down to 400V, and it is a guess as will be shown later, the initial load line calcs are given in the following clip.

Fig 1:

Fig 1 suggests an initial load line from cutoff at 3000V to peak anode current of 2.6A at 400V.

Fig 2:

Fig 2 shows that load line laid over the GS31B anode characteristics.

The lower 14% and upper 43% of the loadline do not have grid curves, more than half the load line is essentially in uncharted territory.

The lack of published data prevents design proceeding any further that this initial load line, prototyping the amplifier is necessary though the estimated currents and resonant load impedance are probably good estimates.

Note that the estimated RMS anode current is 1.3A, so it is likely that this operating point will exceed the specified maximum RMS cathode current. A design for 1200W output is a more conservative design that is less likely to exceed published specifications, though the published anode characteristic do not allow more detailed design at that power level either. Calculating and charting a load line for lower output is left as an exercise for the reader.

The absence of data that might give confidence about the extent of non-linearity at the high end and at the low end might reasonably be viewed by some as selecting a pig in a poke rather than design in an engineering sense! Of course, that could be changed if someone was to measure a sample of valves and publish a more comprehensive average anode characteristic.

Links

Changes

Version Date Description
1.01 23/05/2011 Initial.
1.02    
1.03    

 


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