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Designing with the GI7BT

Here are some hints on how to use the RF Power Amplifier Tube Performance Computer for design of a GI7BT based amplifier.

Limits

Limits on operation of a GI7BT in AB2 include:

  • anode dissipation 350W max;
  • cathode current 1.25A max;
  • grid dissipation 7W max;
  • anode voltage 2.5kV DC;
  • anode voltage peak 5kV;
  • various component temperature limits.

The anode dissipation might suggest that the valve may be capable of up to 1000W continuous or so, but the other limits are more constraining.

The specification of maximum cathode current in useful, but 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 500W output from amplifiers based on this valve. Taking that as a design objective, and limiting the DC anode - cathode voltage to about 2.2kV, 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 200V, 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 2250 to peak anode current of 1.05A at 200V.

Fig 2:

Fig 2 shows that load line laid over the GI7BT anode characteristics.

More than a fair degree of interpolation is needed to characterise the loadline in RF Power Amplifier Tube Performance Computer. Nevertheless, it is usable.

Fig 3:

Fig 3 above shows the transfer characteristic along the load line.

 

Fig 4:

Fig 4 above shows RF Power Amplifier Tube Performance Computer model of Class AB2 operation along the load line.

Your own scenario

The design above is based on as assumed DC supply and expected power output. For your own scenario, you can follow the same procedure to develop a design suted to your own needs.

Comment

The model is derived from one of the several published datasheets on the GI-7BT. Some are less detailed in the average anode characteristics graph than the one used.

What is notable in reviewing the large number of articles by hams is the huge spread in apparent response. In this model, interpolation of the characteristic around cutoff is a problem due to lack of data in the datasheet, but in reviewing Internet articles, I see bias variations from -1.5V (K9STH on QRZ @ 1800V+ IIRC) to -35V. Some might regard the QRZ posting as lacking credibility, it certainly isn't supported by the published datasheets which calls into question the implementor's credibility or the datasheets, or less likely, a huge spread in the characteristic of manufactured valves.

The Russian military don't seem to have thrown money after quality in the way that the West tends to do, and It may be that quality is commensurate with the low prices that these tubes command.

Likewise I have seen claims of output up to 1000W per tube, but one questions whether operation at that level on a continuous wave exceeds published ratings. It is hard to know since this tube is designed for both pulse operation and linear operation, and may have the reserve to deliver more than implied by the datasheet.

Links

Changes

Version Date Description
1.01 14/06/2011 Initial.
1.02    
1.03    

 


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