This project was to investigate the efficacy of the GS-34 triode for use as an amplifier at 902MHz. John N8OU had already attempted to use it at 23cm and found the thermal instability to render it unsuitable, and although it was strongly suspected that the 33cm band would also be a problem, there was enough doubt to check it out. The PA shown in the pictures above was constructed, knowing that if it did not work, the work would not be totally wasted because it could be converted to use the GS-15B by adding a few more sleeves and a decoupling capacitor. This project showed, yet again, that triodes are not suited for use at higher frequencies which use hi-Q, narrow bandwidth tuned circuits.

33cm Performance:

Initial tests showed excellent gain, with 10W drive and 2300V on the plate producing 500W out. Input return loss exceeded 20dB. Output tuning had a slight effect on input tuning due to imperfect isolation between input and output as is expected with triode PAs. The problem is that any keydown period exceeding 3-5 seconds started thermal run-away. Plate current increased rapidly and output dropped. This means that the PA could be useful for SSB and CW only. Water cooling did not improve performance as compared to air cooling. This indicates that the problem is, as with all triodes at frequencies above 432MHz (and even some there), related to distortion of the grid causing capacitance changes internal to the tube. At these higher frequencies, very small capacitance changes within a tube make large differences in the tuned frequency of high-Q circuits such as cavities, making it impossible to keep the PA tuned at power levels approaching the actual capability of the tube. The GI-7B, for example, is capable of over 400W out on 23cm, but cannot be run over 200 to 250W output without severe PA detuning.

In this case, with 1600V on the plate and 8W drive, 200W out for over 60 seconds was maintained. Again, whether air or water cooled made no difference. At this point, attempts to use the GS-34 triode were suspended, and conversion to the GS-15B tetrode begun. Pictures of the various pieces showing construction for those readers who can live with the limitations of this tube on this frequency are included. Dimensions can be determined fairly well from the drawing which IS to scale - follow instructions at the drawing download links, below.