1. Most of them are medium-mu or high-mu triodes, or else tetrodes with very high transconductance. This tends to result in poor linearity. Tetrodes are usually ok if operated push-pull.
2. We are talking about power supplies of 5-20 kV at 1 amp or more. Filaments need scores or hundreds of amps. The power transformers end up weighing 200 lbs or more...
3. The output transformer will be huge, and VERY costly to make.
4. Large blowers for forced-air cooling, or else water connections for water cooling (and big water bills), are mandatory.
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