I4FAF Amateur Radio HF "really" linear,  rugged and trusted,  Tetrode Tube Linear Amplifier experience.

Our HF TX system philosofy has been, since many years, to  have available an amplifier which needs a drive level  (ONLY when necessary) around  50 to 65 watts MAXIMUM  as RF input  level (to get the maximum power available, and legally permitted, from a linear amplifier accessory), or LOWER, for even better IMD, lower  harmonics levels, and for longer equipment  life and enjoiable use of the system with lower heat from the transceiver-exiter finals. To achieve our goals we used a tetrode tube with 50 ohm grid input drive which has also  lower High Voltage Anode requirements, (we used a conservative anode voltage value of about V=2100-2200 under load, or about V=2500-2600 no load obtained with a power supply transformer with a secondary ac value of 1850 V x 1,41 with full wave rectification)  strictly well inside the maximum Voltages specifications of the tube manufacturer for longer tube life, with stabilised-regulated voltages and overdrive protections. I4FAF started to investigate, along with his friend Paolo I4PZP (owner of HAM CENTER SRL in Borgo Nuovo di Pontecchio Marconi (BO), professional Philips glass tetrode tube for Amateur Radio  in 1974 (after a comparative evaluation of the well famous triode 3-500Z or better 3-500ZG, graphite). Now we use a ceramic-metal rugged tetrode tube. Of course do not use more RF output power than reasonably needed to achieve a qso!.  Try first low power: often  it is more enjoiable to get a qso with a lower power level than the maximum you have available. The transceiver needs to be equipped with a good RF output level control, not only in CW or  during tune, but also in SSB to keep always under control the drive level.  Remember to use a good PEP RF Wattmeter to monitor the power output level when operating in phone SSB mode.  A good low pass filter accessory  that attenuates  harmonic further above 30 Mhz  can be of help.  Our experience proves that we have been able to work all the DX, Expeditions and Contests as well with the one 4cx800A version, but as in a nice FERRARI  car, if you have a fine drive control you can run lower power also with a strong and trusted linear ampl. still with benefits and longer equipment life. Remember that  experience in HF amateur radio operations and improvements in the antenna system can often make the real difference.

A beautiful "ensemble" view about HAM CENTER SRL (Bologna - ITALY) variable capacitors specifically designed for HF RF Power amplifiers with TUBES My father I4FAF helped I4PZP to develop these rugged mechanical units.

your right: close detail of specifically designed split section to better tune 10 meters

your left: one more detailed view of split section variable capacitors to improve 10 meters (28 Mhz) amateur band tune (with lower residual capacity).