Why dont you use a GM3SEK Tetrode board. I have used these on my VHF amplifiers. The regulated protected screen supply is really worth the money he asks for these boards.
The J series of Eimac tubes produce excellent IMD performance. I have a 6 meter amp that uses the 4cx600J using a GM3SEK regulator board. 2 tone 3rd ORDER IMD is -44db below one of two tones, its incredible.
I agree with Tom, the abundance cheaply available Russian tetrodes are really hard to get going with a clean signal. The problem with ham transceivers ALC oveshoot, bad EBS design, poor power control on radios, and generally poor IMD performance from terodes has taken us backwards in terms of reducing IMD on the ham bands.
You hear more crap signals from badly designed tetrode amplifiers than from triode amplifiers. Tetrode amplifiers commercial or homebrew should not be the first amp of choice for new hams. It takes a lot of effort making certain you have a clean signal when using a tetrode. Its so bad now that I can spot people who are using tetrodes on CW and SSB just by listening very carefully using a SDR receiver. The characteristic of RF on the signal and IMD from these tetrode signals makes them easy to spot. Most of these amps have poor EBS design that causes these issues and gives signals such a unique filthy fingerprint. Whatever design you use make sure you dont use any EBS, all the designs have faults and cause problems.
I wish everyone would go back to the good old humble single 3-500 homebrew amps, even the 813's were better than most of these crap tetrode amps using russian tetrodes. If they were all as good as the 4cx350fj, they would of made a good choice.
Recently I have decided that I would try and buit an HF amplifier.
My plans go for SSB output NO more than 1Kw (say 700-800w) and will use tube/tubes. This will be my first HF amp.
I have kept for many years (for future use...) four 4CX350FJ which I dont know if they are any good (externaly are in excellent condition).
Q1. Is there a way to test these tubes (4CX350FJ) and know if they are any good before installing them on the amp?
Q2. Are they a good choice or should I use different tube (e.g. 3-500Z)? Any suggestion is most welcome.
Q3. Bearing in mind that I dont have much experience in such amps, would it be a good idea to built one from the scratch or buy an old cheap one rebuilt/restore it?
Thanks for your time