• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

3 x 3 Tube amp 30 watts

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First try with Tubes and the sound is outstanding
I now have to build the board for it.

12AX7 Voltage amp stage followed by 6922 or ECC88 driving NPN transistors
Surprisingly it didn't need any global negative feedback
I also have to prepare a case for it !!
 

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I would do it with ECC88 front and swap the 2N5551 phase splitter with another ECC88 to make it all ECC88 tube (honestly, it would be 6N23P). I don't think i need the 12AX7 gain. Maybe B+ can be lower a bit that way.

How do you determine "it didn't need any global negative feedback"?
 
Listening test, backed up by good looking waveforms on the scope.
12AX7 sounds really nice. 2N5551 replacement may add extra tube sound but given gain there is unity I don't think it matters much.
Tube swapping may change the tone noticeably
3 x 3 name comes from 3 triodes and 3 NPN transistors
 
I would do it with ECC88 front and swap the 2N5551 phase splitter with another ECC88 to make it all ECC88 tube (honestly, it would be 6N23P). I don't think i need the 12AX7 gain. Maybe B+ can be lower a bit that way.

How do you determine "it didn't need any global negative feedback"?

I agree. 75K does not give the best linearity for ECC83. You obviously made it low to lower the gain. So use an 6N23P which has lower mu, and likes lower voltages.
 
Is there any risk of damage to the output transistor from EMF spikes from the transformer? ........I have not been able to kill it going no load to short at full output.

I have made some similar amps. I tried BJT/tube "darlington pairs" and mosfet/tube pairs. I liked the sound of the BJT better, but hard clipping with a guitar preamp plugged into the input and a guitar speaker load would blow one of the BJT's every time. (MJW21194's). The mosfet version was blow proof but I used some huge 900 volt Fuji Mosfets that are no longer available.

Super Tubes | Tubelab
 
If each transistor dissipates 10W (50ma x 200v) and each triode is about 1W (best guess from the datasheet), we've got 22W class-A, max. Letting the real world in, we would probably get a 240v p-p swing across the OT. With a voltage ratio of 20, that means 24v p-p at output. Power would then be about 12.5 watts if my math is correct. That's about what I simulate for my hybrid power pentode design, although mine is at less voltage and more current.
 
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