So.. what you're saying is after two pages of jokes about the OP's question, there is an actual transistor that can replace a tube.
Yes, there is a high voltage MOSFET that can replace a tube as an amplification stage. There were a few jokes to see what his application was to get there and there's a pretty big difference between a 9v 5088 stage and a full voltage tube circuit. So the real answer is, no there is no tube that can replace a 5088, but there is a work around IF it's an AMPLIFIER you're replacing. BUT, it turns out that even after making jokes to get more information, we weren't given all the information... we had to know about another thread.
So.. what you're saying is after two pages of jokes about the OP's question, there is an actual transistor that can replace a tube.
But it's not a 5088 that's being replaced, and has nothing to do with what the op is looking to do.
The transistors used in the BYOC 5-knob compressor aren't gain stages. They are emitter follower stages and one is a variable resistor (control voltage fed to the base).
deparisn, as I explained in detail on your BYOC thread, the 5-knob compressor's compression is done by varying the bias of a CA3080 chip. The chip is your amplifier, and the amplifier part of the chip is an operational amplifier, which involves quite a lot of circuitry. The CA3080's datasheet gives you its internal circuit (on page 3) -- it's 11 transistors and numerous other components. I'm not smart enough to tell you how to translate the internals of that op amp to a tube, and I don't mean to seem rude, but your questions on the forums have indicated that you are probably not advanced enough to do so, and I would be absolutely flabberghasted if anyone has bothered to create a tube-based CA3080 circuit already, although you could look up some very, very VERY old schematics for a tube-based operational amplifier and figure out how to implement all the pins of the chip.
The bias pin on the chip is the source of the compression. A tube by itself would have nothing to do with the compression.
The transistors in the dynacomp circuit aren't part of the sound. The only ones in the audio path are buffers. You can build a cathode follower and an emitter follower and assuming you build the circuits properly, you will end up with a near-unity gain buffered signal in both cases. I have a feeling this won't tell you anything. And I don't think a tube stage can be used as a variable resistor in the way that a transistor can, at least not without huge changes everywhere in the circuit.
If you just want to analyze the difference between a transistor and a tube, you can scope the output of two circuits created to have identical responses: Create them to have identical output and input impedance, and feed them a sine wave signal that is proportionally the same size in relation to the supply voltage the circuit needs to operate.
So the answer is still no, because we just ran with the assumption that it was an audio gain stage. Building a high voltage dyna comp and swapping in MOSFETs and tubes isn't doable.
I for one am not psychic and didn't read or know about the other thread.
Yes, he mentioned compression, but when engineers speak of compression (which we did have that information) they aren't speaking of a compressor like a dynacomp, they're speaking of the onset of clipping where the tops and bottoms of waveforms start being squashed and you have a drop in output power.
http://www.microwaves101.com/microwave-encyclopedia/320-compression-pointSo it is perfectly reasonable for me to, once I've been given the parameters by an engineer, that they want to test the compression characteristics between tubes and transistors to say "well, you can test the differences between tubes and MOSFETS in the same circuit, maybe you should adjust your test situation" Of course, no you can't replace a CA3080 with a tube, it's pointlessly complicated with too many variables to be a decent test.
Of course as a microwave R&D technician, dragging the the whole story out of PhD engineers is what I do for a living, so I guess I should have known there was more to the situation than what was being stated, there's always a piece of information somewhere else that you aren't being told about.
This thread has taken a pretty ugly turn with the shaming of folks that are just trying to help somebody out and drag some information out of them.
edit: Perhaps there should be an entire class added to the engineering curriculum teaching engineers how to communicate all of their needs to technicians and each other. Not as a slam to the OP, but engineers in general, they seem to be pretty bad at that sometimes.