Sometimes the removal and re-install of tubes will clean up (deoxidize) the sockets.  Maybe that got it working.

Depending on the droning sound, it could be a microphonic tube.  Try tapping them gently one at a time, or holding them with a rag or something to damp the vibration but keep from getting burned.  Generally, microphonic preamp tubes will be higher pitched whistling or squealing, where power tubes will more likely rattle or make low pitched rumbling sounds.

The E to B voltage of T4 (0.3V) seems to not be enough to turn it on.  Maybe checking directly with probes at E and B will give a more accurate reading than checking voltage to ground.
The voltage to turn it on should be developed across R5.  That is created because of current through T1.  What are E,B,C voltages for T1 and T2 ?

Do you have those reversed or is it a typo?  T4 should be the TIP29.

That's not the right schematic for the G.  It's actually for a 1000S (printed on sheet).
100G schematic attached.  Shows 600 ohm impedance for the tank so 4EB2C1B tank should work.

Can you post E,B,C voltages for TR4 and TR6 ?

It is very common for the FM212 (and 65) to develop problems with the differential stage of the power amp (Q9 and Q10), which needs to be well balanced.  That could very well be the cause of your distortion issue, but even if not, it still needs to be dealt with.
Check R83,84,85,86 and replace any that are out of spec.
Replace Q9 and Q10.  This may be tricky as they are no longer made and there will be a lot of fakes available.  Try to get them from a reputable electronic parts supplier, not from Amazon or China.  * edit:  looks like Audiolabs of Georgia has them and they are a legit supplier.
It may be better to find a modern production sub for the 2SC2362K.

Yes, either will be close enough.  Looks like 8EB2C1A is available.

Quote from: Pete on March 22, 2024, 05:19:52 PMSooo ... the 4EB2C1B is an interesting suggestion. Can this be mounted either way? – Open side up or open side down?

I don't think they are all that fussy about it, I have seen manufacturers using tanks 'upside down' to what they are supposed to be.  Maybe when they get old and saggy they might start to bottom out.
But if the correct one is available, you might as well get that.
Studio Sound Electronics seems to have them all, including the 4FB series which would be an even better impedance match.  And they have them available in several decay times.

This listing shows a Gibbs 2 spring unit with about 200 ohms DC resistance at each end.  That would put it in the 4EB or 4FB range of tanks, I think a 4EB2C1B should work.

https://reverb.com/item/51673468-acoustic-control-corp-260-150-136-amplifier-gibbs-hammond-reverb-tank-unit-1969

Service manual attached.
This one is solid state so I don't think either of those tanks would be correct, but it is possible.
Reverb circuit shown on pdf pg.17
Drive is taken through cap C205 from a 40408 transistor.  Return into a MPSA05.

Quote from: JonnyDeth on March 20, 2024, 02:56:58 PMWell, looks like I wasted all that time going to college for electrical engineering, thanks for the clarification 

If you can't apply it, I guess you did.
Amplifiers are rated into resistive loads to take reactive power out of the equation. 
Note that I specified amplifiers in my earlier comment to exclude what you said about speakers.

When one walks into a new room, it's best not to have the flamethrower on and start out with the idea that everyone else has it wrong.

Amplifiers are rated RMS into a resistive load, so VAR is not a factor at the amp side of things.

Yes, sine wave is AC.
Normally we call it a 'signal generator' but it is also an AC voltage generator.  Amplitude and frequency are adjustable.
Function generators may also have other kinds of waveforms like triangle or square, which are alternating but not 'pure' AC.  Pure AC is sine.

How are you going to get both wet and dry outs from the Orange?

The T4 voltages indicate either T3 is bad and not letting T4 turn on fully, T4 is bad, or something is wrong elsewhere in it's string (shown in red).

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