1950's Wurlitzer organ somehow produces an AC signal of the frequency of a vibrating reed by using the reed as a varying capacitor supplied with 200v DC buffered to reed by a Pi-network.

Can't see how this circuit can produce an AC tone signal?

Their circuit description is on page 7 here: https://archive.org/details/wurlitzermodel44organservicemanual/page/n5/mode/2up

It's the same principle as a condenser microphone.

The reed is insulated and connected to the load resistor (which I suspect is common to the entire reed bank). Each reed forms the "lower" side of a capacitor.

When a key is pressed, a voltage is applied to the "upper" side of a capacitor (i.e. a small metal plate placed near the reed), through a circuit which controls the rise time of the voltage in order to avoid clicks.

When the reed vibrates, the capacitance between the reed and the upper capacitor plate varies in proportion to the instantaneous distance between each.

If the upper side of the capacitor has a voltage on it, the variation of the capacitance generates a tiny varying current in the lower side. This is picked up by the load resistor and amplified.

Oh, by the way, as you may be aware, Wurlitzer used to make large organs for silent movie theatres.
I only realised, after seeing one of these restored and working in a local Art Deco theatre, the origin of the term "All the bells and whistles". It was used to refer to a fully optioned Wurlitzer organ, with a full set of pipes (whistles) and a set of bells that could be played from the manual.

Yeah, their "circuit description" doesn't really explain how it actually creates a signal. And the owner who just got this organ, though being a techie himself, didn't know how it works, so he asked me as we both look at Service Manual and I say "I dunno either!"

But when you say "condenser microphone" I have to think "of course!" it must be the same! But, truth be told, I've never considered how a condenser microphone actually works and probably couldn't have explained it if put on the spot, maybe because we're not accustomed to thinking in terms of capacitive loads?

When the circuit description calls the 1 meg resistor in the amplifier the "load resistor" I think it's misleading as I would call the 9.1 meg resistor in reed biasing circuit the true tone generating load resistor?

It makes sense when you say that those filter networks are merely for key-click suppression - no doubt to avoid the 'problem' their major competitor Hammond had at that time! They went to a lot of trouble with this as they say there are 165 such filters installed

There seems to be no series 'mixing' resistors for key outputs for polyphonic play?

Now back to the physics of actual tone generation:
As reed is closing (capacitance increasing) current through 9.1 meg resistor increases? (as reed cap. is acquiring more charge?)
But what about the other half of cycle where (charged?) capacitance is decreasing: How would this cause corresponding decrease in current?

"There seems to be no series 'mixing' resistors for key outputs for polyphonic play?"

There is but one! It looks to me like all the reeds (or at least a group of them) are connected in parallel. The common load resistor is the mixer. Simple!

Re the + and - current, maybe the best way to think of it is remember that energy is never lost, it is just converted to another form. So, if you pull one side of a capacitor away, the energy previously stored has to go somewhere....

Here's a link which might help:

https://www.mediacollege.com/audio/microphones/condenser.html

Re the Hammond organ, the key clicks along with the Leslie speaker are part of its charm!

They say this model series sounds most like the original theatre pipe organ! (I suppose both use actual reeds as sound source)
And this one, even by 1959, was still built in original Tonawanda, NY factory (by the 60's production was shifted south to save)
Also this 1959 one has more powerful Amp using 7027's rather than the earlier 6L6's version (luckily I had some 7027's)
Also missing was 12" JENSEN speaker (naturally) but smaller midrange JENSEN is still present.

That is a special organ indeed, the model 44, as it has the earliest electronic vibrato circuit using a phase shift technique - predating the other main technique used by Bonham and Magnatone. It's not clear who in Wurlitzer, or consultant, developed the vibrato circuit, although Dorf presented an article on it in 1954, and the next most famous user of the technique was in the VOX AC30 guitar amp later in the 1950's. That circuit technique is still in use today in a plethora of guitar pedal circuits, albeit with ss devices and smt caps and resistors!

It is amazing what "new" technology reappears using the principles of the old. I likely still have the circuit but Christie's Wheatstone promoted bridge appears in an antenna rotator I repaired. Its input was applied to a 741 OP amp. If the op went one way it switched on an NPN transistor and a PNP the other way & that caused the motor to go either clockwise or anti clockwise. When the bridge balanced the OP nulled it stopped.

Crude, simple, & effective.

From what I've heard so far, its tones are very clean and pure. And it's a blonde cabinet as well, that matches the blonded wood trimming of the new owner's 1959 house! Will post a youtube tune link here when it's running.

I would strongly recommend checking if the organ has had its power supply restored, and not just the electrolytic caps, but also with added 1N4007 in series with each valve diode anode, and an appropriate fuse added to the HT secondary CT. A lot of collateral damage could occur if the valve diode or output stage fail in a bad way.

I have a 1958 electronic organ - and there are few remaining from that early electronic era.

A fuse can be very overrated as a protection device. One of the things is its rating. Often you have to use slo-blow as the biggest problems can be caused by the initial surge, or "inrush" from Silicon diode rectifiers & charging caps. So even a HT fuse can fail to protect.

Mains side is even worse. I have seen quite a bit of damage over the years where RCD's and fuses have failed to trip & blow because the fuse or, RCD was on the primary side and there was no "earth leakage", nor overload.

The last Thorn radiogram I repaired, has a 5 Amp fuse on the primary. There was a chain reaction fail, starting with a 6GW8 causing the rectifier to fail short (metal) the secondary then melted down damaging the fuse holder, but the fuse never let go.

The 2 1N4007 rectifier diodes now have very low watt 100 Ohm series resistors "sailing with the wind" so they burn on overload.

Yeah, a lot of things were not well fused in the 1950s. Will have to see what its power amp has in this case.

This organ is running now. It has superior, pleasing classical organ voices as well as a "Fast Attack" Tab for jazz/Pop music, which I would say sounds about "right" but is not as aggressive or funky as a Hammond which remained the top choice for those musical genres.

Fusing nowadays has advanced, both in defining the performance of the fuse, and in designing the fuse type and rating needed. Unfortunately a lot of vintage gear only had primary side fusing, and many restorers just find a fuse in the bottom of the cupboard and use that.

Link to article on fusing for valve amps: https://www.dalmura.com.au/static/Valve%20amp%20fusing.pdf

Only safe and reliable way to fuse sub-100VA transformers (and even larger ones) is with a thermal fuse in contact with the windings.

Any conventional fuse large enough to be reliable in service will remain intact at currents that will slowly cook a transformer to destruction.

Imho that perception is based on a lack of fuse design awareness, fault cause awareness, and inappropriate fuse part use. People still buy generic fuses from Jaycar, and put a finger in the air to gauge the fuse rating.