I had the exceptional luck to travel to Oxford in an autistic-spectrum youth project, then to get access to a truly British organ in a most traditional place, and to tinker on it ~10 minutes almost without any guidance ;-)

Balliol College chapel organ, Oxford, UK
1928 Harrison&Harrison, Durham, UK
Electro-pneumatic action [Delay 0.1-0.15 sec]
3 manuals x 58 keys (C2-A6); pedalboard: 30 keys (C2-F4)
Radiating but flat pedalboard = BGO standard

console pistons from left to right, down to up:

Swell [III]
Sub Octave, Unison Off, Octave

Vox Angelica 8 [very delicate, covered by anything else]
Echo Dulciana 8 [quite delicate, covered by:]
Violin Diapason 8 [medium-strong]
Lieblich Gedeckt 8 [less strong, less harmonics]
Gemshorn 4 [medium-strong, peculiar harmonics]
Dulciana Mixture [not very piercing or high; by my ear: 2 2/3+2+1 1/3']
Trumpet 8 [very strong, solo herald-like, not the buzzy-woody German kind]
Oboe 8 [distinct solo, quite strong]
Tremulant [made a weak pulsating mechanical noise, but no effect on pipe sound; maybe it affected only some of the stops that I hadn't tested]

Pedal
Choir to Pedal
Great to Pedal
Swell to Pedal

Open Wood 16 [surprisingly seismically STRONG, also quite rich in harmonics]
Subbass 16 [quite strong, purer fundamental]
Flute 8

Choir [I]
Gt&Ped Combs. coupled
General Foot Pistons
Swell to Choir

Viole d'orchestre 8 [medium, singing]
Hohl Flute 8 [much weaker and pure]
Wald Flute 4 [weak, richer, to go with above]
Clarinet 8 [another singing voice]

Great [II]
Choir to Great
Swell to Great

Double Geigen 16 [medium power, a bit buzzy, cello-like]
Large Open Diap. 8 [surprisingly strong!]
Small Open Diap. 8 [still quite strong and harmonic-rich]
Dulciana 8 [medium power, medium-wide spectrum]
Claribel Flute 8 [much weaker than principals, covered by them]
Harmonic Flute 4 [really pure, to go with above]
Octave 4 [strong, to go with the Diapason(s)]
Octave Quint 2 2/3 [strongish]
Super Octave 2 [stronger, marking the whole chorus]

near pedalboard:
4 unknown foot knobs [the General Foot Pistons referred above ?]
2 volume pedals [likely shutters for Swell and Choir ?]
1 more knob [?]

between manuals:
more buttons, marked 1-3, 1-4, 1-5, some Cancels, more couplers

There were a few surprises in this British design that I discovered after having been accustomed here in Romania to more German-inspired designs:
- Manual order/labeling (Choir down-Great-Swell up)
- Very wide dynamic range, ~1000x sound power between weakest
and strongest stops, de facto on several levels each masking the next
- No Mixture on Great (but the basic 8, 4, 2 2/3 and 2' principals are harmonic-rich enough not to need one - but no high Baroque German sound)
- Swell section with wide palette of timbres and powers, most only 8' pitch [The British Octopod] - to be enriched only by Super/Suboctave couplers
- Choir section quite delicate and singing - not to accompany a large human choir, but to represent one
- No supporting bass reeds: Posaune, Bombarde, Fagott etc. on Pedal or manual but again, principals are harmonic-rich enough together to form a whole
- Pedals quite close; technique and narrower shoes needed for not playing seconds ;-)

It was quite well in tune, except a bit of drift between flue pipes and reeds and between bass and treble [3 hand octaves not being exactly 3 ear octaves ;-)]
A 'Pipe Organ Humidification Unit' by Watson&Watkins Ltd. mounted on top of the blower likely helped.

I had some 10 minutes to test and discover these, without the guidance of a pro organist. Not having much of an 'official repertoire', I still resorted to my pop/rock/metal/light classical pieces of doodlings... from Pachelbel to Bach (&Procol Harum) to Deep Purple to ABBA to Europe to my new improvisation on the Am cycle. Think I've used almost every stop, in various ad-hoc combinations.

There is another older organ (Father Willis, 1885) in the monumental dining hall of Balliol, now rarely used, it seems because it isn't in best shape.
Where they filmed 'Harry Potter'...

I strongly hope I can still get other similar occasions in unexpected places around the world.
Also would be glad to read your comments if anyone finds an interesting organ anywhere !

I love organ spotting whenever and wherever I have the opportunity! I've been researching organs in towns within an hour or two of my small town, with the intention of trying to make arrangements to get my hands (and feet) on interesting ones. I'm aware of nearly a dozen pipe organs in my own town of 30-something thousand (in the southeastern US), and have played the majority, but there are still several right here close to home on my list of organs that I have yet to meet. I play at a church in another town about an hour away, and that town has a few more organs that are on my list - one that I haven't yet had the opportunity to play is a historic American organ from over a hundred years ago (Pilcher - 1 manual, 5 ranks), and the same church that owns that one also has a much larger instrument from about half a century ago (Schantz - 4 manuals including an antiphonal division, 45 ranks). Another thing on my radar is checking out some theater pipe organs (which will be a new type of organ for me to play) - I've made contact with someone who has connections to several venues with such instruments and hopefully will be able to go check out and play some of them in the near future.

Basically, I'm always on the lookout for another great organ! Large or small, new or old, church or theater, tracker or electro-pneumatic, a modern or an historic style, any nationality, etc. One of the great joys of being an organist is that there is always another unique organ with which to become friends, and every new venue where one gets the chance to play will likely have another instrument with which to become acquainted.

Jaani kirik - Tartu, Estonia (St. John's church - Lutheran)
2 manuals x 56 keys (C2-G6). Black natural, white accidental keys
30 pedals (C2-F4)
Alfred Führer, Germany, 1949
Mechanical tracker action {most likely at this size and design}

Hauptwerk
1) Rohrflöte 8'
2) Prinzipal 4'
3) Schwegel 2'
4) Scharff 4f

Pedal
5) Subbaß 16'
6) Pommer 4'

Brustwerk
7) Gedackt 8'
8) Blockflöte 4'
9) Oktav 2'
10) Quinte 1 1/3'
11) Dulzianregal 8'
12) Tremulant

foot controls:
Ped.Kp.HW
Ped.Kp.BW
HW-BW
shutter pedal {for BW}

Principally [;-)] a 4' design, this frugal post-WW2 replacement has an Ikea look, with simple geometry of 'wings' and natural wood colour. Didn't listen to it - but was curious about the Schwegel, Pommer and Dulzianregal (harmonium reeds?).
The whole church is only partly refinished after its WW2 destruction, bricks still mostly visible. At the time of my visit it was used recently as a concert hall (stools for a small orchestra, recording equipment) and also exhibited some weirdly creative/un-Puritan paintings.

Displayed is also a specification and sketch for a new 3-manual, 49-register organ with five distinct 16' [still mechanical tracture!], to be built by Pascal Quoirin, St.Didier, France, to become the only French Romantic Symphonic organ in the Baltic countries. It would cost 794.127 Euro. I gave my modest coin to the Organ collection box.

Ruined for centuries, the Tartu Cathedral (Dom) on the hill didn't show clues what organ it had. Now it's a tourist-visitable 'stabilized ruin' where one can see the sky through Gothic arches and climb inside the two towers.

Other churches I searched from a map of Tartu turned up to be: Russian Orthodox (no organ by canon); 7th day Adventist (open Saturday, not Sunday); Methodist (electronic piano, no organ yet but maybe some plans to get one). Music school also has a practice organ, room locked at the time.

With thanks to same ACE Project for making travel possible.

the European travelers took at least one photo of a huge organ..

let me find it.

Virgin Mary's chapel on Bled Lake Island - Slovenia
A. Dernic, Radovljica - Op. IIII, 1912
Pneumatic action. Delay 0.2-0.3-(0.5) seconds
2 manuals x 56 keys (C2-G6), 27 pedals (C2-D4) straight, German style

Lower manual
1. Burdon 16'
2. Mixtura 2 2/3' {low: II or at most III}
3. Oktava 4'
4. Ostra fl. 4' {"sharp flute" says Google Translate}
5. Principal 8'
6. Burdon 8'
7. Salicional 8'
8. Fl. amabile 8'

9. Super okt. I
10. Sub okt. II-I
11. Man. II-I
12. Super okt. II-I
13. I-Ped
14. II-Ped
15. Super okt. II

Upper manual
16. Viola d'amore 8'
17. Vox coelestis 8'
18. Burdonbek 8'
19. Fl. principal 8'
20. Fl. dolce 4'
21. Prestant 4'
22. Kvinta 2 2/3'
23. Flavtica 2'
24. Terca 1 3/5'
25. Tremolo II

Pedal
26. Burdon 16'
27. Subbass 16'
28. Oktavbass 8'

pistons: Supr.okt.I, O; Reg+, O.; P, MF, F, Tutti, O; Avt.ped., O.

pedals: Crescendo, Zatvornice {Google: "lock", seemed the shutters for II}

This island and chapel is a favorite of whole Slovenia for weddings and a tourist attraction. They use to carry the bride up the 99 stairs. Anyone can also pay 3 Euro to enter to ring the bell. Best luck is if it rings exactly 3 times ! So the guard was a little surprised someone asked to see and play the organ instead...

A bit simplified Romantic-era organ, without the characteristic reed stops. In reasonable tuning and state.
I tested for ~6 minutes. Improved versions on "my" variations on Pachelbel's Canon and my Am improvisation. Glad I made less errors ;-)
A British pop musician also tested some of his own.

Some (but not all) notes on II manual (not I) have a substantially longer delay, ~0.5 seconds ! Pneumatic action of different tube length ? leather ventils close to breaking ?

The Octave, Principal and Mixture are substantially louder than the rest. I expected ~2x louder, but it was ~4-5x, covering most of the weaker stops. "Burdon" is just right for a gentle melody, similar to a Gedackt. The pedals are not much louder.

... So I'm glad this project has allowed me to know more in one of my special interests, the Pipe organ !

Bled, Slovenia - Town church

Ivan Milavec v Ljubljani 1910 op. XXI
2 manuals x 56 keys (C2-G6). Pedal 30 keys (C2-F4)
Pneumatic action {likely from era and features}

1. I Burdon 16'
2. I Trobenta 8' [=Trumpet]
3. I Kornet 8'
4. I Mixtura 2 2/3'
5. I Flûte à pavillon 2'
6. I Cevna flavta 4' [=Rohrflöte]
7. I Oktava 4'
8. I Harmončka 8'
9. I Salicional 8'
10. I Harmon. flavta 8' [=Flûte harmonique]
11. I Gamba 8'
12. I Burdon 8'
13. I Principal 8'

14. Sup. oktava 8'
15. Pedal I
16. Pedal II
17. Sub. okt. zveza II-I [=coupler]
18. Man. zveza II-I
19. Sup. okt. zveza II-I
20. Sup. oktava II
21. Oktavina II [=a partial self-coupler ? different from Sup. II]

22. II Viol. Principal 8'
23. II Burdonček 8'
24. II Viola 8'
25. II Flauto amabile 8'
26. II Dolce 8'
27. II Vox coelestis 8'
28. II Gemshorn 4'
29. II Flauto traverso 4'
30. II Dubleta 2 2/3'
31. II Oboa 8'
32. II Flavta 16'

33. P Principal-Bas 16'
34. P Subbas 16'
35. P Burdon-Bas 16'
36. P Salicet 16'
37. P Oktav-Bas 8'
38. P Cello 8'
39. P Kvintbas 10 2/3'
40. P Pozavna 16'

There are also 2 rotary dials, one on each side, with stops in order of

loudness, likely cumulative selectors/indicators of Crescendo.

A. Zapre {=closes (verb,III sg,present)}
pp II-I, ped-I, I Harm. 8'
pp Burd. 16' Dolce 8'
pp P-II Fl.amab. 8'
p Fl.trav. 4' Burdčk 8'
p Sal. 16' Fl.harm. 8'
p Cev.fl. 4' Viola 8'
mf Subb 16' Salicional 8'
mf Gemsh 4' Burdon 8'
mf Gamba 8' Viol.pr. 8'
f Princ. 8' Cello 8'
f Oboa 8' Okt. 4'
f Fl.a pav. 2' Flavta 16'
ff Dubl. 2 2/3' Oktb. 8'
ff Mikst. 2 2/3' Princb. 16'
Tutti Kornet 8' Burd. 16'
Tutti Sup-sub.okt. II-I
Pleno Trobenta 8' Kvintb 10 2/3'
Pleno Sup.okt.I Pozavna 16'

B. Zapre.
Crescendo II.man. {written near centre of dial}
- Ped.II Dolce 8'
- Burdonb[as] 16' [Ped]
- Fl.amabile 8'
- Fl.traverso 4'
- Burdonček 8'
- Viola 8'
- Viol.princip. 8'
- Salicet 16' [Ped]
- Gemshorn 4'
- Oboa 8'
- Dubleta 2 2/3'
- Subbas 16' [Ped]
- Flavta 16'
- Sup.okt. II

Did not get to test.

Organs like this are good indicators of a community's artistic tastes and financial history - when did they first have enough money and interest to install one, or to replace an older one...

Some mathematical observations on blending stops of different loudness.
Since their phases are independent and often random (due to slight mistuning, accidental or intentional as in Celeste) - their amplitudes add as square-root-of-sum-of-squares for each frequency component.

This makes stops of not too dissimilar power blend still leaving some audible trace of the weaker one, but if the difference is large the weaker one is even more 'covered' (unless it has other distinctive spectral components, like reeds or 'string' stops have).

Example 1: Violin Principal 2x louder than Celeste.
Amplitude = sqrt (2^2 + 1^2) = sqrt (5) ~ 2.236 or 11.8% louder than the louder stop alone

{In this case one should have looked after a Gamba or similar, that may have been _exactly_ as loud as the Celeste, for they to make 'deep' beats}

Example 2: Principal 4x louder than Flute
Amplitude = sqrt (4^2 + 1^2) = sqrt (17) ~ 4.123 or 3.1% louder than the louder stop alone

It results that stops meaning to 'blend' need to be close enough in power. Else they form levels or categories, each covering the lower ones completely or almost. Registration can take this into consideration, and just omit the stops too weak to matter alongside the most powerful ones.