Electronic (valve) home organ Restoration

[Phil G4SPZ]

Thanks for this interesting thread, Lucien, and looking forward to hearing a tune at the end of it. Keep it coming. I achieved Grade 1 organ aged 40+ but never learned to read music as I found from a young age that I could play by ear...

[Lucien Nunes]

Turns out most of the dead oscillators were dead because the coil was O/C at one or more ends. It looks like the flux has eaten through the wire at the tag solder joint, so I've added a 1/8" stub of TC wire and soldered the winding to that. C#, F & F# are now sorted. D & D#, now they are continuous, seem to have some shorted turns in the cathode section as the resistance is much lower than any others. However, the overall inductance of D is about right in the sequence and D# not far off, so I will test them before investigating further - they might be wound with different wire.

E is more of an issue - it was O/C in both directions at the cathode tap where the ends leave the bobbin. I cannot pick them up even under a microscope so the outer section will need to be rewound. That can wait until I am near the coil winder, E is not my favourite note anyway.

[Phil G4SPZ]

Originally Posted by Lucien Nunes

...E is not my favourite note anyway

Me neither, it leaves me flat

[Herald1360]

Phil, you're so sharp you'll cut yourself

[Lucien Nunes]

Naturally, having all the white notes in tune will avoid frayed tempers, but as one seems to have sustained accidental damage, we must accept a short interval during which its performance will be diminished.

Now tuning stability is one of the reasons for using dividers. You'll notice that with the same number of valves the organ could have been equipped with 72 oscillators, instead of 12 and 60 dividers. But down at the bottom end, 32.7Hz on this organ (16'C a.k.a. C1) values start getting rather extreme and generally it can be a challenge to get things to track e.g. with temperature. It is vitally important that the octaves stay bang in tune and the only way to assure this in a basic instrument without going to elaborate lengths to achieve stability, and considerable extra cost anyway for the necessary coils, is to divide one octave of oscillators all the way down. The more elaborate the instrument, typically the greater the number of sets of oscillators provided. You might get one set per division, one per stop or whatever - the more oscillators in use, the fewer tones locked rigidly together in phase and the richer and more realistic the sound. The workaround for dull, locked-phase sound was the moving loudspeaker - Allen Gyrophonic, Compton Rotofon, Leslie (on chorale setting) etc.

Although subject to the same locked-phase issues, tuning stability was one of the noted advantages of electromechanical tone generators particularly in the early days. The relative tuning of a Hammond cannot drift because it depends only on gear ratios; in a Compton, Dereux or Electrovoice (all of which use belt-driven generators) a tiny amount of short-term drift can occur with temperature affecting the lubricant viscosity but it's seldom audible. Similarly, the absolute tuning of a Hammond is locked to the mains by the use of a synchronous motor, as are many Comptons. The others use induction motors that can be trimmed very slightly by varying the voltage (and hence the slip).

[dragonser]

Hi,
this is an interesting thread !
I recently looked at Electrochord Bambi Organ which was early transistor ( 1960's ) . Luckily there wasn't much wrong with it !
there were lot of kit Organs available in the 1960's / early 1970's
there was also a book written by Alan Douglas which gave a design for a Valve two manual Organ with pedals. There are also two later books with Transistor designs in them.
regards, Peter B

[Radio Wrangler]

I think the Farfisa organs went in for a lot of individual oscillators rather than dividers. Along with early transistors and dodgy construction, they got an extra dose of 'Character' which was exploited by the likes of Rick Wright, though I think it was price and lightness that sold it to him.

There's one guy Nick Hirst, on youtube playing something from this era on a Farfisa Compact Duo, as it was originally played...and wringing all the special sounds out of it, and doing a brilliant job:

http://www.youtube.com/watch?v=9egi4-GjBBc

A trip back to 1969 or so and off to the furthest reaches of the universe.

DAvid

[Lucien Nunes]

I don't know about all Farfisas but the ones I've seen, indeed every combo organ I've enountered, used dividers. The Farfisa Professional series gave a few hints that it might have multiple generator sets, e.g. separate vibrato on each division. But these were post-processing phase-shifters not true vibrato. Combo organs using germanium dividers are often in need of many transistors these days!

[Radio Wrangler]

I've never been inside one, but I've heard they take approaching an hour's maintenance per hour playing - just not in any planned way. As an early Floyd fan, the sounds are evocative.

A divider organ with Germanium transistors sounds like a case needing some redesign of the bias and then a wholesale application of 2N3906.

Cheers
David

[Herald1360]

Originally Posted by Lucien Nunes

Naturally, having all the white notes in tune will avoid frayed tempers, but as one seems to have sustained accidental damage, we must accept a short interval during which its performance will be diminished.

Errr, groooaaaaaannnn, I think

(natural, temper, accidental, interval, diminished) -- Have I missed anything?

[cmjones01]

That was a skilful way of avoiding a conversation full of musical puns, by using them all up in one sentence!

[Brigham]

SUSTAINed, possibly.

[Radio Wrangler]

I think we'd better resume normal technical discussion before a rather interesting thread gets, um, stopped.

Anyway, inductors with a cathode tap sort of spell Hartley oscillators to me, and they ought to be fairly well behaved.

David

[Lucien Nunes]

We would indeed be ending on a sad note if we got the third degree over a minor lowering of tone.

Yes, Hartley, quite common in musical instruments, especially in divider generators where the waveform of the oscillator itself may need to be shaped to match whatever the dividers produce. In some circuits, the oscillator runs at twice the highest frequency needed and its output is used only to drive the first divider, from which the top note is taken. This avoids the difficulty of making them similar.

There were all sorts of other circuits used for various reasons, either to produce a particular waveform or to provide greater independence of frequency from load changes. For subtractive tone formation it is often helpful to start with a sawtooth, as both odd and even harmonics are present. For certain families of pipe and orchestral instrument tone, such as stopped pipes and clarinets, an approximation to a square wave where odd harmonics dominate is a better starting point. For additive synthesis a pure sine wave is needed.

One method used especially with semiconductor divider organs is staircasing, where the dividers are all started in phase and added with equal weight to produce a sum increasing monotonically with each cycle of the top octave. This provides an approximation to a sawtooth of good linearity, the fundamental of which is decided by the lowest divider output selected.

I'll take some oscillographs of the generator outputs when on test. In the meantime here is the failed coil dismantled. The arrow shows where the tap leadouts should be.

There won't be any progress on this for a day or two but I will take the opportunity in the next post to show-and-tell about an organ that is as different from this one as it could possibly be.

[Radio Wrangler]

Ah, that looks like a Mullard 'Vinkor' pot core with its metal housing.

The centre post should be ground down below the level of the rim to give it a gap. This reduces inductance, acts to combat saturation from DC component in its current, and makes the inductance much more stable.

Some grades of ferrite (EG Siemens N28) just happen to have a temperature coefficient almost precisely opposite to polystyrene capacitors. This might prove helpful.

The hole in the centre of the core can carry an adjuster. Inductive twiddlers are more reliable than trimmer capacitors as they don't need a sliding contact for the rotor. Though this coil seems to have no adjuster.

The AL value (gap width expressed as nH for a 1-turn winding) gets chosen to give a handy number of turns. One old trick is to pad the bobbin with insulating tape so the turns are further from the fringeing flux of the gap... this aids Q.

That should be a doddle to re-wind.

David

[Lucien Nunes]

The coil does have an adjuster, while the polystyrene capacitor is fixed. Both differ in value for each note, to maintain approx constant L/C, the caps being mostly non-standard values. Earlier organs often used iron-cored inductors, which typically had higher tempcos than the ferrite, that did not compensate the capacitors. I would assume that these ferrites are chosen to suit PS tempco, it will be interesting to see how stable they are on the bench wrt temp and supply volts.

I don't want to set the whole thing up with marginal valves, especially in the oscillators, and about a third of the originals have been changed already. The first few I wound through the AVO revealed that those are down to about 2/3 emission, so the remainder might be purged from the oscillators at this point in favour of the best replacements.

Now, as an interlude and contrast, here's an instrument made with the same objective in view, i.e. to play classical organ repertoire, but at the other end of the scale cost-wise and size-wise. It's a Compton Special Series organ of 1966 (compared to 1962 for the Burge) with 365-type generators and a 3-manual drawstop console.

I won't go into detail about the Compton generators and voicing system here as there are dozens of pages of descriptions on the EK website here: Description of Compton tone generation at Electrokinetica although as this particular organ is a recent arrival it's not shown there yet.

Compare the following points:

*The Compton console is only a 'user interface' - no audio or electronics inside, just thousands of contacts, relays and solenoids.
*Even in the wardrobe-sized generator cabinet, active electronics are used only for amplification.
*Generator is purely additive - no audio filtering or wave shaping required.
*Stops synthesized from harmonics and near-harmonics within even-tempered scale.
*Synthesis is DC voltage controlled, allowing envelope control (impossible on the Burge).
*Voltage-controlled synth system requires over 5700 relay contacts, hence cabinet size.

Quite a different beast, I think you'll agree. You might spot the comedy valve line-up in the amp. That's the least of the problems - the cans of the cathode bypass caps are rolling about in the bottom of the cabinet and the screen stoppers are black and crispy. But at least no osc coils to repair.

[mole42uk]

A while ago I mentioned a DIY organ used at school when I was about 10 or 11.

I have recently noticed that there is a Practical Wireless Blueprint "The PW Monophonic Electronic Organ" (c. 1961) - has anyone seen this one? Was there a polyphonic design as well?

Curious!

[Lucien Nunes]

I would think a polyphonic organ quite a meaty project for PW so it will be interesting to see what if anything they did publish on the subject. However the circuit is arranged, even a basic polyphonic spinet organ with 44-note manuals will require a minimum of 28 valves for the tone generation alone. Nonetheless, the difficulty of fabricating precision parts for an electromechanical generator led to home-builds invariably being electronic throughout, although there was steady competition between electromechanical and electronic generation in commercial products,

Below is a picture of a home-built divider organ; I haven't seen this in person, only in pictures, and I know nothing of its provenance or design. You will see power supplies in the base, the middle shelf containing the generators stuffed full of 6SN7s, and at the top you have from left to right the power amp, preamp / voicing and on the right an electromechanical vibrato scanner and its delay line. It will have been broken up for its valves and transformers by now. At some point I must try to save a home-built example.

[Lucien Nunes]

I haven't abandoned this thread / project, unfortunately all vintage projects are on hiatus due to an excess of real work. Will be back on it in a couple of weeks once a few of the current jobs are completed.

[Hunts smoothing bomb]

I'm watching this thread with great interest as I am a hammond player myself.

cheers.