Electronic (valve) home organ Restoration

[Lucien Nunes]

I'd post about this on an organ forum except that most of the readers are mainly interested in certain specific models, mostly from the US, and not so much in the electronics anyway. Following a reference I made in a post earlier, I wondered if anyone here is interested in the electronics of some of these old beasts?

I have a collection of about 40 instruments, many of which are Compton Electrone models that have little in the way of conventional active electronics as the tone formation is done with electrostatic generators and giant networks of passive components. However some of the other makes are electronic throughout, and one early 1960s all-valve instrument made by Burge of Watford happens to be in the workshop at the moment. It's tiny and basic - one manual, no pedals and a mere 15 speaking stops, intended for home / practice / very small church use. It contains around 45 valves and should be a breeze to repair before it gets moved to the display, as I will take a punt on it not needing a full recap.

If it's not too far O/T I'll post words and pictures, maybe even a tune at the end...

[McMurdo]

Very welcome here as a tonewheel player and enthusiast!
Was sent out to service an electrone a few years ago but politely passed it over when I learned of the symptoms

[Lucien Nunes]

Great! Let's start at the beginning with the amp / PSU chassis. Nothing terribly remarkable here, GZ37 rec (note to self, care when inserting as spigot missing), EL34s in UL with shared auto bias, ECC82 paraphase driver. Note large mains TX with extra heater windings - 12.6V @ 7.2A and @ 3A for the tone generators etc. There are a couple of smoothed HT outputs for those too. In a higher spec instrument the oscillator HT would be stabilised but here it's just filtered separately.

Need to test / replace caps (some have been replaced already) and make a better job of fitting reservoirs etc. Might as well test the valves. Cathode bypass looks original, date code TCB/SL = Dec. 61 which is reasonable.

Update follows when I have sound through it...

PS Mods, is this in the right place?

[Ed_Dinning]

Hi Lucien, I have some stuff on Phillips as well as some US stuff with English readers intros. I'll see what I can find but it may be a week or so.

Ed

[Herald1360]

I like that nice simple Power Amp chassis.

Life being what it is, you could probably sell that for many times what you could get for the complete organ

I'll look forward to hearing it.......

[julie_m]

I for one would certainly be interested to see something of how these instruments actually work.

[Lucien Nunes]

This one is what we would term a 'divider' organ, because all the necessary frequencies are derived from a set of twelve 'top-note' oscillators, one for each step of the scale, followed by a chain of divide-by-twos. In this case there are five dividers per note, making a total of six octaves. Twelve oscillators are needed because adjacent notes are related in frequency by the 12th root of two, which would require an unreasonable number of dividers to derive from a common multiple (although this method is used in IC-based organs of the 1970s).

From the generators, the tones are keyed directly by the key contacts and then filtered by the voicing circuits to produce the necessary timbres. This is generalised as 'subtractive synthesis' in which complex waveforms are filtered, in contrast to 'additive synthesis', as used in tonewheel Hammonds and Comptons for example, where harmonically related simple waveforms are summed.

In either case, one set of oscillators or generators confers the serious limitation that all harmonically related pitches are locked in phase, which tends to make the tone dull and artificial-sounding compard to a pipe organ in which each pipe is its own oscillator and filter. Nonetheless most older commercial electronic organs have only one, or a small number of oscillator sets, simply due to the cost and size of any scheme with individual oscillators per 'virtual pipe'.

This diminutive organ will be a good place to start demonstrating the concepts. I have much larger ones waiting in the wings for attention!

[mark pirate]

I will be following this thread with great interest, I for one find these old instruments fascinating.
I have read a lot online about the Hammond Novachord from 1939, this was the world's first synthesiser. It contained no less than 163 valves and well over 1,000 capacitors!

Restoring one of these would be the project of a lifetime, if only I could find one....

Mark

[Mike Phelan]

I used to work for a Hammond dealer; as well as renting and selling radios and TVs, they dealt with many organs; Compton, Gulbransen, Lowrey (weird valves), Bird (neon dividers) and Selmer, to name but a few.

[Lucien Nunes]

The Novachord is quite a lump. And some of those caps are weird values, which must now be made up of multiples. Stability is crucial so you can't get away with cheap stuff. A couple of my friends have restored Novachords, I don't expect to own one any time soon so I don't have to worry about the work involved!

One of the other small instruments in the collection has neon dividers. The badge says Hohner but I wonder how much of it will turn out to be Philips. IIRC that is a hybrid, the oscillators are transistor, the dividers neon, the audio system valve.

Returning to the Burge, the people I got it from, unfamiliar with the possible pitfalls, had powered it up briefly. It made various noises, the usual hum and crackles and some notes. The reservoir cap has a hole in the side and is O/C. Some of the other electrolytics test OK, others are a bit leaky but might reform. However they are non-original so I might as well replace. A quick random check with an insulation tester yesterday confirmed my expectation that most if not all of the Wima film & foil caps are fine. The output couplers, 0.15@400V were showing >5GΩ on 250V test and their encapsulation looks sound and free of cracks.

The other Burge organ in the collection is the 3-manual job shown in the attached pics. If you look at the row of generator PCBs you will spot mostly metal-cased caps, Dubilier IIRC, rather than Wima and Philips. That does not bode well, some are non-critical and can probably stay but they will make it an order of magnitude more effort to get that into shape.

[Lucien Nunes]

The amp is finished and ready to re-fit in the organ. The first thing I did was to make a quick sketch of the schematic and connector pinouts. Unlike the mass-produced US models, service information is rarely available for these instruments, so we must recreate it as we go along.

Specific faults repaired were:
Reservoir cap O/C (the capacitance had escaped through a hole in the side!)
Cathode bypass cap on its way out.
Intermittent track on PCB due to previous hamfisted repair.
Paraphase grid divider up from 270k to 430k & 100k anode load at 380k .
One EL34 bad.

In organs, anything wrong with the power supply can cause real headaches - especially here where there's no stabilisation - so all the HT electrolytics, ranging in condition from dead to possibly OK, were changed as a precaution. In any case none of them were original and some had been poorly fitted. The main smoother had been made up from two 50µF 350V caps in series, with 100k equalising resistors, a scheme that I doubt was original because they were the only Erie resistors in the instrument. One was spot-on value but the other had gone up to nearly 200k. Predictably, the cap on that side was in better form than the one shunted by the lower resistor.

For reservoirs on HT rails of 200mA and over, and any demanding situations, I tend to use the Evox-Rifa (now part of Kemet) industrial electrolytics. They are expensive but have distinctly superior specs. This 33µF 450V part is rated for 300mA ripple, 125°C and has a rated life of 27,500 hours. They overcome the need, with smaller capacities, to use multiple parts to obtain the ripple rating. For the smoother and decouplers I use the old favourite Philips, now BC Components, erm, Vishay axials. These are sadly losing their distinctive translucent blue wrap as they change the branding over to Vishay.

The 100k I replaced with an Arcol RCC050 0.5W carbon comp, available from RS and similar sources. These are great for repairs of this vintage because they look just like original Allen Bradleys and, apart from the glossy finish, respectably like the Morganites used throughout this organ. In the tighter shot of the finished amp you can see it to the right of the ECC82, where it looks quite at home. Unfortunately they don't do a 270k so I've put a modern-looking metal film in that position for now.

The drive balance is acceptable rather than great. I briefly tried touching a couple of resistors across the inverting driver anode load, 150k tidied the balance up nicely. I might make this tweakable as it's a totally reversible mod.

Of the valves, the rec and ECC83 were fine, the Zaerix EL34 was usable although slightly low, but the Mullard EL34 was very erratic. Its anode current jumped around, especially when touched, indicative perhaps of a loose grid or dry joint in the base. I would have tried re-soldering the base but instead put it in the to-do pile for now as there was a nice used pair of Mullards of the same age as the organ, right next to the tester. These were removed from a US-made amp when I replaced them with the correct 6550s and are a good match for Ia, which is of more importance here where their cathodes are tied together.

The non-original 2.5A fuse in the HT C/T was changed to a more suitable 500mA, the very badly wired Bulgin mains lead remade using my heatshrink mod (will post about this separately) and it was ready for test. With the rest of the organ disconnected the voltages would have been rather high so it was run from the Variac adjusted to put the secondary voltages right. The anode currents are well matched at around 60mA giving approx 20W anode dissipation.

I've pumped some tones and music through it, and it's still here on the bench playing 60's juke box style favourites. There's not quite enough iron in the o/p tx to use the EL34s' full welly all the way down to the bottom note but acceptable. And there's no global NFB. I can't remember the last time I had a non-feedback pentode amp on the bench, although I know that the amps in the other Burge organ are the same.

Next: Check the speaker and sort out the swell pedal, as these can be tested on the working amp.

[mole42uk]

Not wishing to hijack this thread, but when I was at junior school, probably in 1965 or thereabouts, one of the staff built a single-manual organ from a kit. I believe it used valves, I don't remember much else but that he used it in the school hall for a Christmas event.

Does this ring any bells with anyone?

[GMB]

That's great to see those old Burge organs.

I restored and still maintain an example of the next generation of Burge church organs. Looks very similar except the valves have become transistors. But quite early at about 1963.

[Peter.N.]

I seem to remember seeing and hearing Burge organs at the radio show in about 1960. I have always loved the sound of electronic organs but didn't get one myself until I was to old to learn easily. I can play basic stuff but I'm sure I would be better if I had learn't as a kid.

I did build a small organ using EF91s in the .50s but it wasn't very successful

Peter.

[Lucien Nunes]

I need to research Burge's firm, I know little about the man and nothing about his company, hence not sure what models were produced over which dates etc. My impression is that the electronics are fairly consistent in design but the construction is variable. The 3m has a couple of strange economies in its build, including chipboard in the manuals. The 1m has a proper Herrburger Brooks manual. A solid-state 2m I saw that I think was Burge had horrible plastic keys (i.e. not wood core) and very basic cabinetry but was well made electrically.

Kit organs really came into their own with the advent of the top octave synth, and I almost started building one based on the Maplin PCBs. It must have been a much more serious proposition to build an organ with valve generators, although given the date, the one mole42 remembers might have had transistor generators but a valve audio system?

[Lucien Nunes]

Having repaired the amp, the rest of the organ can be powered safely, as capacitor failure will not cause major carnage and most of the caps are in good shape anyway. The results of the first test were rather variable - the voicing circuits generally seem to work and the key and stop contacts are crackly but generally all there. However only six of the twelve oscillators run, and most but not all of the related dividers, so I have not managed to get a recognisable hymn out of it, even choosing the key to suit the functional oscillators. The generators will be the next priority instead of the voicing - they can be serviced on the bench and returned to the console once all outputs are good.

Each of the 12 generator PCBs uses three double triodes, forming one oscillator and five dividers, to output six octaves to the key contacts. All use the same circuit and within each PCB the five dividers are identical in design, but certain caps in each stage are chosen to suit the note pitch and octave. Thus very few of the 72 stages have identical component complements.

The PCBs are mounted in a wooden rack, plugged into a 6-pole connector that delivers two HT supplies (for oscillators and dividers), 12.6V heaters and a tremulant input that detunes the osc. The six octaves of tone outputs are taken via B9A plugs to the key contact busses. Attached pics show the generators in place and removed, and an initial rough sketch of the oscillator and first divider, with component values for note B.

[ronbryan]

Lucian

There is no grid-leak resistor shown for V1a in the sketch. Is that correct?

Ron

[Lucien Nunes]

Well spotted. Looking at the PCB pic (I'm not near the organ) there is a 270k unaccounted for so I expect that's it. I did say a rough sketch!

[julie_m]

How do the stops work on these organs? I know that "flute 4" plays an octave above "flute 8", and some organs have a "flute 2.67" which generates the third harmonic of "flute 8".

I guess they must be adding in harmonics / subharmonics to each of the 12 notes of the top octave, which then get divided down preserving the waveform?

[Lucien Nunes]

There are many different methods of voicing and some instruments use more than one, according to the most important requirements for authenticity of a particular stop.

This organ is very basic and uses simple voicing filters, in which the keyed signals from the dividers already contain all harmonics required in the final sound. It has to be this way round as it is impossible to make a divider that conserves the waveform. The keys make separate contacts for each footage, connecting the appropriate frequencies to the footage busbars. E.g. key C(n) will connect divider C(n) to the 8' bus, C(n+1) to the 4' bus and C(n-1) to the 16'. The stop circuit takes the appropriate footage bus output and subjects it to the necessary filters, from which the outputs are finally combined.

This general method - subtractive tone formation - can be elaborated and improved e.g.
Further subdivision of busbars into pitch groups to allow a greater number of filters tuned closer to the part of the compass they handle.
Multiple formant filtering.
Keying multiple complex tones per note (e.g. square and sawtooth) to provide different harmonic palettes on which the filters act.
etc.

If time permits later, when I get round to the filter PCB, I will try to compare the methods used in this instrument with others.

E2A: Listen to the first noises on YouTube: Video clip of first test