J.C. HALLMAN ORGANS page
The Hallman organ, sometimes mistakenly referred to as "Electronic" is actually a wind-driven reed organ with direct electric action and electo-mechanical unit switching. Common harmonium reeds (also found in accordians and harmonicas), usually 158 of them are housed in a semi sound proof enclosure and are individually fitted with condenser-type microphone pickups which connect to the amplifier/speaker system. Click here Jacob C. Hallman information on the origin of this organ.
The wind generator is actually a vacuum, not a blower. This allows the direct electric chest magnets to be placed on the outside of the windchest for easier servicing, cleaning or tuning if ever needed. Each armature/pallet assembly can be unclipped without tools and the reed can be extracted using a small flat screw driver in the slot or indentation at the base of each reed. Reeds are all grounded but the pickups which also appear outside the chest as #8 Roberston Head brass screws are actually biased with several hundred volts and are coupled to the amplifier input through a capacitor to block the bias voltage. Needless to say, care should be taken to avoid electical shocks from this system.
All switching takes place in the relay assembly mounted on the underside of the console lid. Each stop is provided with a separate contact under each key and cabled to the bronze fingers on the relays. Each stop has its own relay. The silver strips which run horizontally beneath (or above, depending on you point of view) the relay contacts are then connected to the chest magnets. Each strip connects to one magnet and plays one reed. Key contacts are connected to the DC organ supply which is in the range of 20 to 25 volts. Selecting one stop and one key directs this DC to a particular magnet and plays it's reed, which is heard through the speaker. Note that all reeds are connected to the sound system all the time. The stoptab system selects individual relays by switching DC to the large electro-magnets located near the back of the lid.
Problems with this organ are nearly always associated with this complex switching system. It is important to keep the contacts free of dirt and oxide or corrostion onf the metal surfaces, and also to see that contacts aren't bent out of position so that connecting surfaces do not meet. Commercial contact cleaners can be used, but keep in mind that much of the system is set in wood, and care must be taken to avoid having solvents disolve varnish, laquer of shellac and distributing it on the contact surfaces. A fine grit sandpaper can be used sparingly if necessary, but in most cases a piece of stiff brown paper will work just as well without the risk of cutting through the plating.
Typical elecronic problems are generally limited to those found in any other audio system. Potentiometers on the swell pedal become noisy and can be cleaned or replaced, resistors can increase many times in value, especially in plate circuits, capacitors develope leakage and tubes burn out or get noisy. Loss of pickup bias can be caused by leaky capacitors and faulty resistors. Most instruments have already experienced this in earlier years and have been repaired. A pickup adjusted too close to a reed can result in extremely loud and abrupt noises through the audio system, espectially in the large bass reeds. Using a red Robertson screwdriver, clockwise increases volume, counter the reverse. For information on tuning reeds there should be many good websites on harmonium servicing on the internet.
HALLMAN ELECTRONICS - overview
As stated before, the Hallman Organ is not electronic itself, but uses electronics for amplification, control of loudness by the swell pedal and tremulant. Starting at the windchest, signals are picked up from the vibrating reeds when played using a condenser type microphone on each reed. This consists merely of one or more brass screws threaded into the wood and adjusted to come close to but not touch the vibrating reed. Usually the reeds are grounded and the pickups biased with a high voltage. The connections can be seen as bare wire threaded in and out of the screws on the outside. The power supply can be seen nearby on the reed chest and consists of several resistors and capacitors serving as a filter network to remove any trace of AC hum, as this voltage is coupled to the amplifier input through only a capacitor. This input is a very high gain, high impedence input so all components must be shielded thouroughly. The entire reed system is enclosed in a grounded metal case. Normally when this is opened some level of hum can be heard from the sound system.
The usual configuration is a two channel system, one channel for Flute voices and the other for String voices. These are typically kept separate through the pre-amp stages, and may be sub divided into several groups depending on frequency. A series of level controls found on the pre-amp chassis is used to regulate the differnt sections so that a smooth transition from one section to the next can be achieved. Tone controls are usually fitted at this point. The pre-amp chassis is usually located under the lid next to the relays, and can be adjusted without removing the back.
The only signal processing which occurs in the system is Vibrato, which consists of a low frequency oscillator coupled to a multi stage phase shifting network to similate a pitch shift. This also has adjustments on the pre amp chassis.
The pre-amplified signal leaves the pre-amp and is routed through the expression pedal (swell pedal). The element in the pedal is usually a complex linear step-attenuator housed in a metal enclosure, coupled to the pedal with polished brass rods which move a set of contacts over two parallel commutator strips connected to individual resistors selected to provide the desired curve. Capacitors are integrated into the circuit to provide the desired degree of "loudness" effect, where lower frequencies receive less attenuation than the higher ones.
From the pedal, the two signals are connected to the two channel power amplifer which is made up of two almost identical circuits, sharing only a common power transformer and rectifier. Each channel has its own choke and system of electrolytic filter capacitors. Audio cables are usually all terminated using Amphenol tube socket type of connectors which are relatively reliable, but can benefit from occasional contact cleaning. Most organs will have had the original capacitors replaced, and probably will not require further replacement. It is normal to find most of the original tubes still in use, and replacement will not necessarily provide any improvement in performance, other than the power rectifier. Old North American and European tubes last a very long time. Typical output tubes are an obsolete 6F6G, which look like 6L6G's, but it is not advisable to replace them with 6L6's, as these demand a lot more current from the power supply. Probably the best replacement is the current 6V6, a smaller looking tube, but a lot closer to the original specs. Expect only about 15 watts per channel. This is a heavy-duty looking piece of equipment, but not very powerful by todays standards.
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