Back before any of us were born, before Countryman made their op-amp based phase shifter, someone at Hammond designed this and made it work well enough to build into product;
I understand they're pulling current back and forth through the SR 10X transformers windings as controlled by the vibrato oscillator. I understand that those transformers (must be?) mag amps, so when the DC current level is high, they restrict AC from passing through the secondary windings. These days we could use a Vactrol for such a function, but being 1947 or so, they only had "sticks and stones" to work with. (as a former boss once said about our PS design lab)
My question is understanding mag amps are good for things like power supplies, stage lighting, locomotive braking - stuff where you dont care "how it sounds" as the technique introduces a large amount of harmonic distortion, how did they make it work successfully as an audio level control element? I'm pretty sure Hammond would not tolerate the tone of their organ getting all smashed for a little vibrato, even if it beats tremolo.
It cant be that easy. Then why could I not take a cheap 70V line transformer, put some DC / Vibrato frequency AC through the 70V side and render me a "mag-amp" based variable AC attenuator on the speaker side winding? And have it sound just like a variable resistor would? Or, would I need a gapped SE OPT to get close to that ideal? Or - is there truly zero hope of rendering such a thing out of junk you happen to have lying around; you need the custom build transformers Hammond designed for the above circuit? And even then, it only works for miniscule AC currents in the secondary.
I understand they're pulling current back and forth through the SR 10X transformers windings as controlled by the vibrato oscillator. I understand that those transformers (must be?) mag amps, so when the DC current level is high, they restrict AC from passing through the secondary windings. These days we could use a Vactrol for such a function, but being 1947 or so, they only had "sticks and stones" to work with. (as a former boss once said about our PS design lab)
My question is understanding mag amps are good for things like power supplies, stage lighting, locomotive braking - stuff where you dont care "how it sounds" as the technique introduces a large amount of harmonic distortion, how did they make it work successfully as an audio level control element? I'm pretty sure Hammond would not tolerate the tone of their organ getting all smashed for a little vibrato, even if it beats tremolo.
It cant be that easy. Then why could I not take a cheap 70V line transformer, put some DC / Vibrato frequency AC through the 70V side and render me a "mag-amp" based variable AC attenuator on the speaker side winding? And have it sound just like a variable resistor would? Or, would I need a gapped SE OPT to get close to that ideal? Or - is there truly zero hope of rendering such a thing out of junk you happen to have lying around; you need the custom build transformers Hammond designed for the above circuit? And even then, it only works for miniscule AC currents in the secondary.
Back in about 1975 or so I was a Mr. Fixit for all things electronic that were used in the factory at a large Motorola plant in Florida on the 3:30 to midnight shift. One of the neat things that I was responsible for maintaining and repairing was a 100 watt CO2 laser. It had a REGULATED 25 KV power supply capable of sourcing up to half an amp. Now such a supply today would take some fancy parts, but this beast was built in the 1960's with even older technology. This is where I learned about "saturable core reactors." Much like you describe there are two windings on a common core. The primary winding is in series with the line voltage supply to the 240 volt to 25 KV power transformer. The control winding is fed by a variable DC current source. With no DC current the inductance of the primary coil is large enough to seriously impede power to the main step up transformer, resulting in about 10 KV out. Apply some DC to the control winding and the core begins to saturate reducing the inductance of the primary coil. This variable DC current comes from a control loop that used some germanium power transistors. All of this worked perfectly and never failed until I was asked to get MORE POWER from the old laser. I simply placed a jumper across the primary coil, bypassing the control system which sent the 30 KV meter to its stop and resulted in nearly 200 watts of laser power. I must state that the Saturable Reactor in this laser probably weighed as much as I did at the time, but I was a skinny beach bum.
Here the variable inductance plays with parallel RC connected to the plate to form a current controlled phase shifter.
I have tinkered with the idea for use in a music synthesizer's voltage controlled filter with more experiments to come. From reading the above discussion it should be obvious that a variable INDUCTANCE system can be used to control an AC signal. It will however not have the same effect on all frequencies, so it makes for a variable tone and volume control at the same time. Here I'm looking for a variable frequency parallel resonant circuit in the 100 Hz to 5 KHz range that can be made with parts capable of fitting on a PC board full of vacuum tube circuitry.
Preliminary experiments seem to favor smaller cores that can be saturated easily that have windings capable of passing sufficient current to saturate the core without smoking. So far some older common mode line chokes look worthy of more experimentation.
Here the variable inductance plays with parallel RC connected to the plate to form a current controlled phase shifter.
I have tinkered with the idea for use in a music synthesizer's voltage controlled filter with more experiments to come. From reading the above discussion it should be obvious that a variable INDUCTANCE system can be used to control an AC signal. It will however not have the same effect on all frequencies, so it makes for a variable tone and volume control at the same time. Here I'm looking for a variable frequency parallel resonant circuit in the 100 Hz to 5 KHz range that can be made with parts capable of fitting on a PC board full of vacuum tube circuitry.
Preliminary experiments seem to favor smaller cores that can be saturated easily that have windings capable of passing sufficient current to saturate the core without smoking. So far some older common mode line chokes look worthy of more experimentation.
I doubt they cared. The tone is reasonably pure without vibrato, but how are you going to measure THD with vibrato?
Hammond didn't want to use Leslie's speaker (doppler vibrato) so designed something else to try to get the effect.
Tone wheel organs were all about adding harmonics to enhance the fundamentals, that would have been a feature, not a "tone smasher"...
Some pretty interesting history here; https://www.effectrode.com/knowledge-base/history-of-vibrato/
I certainly never knew that much was going on before op-amps and matched FETs comprised these things. The Hammond repair tech I was friends with in my high school days never mentioned the delay line / rotating plate capacitor selector, which I'm sure he would have been familiar with.
Anyway, I still dont know if these saturable reactors can be realized from the junk box, or if you'd need a transformer alchemy lab to build one that'll attenuate audio - without trashing the signal. Maybe that only happens if a lot of current flows...
Any transformer with a magnetic metal based core can be saturated with DC current or excessive signal. A The magnetic properties of the material have a BH curve and some have some hysteresis associated with that curve. Tinkering with small iron core transformers has yielded mixed results, and my intended application is quite different than yours.
I have been tinkering with electronic music and synthesis ever since I bought the PAIA 2700 kit in the early 70's. The most important feature, and the one that defines the sound character of a subtractive analog synthesizer is its Voltage Controlled Filter. The most well known filter is the Moog ladder filter. Here a 4 pole "ladder" is created with 5 pairs of variable resistance elements as the vertical elements of the ladder and 4 fixed capacitors as the rungs. The resistance elements need to be variable by some means of voltage, current, or other type of control. The original Moog filter used BJT's and varied the current through them. I have tried BJT's, mosfets, silicon, germanium, and vacuum tube diodes and triodes, and CDS photocells or Vactrols (CDS cell-LED pair). Each of these have advantages and disadvantages. Many years ago, I experimented with small iron core transformers in place of the resistors.
These "subtractive" synthesizers work exactly the opposite of a typical Hammond tone wheel organ. The tone wheels generate a good clean sine wave of a single frequency. The drawbars are used to mix in selected harmonics or subharmonics of the main pitch. A subtractive synth starts with a harmonically rich single frequency signal, usually a square or ramp and filters out the unwanted harmonics.
The Moog filter uses 4 RC poles in a balanced diff pair circuit similar to an opamp, with a variable "negative" feedback loop applied from the output to the input. Since we now have reactive elements inside the loop, each of which provide increasing phase shift as the corner frequency is approached, there will be a frequency where the negative feedback becomes positive. Advance the variable feedback knob enough and the filter becomes an oscillator. The feedback knob is usually labeled "RESONANCE" but "Q" is also seen on small front panels. 4 RC poles provide the right balance between response and stability.
My experiments many years ago attempted to make 4 poles with inductors and capacitors. Those experiments used small dual primary - dual secondary power transformers in the 1 to 10 VA power range. The two primaries were used as the vertical elements of the ladder with fixed capacitors as the rungs. A variable DC current was applied to the secondaries to reduce the inductance by partially saturating the core. The idea was that 4 poles can be realized with half the reactive parts. The concept works, but I could never get enough frequency range (several octaves) with the transformers I tested.
I have been toying with the idea of making a vacuum tube based music synth for years. The project gets some attention, I hit a roadblock or detour, and it goes back in its box. The VCF is however back on the bench at this moment being rewired. This time I'm trying a design that seems to show promise in LTSpice.
Most iron core transformers can be used as saturable core reactors. The question is whether or not you can get enough inductance range for your intended application without smoking one of the coils. I doubt that it is possible to create something that can attenuate a wide range of frequencies equally in this manner. Any inductance will always attenuate the high frequencies more than the lows.
I vaguely remember a wild sounding wah-wah pedal from the late 60's that used an iron core solenoid as the variable reactance element. The pedal moved the laminated pole in and out of the coil.
I have been tinkering with electronic music and synthesis ever since I bought the PAIA 2700 kit in the early 70's. The most important feature, and the one that defines the sound character of a subtractive analog synthesizer is its Voltage Controlled Filter. The most well known filter is the Moog ladder filter. Here a 4 pole "ladder" is created with 5 pairs of variable resistance elements as the vertical elements of the ladder and 4 fixed capacitors as the rungs. The resistance elements need to be variable by some means of voltage, current, or other type of control. The original Moog filter used BJT's and varied the current through them. I have tried BJT's, mosfets, silicon, germanium, and vacuum tube diodes and triodes, and CDS photocells or Vactrols (CDS cell-LED pair). Each of these have advantages and disadvantages. Many years ago, I experimented with small iron core transformers in place of the resistors.
These "subtractive" synthesizers work exactly the opposite of a typical Hammond tone wheel organ. The tone wheels generate a good clean sine wave of a single frequency. The drawbars are used to mix in selected harmonics or subharmonics of the main pitch. A subtractive synth starts with a harmonically rich single frequency signal, usually a square or ramp and filters out the unwanted harmonics.
The Moog filter uses 4 RC poles in a balanced diff pair circuit similar to an opamp, with a variable "negative" feedback loop applied from the output to the input. Since we now have reactive elements inside the loop, each of which provide increasing phase shift as the corner frequency is approached, there will be a frequency where the negative feedback becomes positive. Advance the variable feedback knob enough and the filter becomes an oscillator. The feedback knob is usually labeled "RESONANCE" but "Q" is also seen on small front panels. 4 RC poles provide the right balance between response and stability.
My experiments many years ago attempted to make 4 poles with inductors and capacitors. Those experiments used small dual primary - dual secondary power transformers in the 1 to 10 VA power range. The two primaries were used as the vertical elements of the ladder with fixed capacitors as the rungs. A variable DC current was applied to the secondaries to reduce the inductance by partially saturating the core. The idea was that 4 poles can be realized with half the reactive parts. The concept works, but I could never get enough frequency range (several octaves) with the transformers I tested.
I have been toying with the idea of making a vacuum tube based music synth for years. The project gets some attention, I hit a roadblock or detour, and it goes back in its box. The VCF is however back on the bench at this moment being rewired. This time I'm trying a design that seems to show promise in LTSpice.
Most iron core transformers can be used as saturable core reactors. The question is whether or not you can get enough inductance range for your intended application without smoking one of the coils. I doubt that it is possible to create something that can attenuate a wide range of frequencies equally in this manner. Any inductance will always attenuate the high frequencies more than the lows.
I vaguely remember a wild sounding wah-wah pedal from the late 60's that used an iron core solenoid as the variable reactance element. The pedal moved the laminated pole in and out of the coil.
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Practical Electronics magazine (UK) published a project they called a Glissandovibe in the late '60s.
It was an audio oscillator "musical instrument " with a glissando effect achieved by moving an iron core in and out of a coil.
It was an audio oscillator "musical instrument " with a glissando effect achieved by moving an iron core in and out of a coil.
