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Delta-Trem Tremolo-Panner In-depth

The Delta-Trem™ adds beautifully rich and luscious tremolo (amplitude modulation) to electric guitar. As with all Effectrode pedals, vacuum tubes play an intrinsic role in generating the effect, however another vital component is also involved here, a long forgotten, archaic electronic device known as a ‘Raysistor‘. The Raysistor enables the Delta-Trem to reproduce smooth volume fluctuations, like the ‘bias’ tremolos found in the early Fender^® ‘Tweed’ Tremolux guitar amps, and also the deeper throb of the ‘optical’ (neon/photocell) tremolo of their later ‘Blackface’ Deluxe Reverb and Twin Reverb amplifier models. But the Raysistor can do much more than merely replicate bias and optical tremolo… with the right kind of LFO (low frequency oscillator). Read on, to discover how we captured the magic of vintage tube amp tremolo, improved upon it and canned it in an aluminium box.

Fender 'Tremolux' amp — A 1959 'Tremolux': Fender's first tremolo-equipped guitar amplifier .

Before the dizzying array of digital delay and reverb guitar effects we have today. Before the myriads of analogue modulation ‘chorus’ and ‘flanging’ effects that multiplied and mushroomed in the early 1970s. And long before the advent of digital signal processing, when the transistor was just an idea on the drawing board at Bell Labs, there was only guitar amp reverb and tremolo. These primitive guitar effects weren’t generated by the cool, calculating, computerised core of some DSP (Digital Signal Processing) chip or within the cold, crystalline circuits of a transistorised circuit, but by high voltages and currents flowing through copper wires and vacuum tubes—they ran hot! In fact, so hot that the plates of an output power tube would glow a deep, sombre red if incorrectly biased—not an uncommon occurrence in some older tube amps’ carrying ‘bias’ tremolo circuitry.

Bias tremolo and optical tremolo, are the two main types of tremolo found in vintage Fender^® guitar amps, although Leo Fender did develop and patent a third type, which he called ‘Harmonic Vibrato‘. His novel circuit worked by modulating high- and low-pass filters and mixing their outputs together to achieve the tremolo effect, however his thermionic invention also produced a not insignificant amount pitch shifting too so, strictly speaking, not a pure tremolo (amplitude modulation), but also a vibrato (frequency modulation). Although they’re quite different in their design and tone, all three of these tremolos do have one thing in common—the LFO (low frequency oscillator).

The LFO

The LFO utilised in Fender’s old amps is a primitive ‘phase-shift’ oscillator arrangement as shown in the diagram below. Essentially, it’s an inverting tube gain stage (it’s proper name is a ‘grounded cathode’) where the output signal is fed back to the input via a network of capacitors and resistors. Each of the three pairs of resistors and capacitors introduces 60° phase-shift giving a total phase-shift of 180°. As the output signal is already 180° out phase with input (this is an inverting amplifier) this means the signal being fed back to the input now is 360° out of phase, a.k.a. positive feedback, which causes the circuit to oscillate, generating a slightly cockeyed, low frequency sinusoidal control voltage.

The speed of the LFO is altered by varying the resistance of one of resistors in the phase-shift network. However its range is very limited, typically 4 to 8Hz and at lower speeds the oscillation collapses and ceases to operate. This restricted range is okay for use as a tremolo modulator but not so great for achieving long, smooth sweep rates in a phaser or even a ‘vibe’ pedal.

Bias Tremolo

Sylvania 6V6GT Beam-Power Tetrode Vacuum Tube — Sylvania 6V6GT beam power 'tetrode' vacuum tube: Bias tremolo works by wiggling the bias voltage of the Fender 'Tremolux' amp's 6V6 output tubes causing amplitude modulation (AM).

With ‘bias’ tremolo the LFO ‘wiggles’ the bias voltage on the grids of the power tubes in the push-pull output stage of the amp, which results in variations of the guitar’s volume level. Early Fender^® guitar amps, such as the tweed-covered ‘Tremolux’ introduced back in 1955 (Fender’s first tremolo-equipped guitar amplifier) and other models manufactured prior to 1963, such as the brown tolex-covered ‘Princeton’, utilised this type of tremolo circuitry. The class-B push-pull tube power amplifier in these amps employs two tubes; one tube pushes current (electrons) into the amp’s speaker (causing the speaker cone to move forwards and push air) and the other pulls it (making the cone move backwards). A bias voltage needs to be applied to the tubes to set an ‘idle’ current flowing through them so they are ticking over, ready to do the serious work of moving all those electrons. This is analogous to setting the idle speed of a car engine—if the rpm (revolutions per minute) is too low, then engine will stall; if too high, the engine is burning excessive gas and overheating.

The bias voltage for 6V6 and 6L6 ‘Tetrode’ power tubes in Fender amps is around -35V and is adjusted about this point for minimum idle current without introducing crossover distortion. Now herein lies the fundamental problem with bias tremolo: power tube biasing really isn’t something you want to mess around with—once set up properly, it should be left alone and not interfered with. But bias tremolo, by design is constantly interfering with the power tube biasing. On every tremolo pulse cycle it applies positive voltage push to the bass-line negative grid bias increasing current flow through the tubes to increase gain and causing them to run hotter.

Wiggling the power tube bias up and down generates the tremolo effect but introduces some pretty serious, potentially catastrophic reliability issues in the amp. For instance, if the ‘Intensity’ pot happens to go open circuit—as pots are sometimes prone to do—or there’s just a speck of dust trapped between the carbon track and the wiper within the pot, then the negative bias voltage will be lost and the tubes will go into meltdown. The plates will begin to glow a deep, cherry-red (“red-plating”) as they’re relentlessly pummelled by an uncontrolled flow of electrons jostling against each other in their frenzied attraction to it, ultimately destroying the tube and quite possibly damaging other components too.

Additionally, even when operating correctly ‘bias’ tremolo has another drawback: crossover distortion. Now unlike tube clipping distortion, crossover distortion sounds highly objectionable, especially audible at low volume levels. A ‘cold-biased’ push-pull tube output stage will generate low-order odd harmonics, mainly 3rd and 5th harmonics of the fundamental, but it’s not so much the harmonic content that’s so objectionable, it’s the context. Being more apparent at low volume levels and masked as the amp is cranked up is unnatural, not what we’d expect to hear, being the exact opposite of how clipping distortion manifests itself. And in practice crossover distortion has a thin, ‘raspy’, ‘buzzy’ quality about it, like a ripped loudspeaker cone or a failing electronic circuit. Like ‘blocking’ distortion it’s useful to blend crossover distortion with tube clipping distortion to obtain a more complex or even ‘gritty’ overdrive/distortion, but by itself it sounds pretty nasty. Not the type of warm, full-bodied, rich overdrive you’d normally associate with a tube amp.

There’s about as much mojo in crossover distortion as there is in misaligned tape echo heads, or noisy carbon composition resistors, and the reliability issues associated with modulating the bias of power tubes were undoubtedly a customer relations nightmare for the big amp companies. So these ‘idiosyncrasies’, or to be more exact, defects are probably not the qualities of bias tremolo an audio engineer would seek to build in to a pedal, even if it were feasible to house two 6V6 power tubes in a stompbox and power them with a wall-wart. But despite its technical shortcomings, it has to be said, the bias tremolo of a Fender ‘Princeton Reverb’ amp has a beautiful, lilting quality about it, and consequently is highly sought-after by guitarists for its smooth, buttery character. The tube bias tremolo in this old amps is musical and, like a great deal of other classic tube gear from this era, inspiring to play through. And, it doesn’t overly impose itself on the guitar, creating a wavering effect, rather than the ‘choppy’ throb of ‘optical’ tremolo.

Optical Tremolo

fender_photo_cell_assembly2_320px — The Fender "Trem-bug" consists of a neon lamp and phototcell inside a short length of heat-shrink tube.

Fender’s ‘bias’ tremolo amps were all eventually superseded by newer models fitted with ‘optical’ (neon/photocell) tremolo. This design change was almost certainly catalysed by the problems customers had experienced with weak/distorted tremolo in the older amps and the much more serious problem of power tubes going in to thermo-nuclear meltdown. Optical tremolo resolved both these issues as the output power tubes were kept set at their optimum bias current and left there, however the new circuitry sounded different to bias tremolo because it worked differently.

Both types of tremolo utilised exactly the same LFO circuitry [shown in the schematic diagram below]. However, rather than wiggling the grid voltage of tubes in the amps output stage, the LFO controls the brightness of a neon lamp (a cold cathode tube). The lamp is mounted in close proximity to a photocell and both are covered in a shroud to prevent any stray external light from entering. The whole assembly is technically an early form of ‘opto-coupler’ or ‘opto-isolator’ and back in the day radio repairmen dubbed the device a “Trem-bug”, a quirky, yet somehow appropriate nickname.

As the brightness of the lamp fluctuates, the amount of light falling on the LDR changes, and so does its electrical resistance. This results in the guitar signal level being cyclically attenuated, creating a deeper, more ‘choppy’ tremolo effect than bias tremolo. This more choppy tremolo is entirely down to the physics of how neon lamps work. The lamp, which is essentially a small, sealed glass envelope containing two tiny carbon electrodes and a small amount of neon gas, requires a certain minimum voltage in order to strike (light up). With no voltage across its terminals the lamp is not only dark (unlit), but also non-conducting. If the applied voltage is gradually increased from zero there will come a point (the ‘breakdown’ voltage) where the lamp suddenly bursts into life and glows. In practice this happens at around 90V for a neon lamp (different breakdown voltages for tubes filled with other inert gases such as argon, xenon, etc).

So, in an optical tremolo circuit, as the LFO output reaches the 90V breakdown voltage, the neon gas within the envelope ionises, conducts electricity and the lamp emits its characteristic orange glow. The physics of neon lamp operation brings us to the first shortcoming of optical tremolo: the is sudden jump in light intensity resulting from the neon gas striking, and then after this point the light intensity varies relatively linearly with the LFO control voltage. There is absolutely no way to adjust this, it’s intrinsic to the design and entirely dependent on the physical properties of neon. Even though the phase-shift LFO in the tremolo circuitry generates a smooth sinusoidal wave control voltage, the LDR does not “see” it because the transfer characteristic of the neon lamp transforms it into something more pulse-shaped. Subjectively this gives the deep, throbbing tremolo sound guitarists associate with the opical tremolo circuitry found in Fender’s ‘Blackface‘ ‘Deluxe Reverb’ and ‘Twin Reverb’ amps. But there is no control over the tremolo pulse for smoother, more fluid or shimmery tones.

Type NE-2 neon glow lamp — Type 'NE-2' neon glow lamp: When high voltage is applied the neon gas within the lamp ionises, radiating its characteristic orange glow. [Photo by Iacopo Giangrandi].

Close-up photograph of a TO-8 cadmium selenide (CdSe) photocell — TO-8 cadmium selenide (CdSe) photocell. The electrical resistance of the CdSe strip (the wiggly bit) decreases when light (photons) falls on its surface.

And the second problem is the current surge generated by the ionisation of neon, an electrical spike that finds its way into the audio signal path. This manifests itself through the amps loudspeaker as an annoying “ticking” noise at the same frequency as the tremolo speed. A well-known remedy for this is to solder a small 10nF capacitor in parallel with the lamp. This fix is not always 100% effective at tuning out the noise though and further adjustments invariably need to be made to the component lead dress and their layout on the turret board in the oscillator circuitry. The idea is to minimise, or null-out any striking noise being capacitively coupled from the neon lamp into the audio circuitry. It’s tricky to get this just right, however, with a little time and patience, the ticking noise can be reduced to a level where it’s good enough for Government work… or gigging.

The Secret Life of “Trem-bugs”

Even today, the old-school tremolo tube circuitry within Fender’s ‘Blackface’ and ‘Silverface’ amplifier models still harbours some secrets. Secrets that have a direct bearing on its function and tone. Secrets that amp gurus and proud owners of these gorgeous guitar amps will undoubtedly find fascinating. Firstly, the “NE-2” glow lamp used to make the trem-bug. It should be a type “NE-2U”, as opposed to the more common “NE-2H” lamp. The NE-2U is a high brightness indicator lamp and was specially designed for the purpose of controlling photocells.

A small amount of a radioactive additive was added to the neon gas, specifically Krypton-85 gas, which improves the lamps start-up reliability when operating in complete darkness (a.k.a. the “Dark Effect”). Being radioactive, Krypton-85 emits β-particles (electrons) and these help the neon to ionise and strike more reliably when a high voltage is applied. Unfortunately, Krypton-85 has a half-life of 11 years so it’s effect gradually diminishes over the years: after a few decades, these lamps still work, but don’t strike as enthusiastically as when they were brand new. This is one of the rare cases where it’s better to use a new tube rather than NOS one.

Characteristic curves of photocells and the human eye

Secondly, the photocell in the trem-bug is a TO-8 (approx 11mm ⌀) CdSe (cadmium selenide) type, not CdS (cadmium sulphide). CdSe photocells typically exhibit a significantly shorter time constant than CdS photocells (approximately 10 milliseconds in contrast to 100 milliseconds); in layman’s terms means they’re faster. They’re also more sensitive to red light in the colour spectrum, as shown in the graph above. Ionised (glowing) neon gas emits distinct frequencies of light, with its highest spectral peaks being between 600nm-690nm, which means both types of photocell will exhibit resistance changes in response to light, however the CdSe device will respond more rapidly. Finally, a CdSe photocell can easily be distinguished from a CdS type as they’re black in colour, rather than bright orange.

Vintage Tremolo Re-engineered

Engineers love to change things, not only to aggravate the likes of Dr “Bones” McCoy, but in the pursuit of improving the function and eliminate the shortcomings of some gadget, or electronic gizmo, such as bias and optical tremolo. There’s no doubt these vintage amp tremolos possess wonderful, rich, sonorous qualities but both types have their shortcomings too. Bias tremolo would be better without the ‘cheap’ sound of crossover distortion and having to live in constant fear of catastrophic, terminal output tube meltdown. And optical tremolo would sound even better, even sweeter without those annoying background ticking noises and some control over the smoothness of its pulse shape. Seen through an engineer’s eyes it’s obvious that changes need to be made; first off, the power tubes and neon tube must go.

Now, we’re left with just the photocell, which has the potential to be a beautifully linear, low-distortion voltage control device, if used correctly. The key here is to use a light source that can be varied smoothly and continuously without glitches or sudden jumps in brightness. Fortunately, those clever boffins in white coats at Raytheon developed the perfect electronic device for smooth linear control of voltage signals way back in 1960s–the ‘Raysistor‘. The Raysistor is essentially a small metal can housing a filament lamp and photocell. The brightness of the filament lamp can be controlled with considerably more precision than the neon lamp, especially at low light levels; such fine control is of vital importance if you’re looking to generate a subtle, shimmering tremolo.

Unfortunately Raytheon are longer in the business of manufacturing Raysistors (or sub-miniature tubes for that matter) and NOS Raysistors are rarer than hen’s teeth. The closest matching device available today is the ‘opto-coupler’, which consists of an LED (Light Emitting Diode) and an photocell housed together in a single epoxy case. However, an LED is an extremely efficient light emitter, too efficient in fact, making that fine control for super-smooth tremolo impossible. The filament bulb really was the only device suitable for achieving the smooth tremolo effects we were looking for.

So we set to work developing our own custom Raysistor based on a 3mm ‘grain of wheat’ bulb and cadmium sulphide (not cadmium selenide) photocell. These two electronic components were specially chosen to give the Effectrode ‘Delta-Trem’ tremolo pedal range and versatility. The tiny filament in light bulb, has very low thermal mass meaning the light can be switched on and off rapidly to mimic the neon lamp in optical tremolo; and it can also be turned on and off slowly too, where its brightness transitions more gradually, replicating the smoothness of bias tremolo circuitry.

Further, the Raysistor attenuator circuitry was sandwiched between two triode tube stages—a grounded cathode gain stage and a cathode follower—thus ensuring super low-noise performance and buffering the output. House two of these amazing high voltage, thermionic tremolo circuits in retro-cool aluminium alloy box and you have the makings of an incredible vacuum tube tremolo-panning pedal! But we’re not finished yet—there’s just one more vital ingredient to add—the LFO.

Making Mixed Waves

As described earlier, the primitive phase-shift LFO circuitry found in vintage tube amps is only capable of generating sinusoidal wave amplitude modulation. Further, the speed range is severely limited as the oscillator loses gain at lower speeds, resulting in a weaker intensity of effect (less depth to the tremolo). The original ‘Uni-Vibe‘ pedal has a similar phase-shift LFO configuration (built using transistors rather than tubes) and therefore exhibits this drop-off of intensity as the oscillator begins to collapse. In hindsight it’s easy to pick fault with these archaic old tremolo circuits, however it should be recognised that some of the designs being discussed here are over half a century old—at the time they were the latest thing.

Today, with hindsight, basking in the after-glow of all the wonderful discoveries and inventions made by electrical engineers at RCA, Leo Fender and, relatively recent developments in electronics technology, comes the realisation that the old phase-shift LFO can be improved… a lot. There’s no audio signal passing through the LFO, it simply generates a sinusoidal control voltage, so it doesn’t really matter if that voltage is generated with tubes, transistors, mechanically or digitally—after all it’s only turning the brightness of a lamp up and down. Technically, that control would be much better achieved digitally using DDS (Direct Digital Synthesis). DDS makes it possible to synthesise any type of analogue waveform—sine, square, triangle, rising or falling ramps, pulse, arbitrary wave forms, that is, any wave shape that can be imagined. Like Dr McCoy said, “I know engineers—they love to change things.”, and we do—so we lashed togehter a novel mixed wave generator and shoehorned two of these cool control circuits into our new model “DT-2A” Delta-Trem Raysistor Tremolo-Panner pedal.

But limitless possibilities would surely create chaos and madness for any working guitarist attempting to dial in their favourite tremolo setting whilst gigging. So to make things more straightforward and intuitive we limited the Delta-Trem to three fundamental waveform types (‘Filament’, ‘Fluid and ‘Neon’). Each of these can be selected via a three-way toggle switch on the rear panel of the pedal. Further, the waveform type can be twisted and morphed into radically different pulse shapes using a ‘Shape’ knob. This, in combination with the ‘Width’ and ‘Speed’ knobs, makes it possible to sculpt an array of different types of tremolo quickly and intuitively.

Filament waveforms — Filament lamp waveforms

With ‘FILAMENT’ pulse shapes selected the Delta-Trem produces a a range of smoother amplitude modulation tremolo effects. With the ‘Shape’ knob set at 12 o’clock the LFO generates a roughly sinusoidal shaped wave (just a real phase-shift oscillator) replicating the buttery pulse of the bias tremolo found in the 1955 Fender ‘Tweed’ ‘Tremolux’ amp. When the ‘Shape’ knob is rotated anticlockwise the LFO pulse shape gradually becomes even smoother, morphing into a triangle wave and, when fully anticlockwise, becomes a unique “crossover” wave, creating an exceedingly silky, smooth tremolo. This can be used to add a subtle shimmer to chord-work.

As the knob is rotated clockwise from the 12 o’clock position the sides of the LFO pulse shape get steeper and steeper until it becomes more squarish is. This intensifies the tremolo throb so that it becomes more reminiscent of an old Valco amplifier.

Neon waveforms — Neon lamp pulse waveforms

With ‘NEON’ pulse shapes selected the Delta-Trem LFO generates a pulsed waveform, replicating the neon glow lamp optical tremolo of a Fender ‘Silverface’ or ‘Blackface’ Deluxe Reverb amp. As described earlier, photo-optical tremolo produces a more intense, ‘choppy’ sounding amplitude modulation than bias tremolo. Essentially, the ‘Shape’ knob controls the mark-to-space ratio of the LFO pulse shape. When the knob is set fully anticlockwise the pulse is narrow, emulating a tired old neon lamp that’s not working particularly well. Then, as the knob is rotated clockwise, the mark-to-space ratio increases generating the extremely deep throb of a factory-new Deluxe Reverb amp from 1966!

Fluid waveforms — Fluid rising and falling waveforms

With ‘FLUID’ wave shapes selected the Delta-Trem pedal generates variable rising and falling sawtooth waves where the ‘Shape’ knob sets the ramp rate. This adds lopsidedness to the throb of the tremolo creating a unique “Flyback” amplitude modulation effect, where the sound level gradually increases (or decreases) then suddenly jumps back. In the anticlockwise position a rising sawtooth is produced. Rotating the ‘Shape’ knob clockwise gradually alters the characteristic until a triangle wave at 12 o’clock. As the knob is further rotated the wave changes to a falling sawtooth.

So there you have it. The tale of how the boffins at Effectrode developed and refined vintage amp tremolo, canned it in an aluminium alloy box, and discovered some incredibly cool and useful stuff about vintage amp tremolo along the way—effects pedal design really is a voyage of discovery. Anyway, if you enjoyed reading this article then you might also like to find out how we captured the magic of amp vibrato in a box too—built with vacuum tubes, of course!

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Delta-Trem Tremolo-Panner In-depth