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
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.
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.
Types Of Tremolos In Tube Amps — Carl's Custom Amps
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).
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.
Fender Tremolux Blonde Piggyback Tube Amp Head 6g9 Circuit W/ El8
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
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.
Origin Effects Modulation Pedals
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.
Au7 Tube Tremolo
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
‘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.
‘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
Delta Trem Tremolo Panner In Depth
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
0 Response to "Guitar Amp Tremolo Circuit"
Posting Komentar