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Ticked off with Tremolo?

Does the tremolo (a.k.a. vibrato by the engineers in Fender’s R&D dept.) in your vintage Fender^® ‘Deluxe Reverb’ amp make an annoying ‘ticking’ noise? It’s not an uncommon problem. My Deluxe Reverb tremolo ticks too, and I tried every trick in the book to get rid of it; adjusting lead dress, adding a 10nF capacitor, changing tubes, even cleaning the component board. But although some of these remedies improved things, they did not completely cure that infernal ticking.

This got me thinking as to whether there might be some underlying cause, some fundamental flaw or design oversight in Fender’s tremolo circuit that was the root cause of the noise, and if it could be completely eradicated. After all, the ‘Deluxe Reverb’ and Fender’s other amps that employ photocell circuitry, such as the ‘Twin Reverb’ generate a beautifully deep and lilting amplitude modulation effect. They really are something special, and inspiring to play through, so it seems a pity to have to put up with a crazy runaway clock ticking away in the background. Surely something can be done to fix this problem—let’s see if there’s some way to stamp out those ticks good and dead.

Crosstalk

Noise is a well known and understood phenomenon in the audio electronics world. Many, many textbooks have been written about it and many guitarists are aware of the importance of shielded cables, earth loops and suchlike, but this ticking problem smacks of something a little different, something called “crosstalk“. Crosstalk is an archaic term from a bygone analogue age, where conversations on telephone lines could become crossed or the ‘Left’ channel on a stereo record player could be heard on the ‘Right’. In short it describes any unwanted noise that finds its way into an audio signal. If two conducting wires, or printed circuit board tracks, are separate then electrical signals should not propagate between them. However, if the two wires are in close proximity to one another and/or running parallel, then stray (parasitic) capacitance and mutual inductance exists between them. This results in noise being propagated or coupled from one wire conductor to another.

Fender vulcanised fibreboard — Before the advent of Printed Circuit Boards (PCBs) electronic components used to be mounted on fibreboard.

These days crosstalk is more usually referred to as “signal coupling”. Signal coupling can be “inductive” (this is how tape heads record/playback a signal on magnetic tape), “capacitive” (like the coupling caps in a guitar amplifier) or ‘radiative’ (like how a radio picks up a signal from a station transmitter). One of the ‘ticking’ fixes mentioned above suggests cleaning the component board to help reduce capacitive coupling. The reasoning behind this is that these early ‘electronic circuit boards’ were manufactured from vulcanised fibreboard. Fibreboard is known to be hygroscopic meaning it’s prone to absorbing moisture from the atmosphere. As its moisture content rises so does its conductivity, and its capacitance, causing the fibreboard’s electrical impedance to fall. Small electrical currents begin to flow and find their way into areas where they shouldn’t be. In this case control currents from the tremolo phase-shift oscillator induce voltages onto the audio signal path, contaminating it with the noise. Cleaning the fibreboard with pure alcohol is supposed to restore the fibreboard’s original high impedance performance.

Fender's Fixes

However, moisture in the circuit boards isn’t the only thing causing these amps to tick. Back in the 1960s large numbers of ‘Deluxe Reverb’ and ‘Twin Reverb’ guitar amps rolled fresh off Fender’s production line with ticking troubles right from the day they were made. And Fender were aware of this. In fact, it was such a huge problem that they released a service bulletin (service bulletin #9) outlining fixes to mitigate the ticking. One of the fixes involves “bunching” the wires running from the tube socket associated with the tremolo circuit to the component board (shown in the video demo below).

Fender provide no explanation as to physics of how this works, but adjusting the position of these wires relative to each other and the chassis definitely nulls out the ticking noise, to a certain extent. This may be due to capacitive noise cancellation or because the noise is being dumped into the amp chassis and to ground (earth). This ad-hoc “calibration” of the amp’s trem circuitry to make it work properly is a bit hit and miss though, and unsatisfactory to say the least.

Further, Fender advise moving the wires from the ‘SPEED’ and ‘INTENSITY’ pots away from the tone control wiring. There’s a certain logic to this as, in theory, it should help prevent noise being coupled into the tonestack, which is a high impedance audio circuit and therefore sensitive to noise pick-up. But in practice moving the SPEED and INTENSITY wiring away from the chassis results in an increase in ticking noise, not a decrease—probably because the chassis is no longer partially screening the wires and “sinking” the noise to ground. So much for that ‘fix’.

Fender’s final fix recommends installing a 0.01μF (10nF) 600V capacitor from the point between the 10MΩ resistor and neon glow lamp to ground. Each time the lamp “strikes” and illuminates it releases a burst or pulse of noise, splattering the audio spectrum with high order odd harmonics. The capacitor works to “shunt” most of these harmonics to ground. This fix reduces the noise substantially, but as Fender point out in their service bulletin, it’s just another partial cure.

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].

Shaky Ground

So, none of the fixes discussed are a ‘silver bullet’ that can kill ticking stone dead. They do, however give us a clue as to how one might be manufactured. Notice how the subject of grounding keeps cropping up in some form or another, whether it’s screening wires or sinking noise to earth. These factors point to a problem with Fender’s grounding scheme, or to be more exact, lack of it.

Fender's adhoc earthing scheme — Fender's free-form jazz exploration in grounding.

An examination of the component layout within the amp reveals that resistors and capacitors are soldered to the amp chassis at what appears to be the nearest convenient point. It’s unlikely this kind of ad hoc grounding regime will yield optimum noise performance. More consideration needs to be given to the return paths of currents flowing through these components.

For example, the SPEED pot is connected to the other components in the tremolo oscillator circuitry via the chassis. As a consequence low frequency control currents end up flowing near sensitive audio circuitry. Then, because the chassis has electrical resistance, these currents generate voltages, which become coupled into the audio signal path.

This is bad news, as once the audio signal is convolved with low frequency noise from the tremolo oscillator it’s very, very difficult to remove it using tried and tested noise reduction tools such as filters or noise-gates. And even advanced digital noise reduction techniques, such as those based on Fourier analysis, invariably introduce some kind of ‘artefact’ into the signal they are attempting to restore. A real world solution is practically impossible: like trying to separate mixed orange paint into its component red and yellow colours. From an audio engineer’s perspective it’s always best to prevent noise from getting into the audio in the first place.

The Perfect Earth

Incidentally, at absolute zero (-273°C) metals become perfect electrical conductors—the atoms in the the metal cease vibrating and electrons can flow unimpeded without bumping into them. At such incredibly low cryogenic temperatures the folded steel chassis in a ‘Deluxe Reverb’ amp would have no electrical resistance. A curious consequence of this is that there would be no need to adhere to a strict grounding regime. Noise voltages (or ‘potential differences’ as they used to be called in the old days) cannot exist without resistance and without resistance there can be no voltages to induce currents—there would be no capacitive coupling. So, at absolute zero Fender’s ad-hoc grounding scheme would work fine—there would be no ticking noise, but few guitarists would be enthusiastic to gig in such chilly conditions.

The Cure

The good news is that capacitive noise coupling can be reduced to vanishingly low levels, even zeroed out completely by paying careful attention to the ground layout. One approach is to localise all grounding in the tremolo circuitry. Take a look at the schematic of the tremolo circuit below. The SPEED pot is physically connected to point ‘B’ (the ground eyelet of the cathode resistor and capacitor in the phase-shift oscillator) by a length of wire rather than to the point ‘A’ (amp chassis near the front panel). This prevents the oscillator noise from permeating the chassis—if there’s no noise in the chassis in the first place then there’s no way it can become coupled into the audio signal path.

photocell_tremolo_circuit_earthing — Localised grounding (or ground partitioning) is the key to solving 'ticking' noise problem in Fender's vintage photocell driven tremolo circuits.

Technically this is known as “ground partitioning“. This technique is often used in digital electronic audio devices such as CD (Compact Disc) and DAT (Digital Audio Tape) players, computer soundcards and digital effects pedals too. Analogue and digital grounds are kept separate (partitioned) and only connect at one point. This is the single best line of defence to prevent noise from getting into places that it shouldn’t and degrading audio performance.

Audio equipment often contains more than just two grounds. There can be separate grounds for preamp sections, power-amp, power supply, control circuits, motor/relay switching, ADCs (Analogue-to-Digital Converters), etc. All these grounds are still connected to just one single point—the “Star Earth”. This ensures noise remains localised and won’t affect sensitive circuitry, such as a high gain preamplifier input stage.

The star earth is then connected to the earth pin of the power connector inlet, which in turn connects to a mains power outlet in your home. Finally, all mains outlets are physically connected to ground (the actual ground or earth you walk on), preferably by means of a solid copper rod hammered a couple of feet into the soil next to your home—all the noise should ultimately flow down there and into the Underworld never to be heard from again.

Speed' pot 100K resistor — Disconnect the leg of the 100K resistor on the 'Speed' pot from the chassis and connect it directly to the 2K7 cathode resistor in the phase-shift oscillator.

In Closing…

And that’s about it. I think we’ve made more than a fair stab at stamping out a particularly stubborn and nasty old bug that has been lodged within the electronic guts of Fender’s tube amplifier tremolo circuitry for almost sixty years. Grounding is the key. It’s vitally important, but is often overlooked, misunderstood or regarded as a trivial. Far from it. There are many subtleties to ground layout: Tiny, almost immeasurable currents can cause a great deal of mischief if they find their way into sensitive audio amplifier circuitry. It’s not an easy thing to get right, as Fender realised to their dismay when customers began to complain about strange noises emanating from their ‘Deluxe Reverb’ and ‘Twin Reverb’ Amps. The take home message here is: whether you’re building, or tweaking a guitar amp always stick to the ground rules—that way you’ll remain on terra firma, a.k.a. solid ground and your amp will tick over nicely, and more importantly, quietly.

If you enjoyed reading this article and want to find out more about the inner workings of photocell tremolo in vintage amps then check out our article: Delta-Trem Tremolo-Panner In-depth

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Ticked off with Tremolo?