Tag Archives | fender

Tone Secrets: Harmonic Tremolo

With the recent introduction of a new product, the Coldcraft Harmonic Tremolo, I feel there’s a need for a concise explanation of what this circuit does and how I came to its design.  First, its no secret that the Harmonic Tremolo was inspired by one of Leo Fender’s early amp vibrato designs.

Whoa, Vibra-what?  You said we were talking about Trem?

So first things first.  Tremolo is the continuous modulation of Amplitude (Volume, loudness), first present in tube amps from the 50’s.  Vibrato is the continuous modulation of pitch.  However, Fender mixed all of this up by naming the tremolo channel of his early amps as “Vibrato”, ie Vibrolux.  He also labeled the “whammy bar” on this Stratocaster design as Tremolo.  With that in mind, it will hopefully be clear that the Coldcraft Harmonic Tremolo was inspired by an on-board amp effect known as “Harmonic Vibrato”.  In reality, both are Tremolo-like effects in the definition of the word.

Now dig, Leo used numerous different Tremolo circuits in his amps.  More traditional amplitude modulation design include Bias Tremolo and Photocell/Opto Tremolo, which have distinctly different feels to the waveform.  For a more depth explanation of all 3 types of Amp Tremolo, I humbly refer you to a Strymon Whitepaper on the subject.  For this Tone Secrets blog post, I will be concentrating on the Fender Harmonic Vibrato and how it led me to design the Coldcraft Harmonic Tremolo.

Ok, so.  The Fender Harmonic Vibrato takes the guitar signal post-preamp, splits it into high and low bands, and modulates the amplitude of each using 2 out of phase LFO signals.  How out of phase?  Exactly 180 degrees out of phase, or completely inverted as demonstrated below.

Now lets say that the modulation of the Treble/High path is driven by the Grey wave, and the Bass/Low path uses the Blue wave.  As the Treble gets louder (toward the peak of the wave) the Bass is subsequently getting softer/quieter.  So at first glance, the tone is modulated from thick and warm to bright and chimey.  Here is a block diagram representation of the circuit, courtesy of Strymon.

Now dig, in the original implementation of this effect by Fender, the modulation was created by varying the gain on a pair of tubes.  When you change the gain on a tube or similar gain device, you can also change the harmonic content added to the signal by the gain medium.  In this case the harmonics added by the tube, enhance tone of that path (Treble or Bass), and are likewise modulated with the amplitude as well.  So not only is the tone alternating bright/dark, but the harmonic content is alternating between bass-driven and treble driven components.  DYN-O-MITE!

But there’s a lot more going on in the original Fender circuit that’s not so apparent.  The crossover used to filter the signals is nothing more than a low-pass filter on one, and an high pass filter on the other, centered near some frequency.  Now if you look through the Fender schematics from the ’59-’63 era, you will see different filters values used.  It seems like Leo was still experimenting with the EQ curve, and this has a drastic effect on the sound.  I have seen it centered at 330Hz as well as 440Hz, sometimes with significant scoop to the output eq curve, maybe 10 dB.  You see, the filters both roll off signal at 6db/Octave, which is a much less than perfect crossover.  Even more important however, when recombined at the output, there is a significant portion of the Mids that take both paths, and this can cause a phasing effect when summed at the output.  This relative phase between Mids taking the Treble path and Mids taking the Bass path will also change with the modulation, which gives this effect a distinctive, but subtle chewyness in the output.  BONUS!

Now we have the formula, alternating Tremolo on Treble & Bass, Phasing in the Mids, and the subtle addition and subtraction of harmonic content from the two gain stages.  For the Coldcraft Harmonic Tremolo, I chose to center the crossover at 330Hz, with a more or less flat EQ curve when recombined at the output.  The signal is first buffered by opamp, then split and filtered.  For the gain stages, I chose discrete MOSFETs for their simplicity and nice, punchy tone.  The AC gain of the stages is modulated for the tremolo effect, similar to the popular “EA Tremolo” design published in an electronics magazine long ago.  Here is a nice layout and clear schematic of the EA circuit, courtesy of Beavis Audio.  Other notable examples include the Runoffgroove/home-wrecker.com Modified EA Tremolo.

DSC_0042Modulating the gain in this way also adds/subtracts harmonic content as the gain changes.  The two inverted outputs are then summed by an inverting op amp gain stage that provides overall level control, low output impedance and make the effect overall non-inverting.  The LFO driving the gain control also has a shape control, letting the wave smoothly morph from “soupy, swampy” sine to triangle and smoothed off square wave.  In my opinion, the harder shapes can unlock some really sweet, almost Townshend/Who Mellotron tones.  Can you dig it?

So this is all talk until you can really hear it, so I’ll be updating this post with a video/audio demo in the near future.

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4-Way Hum-Free Strat Wiring

After the post on my 4-way Tele wiring scheme, someone asked how I would prefer to wire up a 3-pickup strat.  Not really being a strat-guy I tried to come up with something unorthodox using the same 4-way switch as my Tele wiring.  Traditional Fender wiring for the Strat gives the following options,

  1. Bridge (No Tone)
  2. Bridge/Middle (Tone 1)
  3. Middle (Tone 1)
  4. Middle/Neck (Tone 1+2)
  5. Neck (Tone 2)

Now, my 2nd guitar growing up was a Strat-style, so I am quite familiar with the ice pick bridge that comes from the lack of tone control.  I’m also not a big fan of hum, so after spending some time brainstorming, I came up with the following 4-way wiring scheme for a 3 single-coil Strat.

4way Strat Wiring

I’ve opted for a Master Tone control, and a Master Bass control, as used by Leo Fender when he designed the G&L Legacy.  The Legacy was Leo’s final say on the Strat-style guitar.  You may also notice that this wiring includes an extra cap to ground (C3).  I have not tried out this wiring yet, but it would seem to me that it would be quite smooth, without the ice-pick that annoyed me so much.  The bass control would definitely keep the guitar from sounding muddy.

On to the pickup combinations.  What I’ve done here is only used 2-pickup combinations, either in series or parallel, and always with the bridge pickup.  The combination of the bridge with either the neck or middle should work to balance out the highs and lows for a full sound.  In my experience with the Tele parallel/series, I would expect the series settings to be slightly louder and warmer, whereas the parallel combinations would be brighter and punchier.

One last caveat.  To achieve true, hum-cancelling operation, you must use/wire reverse wound, reverse polarity Middle/Neck pickups relative to the Bridge pickup.  You may see this used on a parts-caster in the near future.  Actually, its the refinished body of the Strat I mentioned.

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Super Custom Tele Wiring

I thought I’d share my own personal Tele wiring recipe.  My main guitar for the past 10 years has been an American-made Fender Highway 1 Tele.   A few years ago I installed Seymour Duncan Five-Two single coil pickups in both the Neck and Bridge, and I was instantly pleased with the smooth, punchy bridge.  At the same time, I installed a 4-way switch giving me the following wiring options,

  1. Bridge
  2. Neck + Bridge in Parallel
  3. Neck
  4. Neck + Bridge in Series

Although I was less than thrilled with the neck pickup itself, I found the Series setting useful for leads high on the neck.  The neck alone was just too muddy. It was a few years later that I found a simple mod for the 4-way wiring that would allow the bass to be rolled off a bit in the Neck position (#3) only with the addition of 1 capacitor.  This satisfied me for about a year before I decided to give my guitar a nice cleaning as well as modify the pickguard so that the neck pickup height could be set from the outside.  I never quite realized how crucial pickup height was until I was finally able to adjust it without loosening strings and removing the pickguard.  You see, my particular Tele has the 22 fret extension, which makes it impossible to remove the pick guard without loosening the neck itself, and you can’t adjust a neck pickup without removing the pick guard, at least in the stock configuration.

So with control over the neck pickup height, I finally had a neck and neck + bridge sound I was proud off.  This is how the guitar is currently wired.  For most of the last 10 years, I played on the bridge pickup exclusively, with tone and volume knobs at full, and I just set my amp and pedals to work best with that sound.  Set this way, the rig does not respond well to the neck and mixed positions, so I would just ignore them, and that’s a shame.

Recently, I’ve been rolling off the tone a bit on the bridge, and setting the amp and pedals to gel with that.  However, to use the same settings with the neck and mixed positions, I need to roll the tone back to full, and that’s too many changes to make when switching pickups.  All this has brought me to the following wiring scheme,

Coldcraft 4way Tele Wiring

This schematic can be interpreted in two ways with the same goal in mind.  In the first arrangement, ignore R2 and use a 250kA Volume pot.  This allows the tone control to only act on the Bridge setting.   Rather than bypass it in positions 2-3-4, the pot wiper is opened and the tone control is set to full (even though it is physical somewhere in the middle).  This is a great way to get the sweet spot rolloff for the bridge, while retaining full treble chime for the other 3 positions.

The 2nd arrangement mirrors what some people like to do with humbuckers.  Rule of thumb is, 250k Pots for single coils and 500k or 1M for Humbuckers.  Well what do you do if you have both, or switch settings that combine pedals into both?  Well, you can have the switch select the overall load on the pickups depending on its position.  To make this happen, the Tone Pot wiper (lug 2, should be disconnected from the switch and grounded to lug 1.  R2 should be used along with a 500k Volume pot.  The pickup options are the same as before, but when selecting the bridge pickup only, R2 will be grounded creating an overall load of ~250k (R2//Volume Pot).  In the other positions, the connection is open and they will see a 500k load.  The Tone Control works in all positions.

In both arrangements, R1 and C2 are optional “volume mods” that keep the guitar bright when the tone is rolled off.  I have not included values as I don’t have this in my guitar, but will probably add it when I rewire it next.  C1 is the tone cap, and is often 22n-47n.  C3 is the bass roll-off cap.  In my current wiring scheme I used 100n, but I may lower it in the future to cut even more lows from the neck pickup.

So there it is. This is the wiring used in Coldcraft’s #1 rock stick.

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