Amplifier Tremolo Technology White Paper

Flint Tremolo & ReverbSometimes to understand who you are, you have to go back to the beginning, back to where it all began. Before smart phones, before computers, before integrated circuits and the transistor—the only effects available to guitarists were tremolo and spring reverb. The guitar players of the day didn’t have the rainbow of colors that we have now.

But like a charcoal sketch, there is a stark beauty to the tone without the wash of effects that are now possible. Stripped down to the bare necessities, the contrast of the different tremolos becomes apparent. You feel the beating heart of the photo trem, the rolling waves of the tube trem and the hypnotic swirl of the harmonic tremolo.

Given the storied history of these circuits found within classic amplifiers of the 1960s, there was no doubt that we wanted to develop a studio-class pedal that faithfully delivers three of these iconic and unmistakable tremolo effects. We examined the sonic complexities and tonal interplay, and accounted for every last detail in our hand-crafted algorithms.

The result is the technology found in Flint Tremolo & Reverb. Pete Celi, our Lead DSP Engineer and Sound Designer illustrates the research and sound design process in the White Paper below.


Strymon Amplifier Tremolo Technology White Paper

Amplifier Tremolo Overview

Still incorrectly labeled as ‘vibrato’ in many cases, the tremolo effect is a cyclical amplitude (volume) modulation of the input signal. Although there are many cool tremolo effects that can be had by using a simple VCA (voltage-controlled amplifier) circuit and applying geometric waveforms (like sine, triangle, square, ramp) to modulate the amplitude, our interest is in exploring the unique, soothing, pulsing, hypnotic effect that comes out of vintage amplifier tremolo circuits.

There were three main variations that came about in the late ’50s and ’60s. The three types can be referred to as Harmonic Tremolo, Power Tube Tremolo, and Photocell Tremolo. These variations have unique characteristics that result from the very different ways that the effect is achieved


One thing that these vintage trem types share in common is the LFO (low frequency oscillator) circuitry, which is generated by a classic positive feedback ‘phase-shift’ oscillator. A network of resistors and capacitors determine the rate of oscillation, and the resultant LFO signal is a mildly distorted sinusoidal signal.



As the LFO circuitry is common to all three trem types under investigation, we can see that LFO waveshape is not responsible for the very different sounds that result from the three implementations. Let’s look closer at the three types.

Harmonic Tremolo

The Harmonic Trem is actually not a pure tremolo effect. It is really a dual-band filtering effect that alternately emphasizes low and high frequencies. The end-result is a soothing pulse that has shades of a mild phaser effect combined with tremolo due to the nature of the frequency bands that are alternated. This circuit required two tubes to create a two-phase differential LFO that controls the gain of the two frequency bands, and then another tube to sum the two bands together. This implementation had a rather short period of availability perhaps due to the somewhat ‘expensive’ implementation. The basic idea is shown below:



One phase of the LFO signal is added directly with the low-band input signal, while the other phase gets added directly to the high-band signal. Essentially, the filtered signal ‘rides’ on top of the LFO signal on its way into the tube summing amplifier. This effectively changes the small-signal operating point of the filtered signal along the tube gain curve. When the LFO signal is at low voltages, the filtered signal will have more gain as the tube operates in its steepest gain region. Conversely, when the LFO is at higher voltages, the tube gain-curve flattens out, and the input signal experiences reduced gain. Since the two bands have opposite phase LFO signals, when one band is experiencing high gain, the other is experiencing low gain. When the two are combined, the opposite phase LFO signals cancel each other out, and the two alternating amplitude-modulated filtered signals comprise the output. This produces the tremolo effect of hearing a loud (bright) signal alternating with a soft (dark) signal.

Also, as a consequence of riding up and down the tube’s gain curve, the filtered signals experience slight changes in harmonic content due to the changing nonlinearities of the gain curve around the signal. This adds further complexity to the trem’s sound.

Power Tube Tremolo

Next in line was a more cost effective circuit that eliminated two tubes from the Harmonic Trem implementation. It used the LFO signal (no longer a two-phase LFO) to directly influence the power tube bias of the push-pull output stage.



In a push-pull power amplifier, two tubes are employed and biased so that they idle at substantially less than full power. This keeps power dissipation to a minimum when no signal is going through the amp, allowing them to provide power to the speaker more efficiently while increasing tube life. The guitar signal is split into opposite phases so that one tube conducts when the signal is positive, and the other tube conducts when the signal is negative. The two outputs are added together through the output transformer.

By applying the LFO to the bias, the power tubes are being biased into lower and higher idle currents. At low idle currents, the tubes are shutting off and signal gain (volume) is reduced. At higher currents, the tubes are running hot and higher gain results. This alternating gain produces the tremolo effect.

But there is more going on than just a change in volume. Secondary effects coming into play are crossover distortion as the tremolo volume heads towards zero and the tubes are shutting off. At the other end, increased power tube harmonic distortion occurs as the tremolo nears its maximum volume. The effects of power-supply sag also contributes to some of the dynamic response when playing through this kind of tremolo circuit, as it influences the relative bias point of the power tubes. All these things add up to contribute to the ‘magic’ of this trem circuit.

Photocell Tremolo

The Photocell tremolo uses a light-dependent resistor (LDR) to attenuate the input signal. The LDR is coupled with a miniature light bulb that is connected to the LFO. As the LFO oscillates, the bulb gets brighter and dimmer which in turn varies the resistance of the LDR. The varying resistance works with other circuit impedances to change the signal level.



The light element used in the classic photo-trem circuits in the 60s was a neon bulb which has a very fast response time, meaning it turns on and off very quickly and spends very little time in between. This produces a characteristic ‘hard’ sounding tremolo that is moving between two levels, almost like a square wave. The duty cycle (symmetry) of the tremolo depends on the characteristics of the bulb relative to the LFO voltages, but the classic Photo-trem circuits were tuned to spend most of their time at the higher output level (duty cycle >>50%, bulb is ‘off’), switching to the lower level only briefly during the cycle. At maximum intensity, a choppy trem results.

Also, as the photocell trem circuit is not buffered, the tremolo creates a varying load resistance in the signal path as the bulb changes the resistance of the LDR. This in turn has secondary effects on the signal’s frequency response that contribute subtle characteristics as well.

Capturing the Magic

We can see from the discussions above that the end result of these vintage tremolo circuits is much more than a simple cyclical volume fluctuation. The depth, warmth and overall vibe of each one of these tremolo types can only be created by giving consideration to the entire circuitry used in the process. For the harmonic tremolo, the interaction of the LFO with the input signal in relation to the preamp tube’s operating characteristics must be accounted for. The Power-tube tremolo must recreate the vintage push-pull power tube section including the phase-splitter, tube characteristics, and power supply considerations. The photocell trem must involve the proper bulb-LDR characteristics in relation to the LFO signal, along with secondary consideration of variable loading in the signal path. When these things are all properly accounted for, the difference from a simple VCA tremolo is apparent. The complex and subtle nuances come to life, producing the mojo of their vintage amp brethren.

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30 Responses

  1. Trem looks super badass. Stoked to hear some samples. I would trade the 70’s and 80’s reverb with just more spring choices if i could, ha. Love that off brand vintage trashy short spring sound, and not sure if that has been captured in stompbox form yet… looks super cool though.

  2. Interested to know what the hidden controls are, also will the settings of each time of trem and verb be saved? So you could set up your fav. LFO, photocell and harmonic trem settings and change to them using the dip switch without having to dial it in again. Would be a wicked feature.

  3. Would that there was an on-board tap…


    Still sounds great, though — but the need for external tap may be a deal-breaker for me.

  4. @Maury – we wanted to keep the pedal size small, and didn’t want to have an alt function for tap, thus why we opted for external tap. There are several very small and inexpensive tap pedals that can be used.

    And remember, many of the greatest examples of tremolo in music are not synced to the tempo of the song. So definitely don’t feel like you always have to be locked in! It’s always good to experiment! 🙂

  5. I’m totally amazed by this pedal!!! It’s gonna be a must for me!
    But can I somehow use the TAP knob on my Timeline as an external TAP for the FLINT?

  6. Can’t wait to hear this in person. Judging from the concept, white paper, and sound files, you guys hit it out of the park again. You’ve truly gone from making things I always wanted to making things I wanted but didn’t know I needed yet!

  7. Could you tell me at least an approximate date of release? I’m selling the Tremolo and Reverb on my board to get the Flint!

    Thanks guys

    1. Hi Saul – we’re hoping to release Flint within the next 30 days or so. We’ll start taking pre-orders on our online store within the next couple weeks. Thanks! 🙂

    1. Hi David— in the US Flint will sell for $299. Pricing will vary slightly overseas but should be roughly equivalent to our other pedals. Thanks! 🙂

  8. Can’t wait for this pedal. The sound clips are awesome! Especially the infinite sounds. Would love to add the Blue Sky’s “Shimmer”. It’s definitely on my next pedal list.

    Question on the tap tempo: Do you need a “NC” or “NO” switch? I’m working on building a 2 input tap tempo, one for my DD20 and one for this, hopefully controlled by the same switch. I’m just wondering if I could do it or not. I believe the DD20 uses a normally closed.


  9. This looks like the perfect companion for my hand-wired Vox AC15, and those sound-clips are fantastic. I’m new to your gear, but after one site visit I’m planning on purchasing both the Flint and Brigadier….I’m thinking seriously about the OB1 too!

    Really nice gear guys, can’t wait to try it!

  10. Holy T-Bone Burnett in a box! Flinging old gear at anyone who will give me money so that I can buy this pedal. Love my Timeline – can’t wait for this little dude.

  11. Would love to also hear some synth, drums etc. examples also!

    I know I’m in the minority here but I love your pedals and am a big Timeline fan.
    I actually put it over the master bus of a full track, it’s that good.

  12. Not to sound impatient, but it’s been a month. I need this to go into the effects loop of my Ethos. Please.

  13. What is the interaction between stereo and the tremolo? The Möbius demos include some sweet sounding combinations of the Harmonic Tremolo algorithm and panning, but I’m guessing the Fint doesn’t offer this? Will the tremolo just slam a stereo input (or a stereo reverb) down to mono?

  14. @Mark, the Flint tremolo will output the same way out of both the left and right outputs if you’re going into two separate amps or monitors however, it won’t pan back and forth in the stereo field like the tremolo machine on Mobius.

  15. @Rick: “the Flint tremolo will output the same way out of both the left and right outputs if you’re going into two separate amps or monitors” So does this mean that if you do Reverb > Tremolo that you end up with a mono result? Or is it just that the left and right channels from the reverb get modulated in the same way? (I assume it’s a stereo reverb or else there would be little reason to support stereo out.)

  16. Is the harmonic trem on the Mobius and Flint the same technology/sound? I tried the Flint out and it was exactly what I’m looking for.

  17. Just wanted to say thanks for a superb recreation of the tremolo in my 1960 Fender Vibrasonic amplifier. I used the amp for many years and mostly because of the incredible tremolo circuit. Now having the option of that tremolo on my pedal board is a luxury. It became a signature sound for me. The Vibrasonic and the Fender tube reverb tank were a bit beastly to pack around. Now I have that same effect through my much more practical modern amp. I’m very happy with this product any other unit pales in comparison.

    1. Thank you so much Bill for the wonderful comment. I will pass this along to Pete! He will appreciate hearing that 🙂

  18. Another interesting modulation approach was done on the 1953-1963 Wurlitzer Electrostatic Continuous Free Reed organs. The North Suburban Hammond Organ society ( NSHOS ) has an excellent animated article on how the vibrato was created electronically. Google> NSHOS Wurli 4600 . Models to save are 1953-1963 Wurlitzer Electrostatic Continuous Free Reed organs- 44 spinet, 4410, 4420, 4430, 4600, 4602, and 4800. [4410 spinet, 4600 console] -both the first organs to ever have Sustain, you’ll see how it was done and became the method all manufactures used. Of course Hammond tone wheel (A, BV,B2, C2, RT2,B3, C3, A100, D100, H100, T100,M2, M3, M100, L100, etc) couldn’t support this DC Keying bleeder capacitor/ resistor type sustain], These aforementioned Hammond and Wurlitzer electromechanical organs will never again be made. They are easily capable of operating many hundreds of years with simple capacitor changes every 40-60 years or more. Save them please!

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