The Variable Varitone Chris Wargo 1-29-2012

Introduction This paper is a follow-up to the paper I wrote called “The Gibson Varitone - Where’s the Disconnect?” In that paper, I discussed the operation of the Gibson Varitone and walked through a technical analysis of the circuit in order to explore the common assertion that the circuit colors the sound of the guitar, even when in position 1 (bypass). That paper is available at http://www.onlyandnothing.com/varitonepaper.pdf Using both theoretical models and actual sound tests, I proved to myself that there was no detectable difference between position 1 bypass on a Varitone and a true-bypass setup. However, after I wrote the paper, I had communications with multiple ES-345/ES-355 owners who insisted that the tone of their guitar changed after removing their Varitone. There was enough insistence from seemingly reasonable people (and one or two unreasonable persons) to make me wonder if I’d missed something. I discussed this issue with ES-335 expert Charlie Gelber (es-335.org, Opporknockity Tunes Guitars). It seemed like Charlie and I were thinking along the same lines. From Charlie’s personal experiences, and from the experiences with those I’d been in communication with, we noticed a trend. The Varitone guitars that seemed to sound a little wonky to Charlie, and the guitars that appeared in my reader’s before/after stories all seemed to be from the mid to late ‘60’s. My modeling and testing was done on ’59 and ’61 ES-345s. Charlie and I knew that the Varitone construction changed in the early/mid 60’s, when they went from discrete components to a ceramic chip package housing the majority of the circuit. It was now obvious that if there was a coloration effect from the Varitone, it was probably a phenomenon associated with the newer chip package and we needed to do some tests to explore the possibility.

Experimental In order to study the chip based Varitone, I bought an unmolested wiring harness on eBay from a 1965 ES-345. I also borrowed an inductor and a 1959 switch assembly from Charlie. This allowed me to make directly comparative measurements and tests on the two types of Varitones found in 1950’s/60’s Gibsons. Figure 1 shows the differences between the two types of Varitones (more on the foot pedal later). In 1959, the Varitone was made from a rats-nest of discrete components. In other words, individual capacitors and resistors had to be soldered to the switch, without having any shorts between the components. I’m sure that this was not a quick process by any means. At some point in the early 60’s, the switch was made to a ceramic chip package made for Gibson by Sprague, which no doubt saved assembly time for a Varitone harness. Instead of positioning twenty separate components into a tight space, only two chips that laid flat on top of each other were required. Although the same number of solder connections was made, the reduction in components surely made this a quicker process, and reduced the chances of shorts. To test the two separate Varitone constructions, I took two approaches. The first was to measure both assemblies and dissect the 1965 chip to see how it was made. The next was to build both Varitones into a foot pedal with an A/B selector and a true bypass switch. This allowed me to directly compare the 1965 against the 1959 with the same exact guitar in fast succession.

Figure 1 – Varitones from 1959 and 1965 mounted into a foot pedal.

Figure 2 – Top view of the Varitone pedal.

The 1965 Chip Package Dissected I wanted to see what was inside the 1965 chip package before I went any further with my testing. Since the ES-345 Varitone is stereo and I was going to make a mono Varitone pedal, I was able to steal one of the chips packages from the assembly for dissection. In the past I’ve had good luck swelling/softening some epoxy chip packages by soaking in acetone for a couple days, but that didn’t work on these (the encapsulent doesn’t actually seem to be epoxy). Instead, I had to chip away at it which caused some damage. Still, I was able to save enough to see what was going on. The assembly is made on a 0.025” thick ceramic wafer. The anti-pop resistors are made with screen-printed carbon traces. The small value capacitors are made by using the substrate as the dielectric in a parallel plate configuration and having printed/etched capacitor plates on each side of the chip. For the larger capacitors, discreet components were used to achieve the desired capacitance (most likely MLCC packages). While I was poking around with some resistance and capacitance measurements on the chip fragments, I noticed something very important. There was a huge amount of stray capacitance between adjacent traces on one fragment of a chip. There was no adjacent capacitor plate on the back, but the capacitance still came in at 132 pF over a very small section of trace. This is not an insignificant value for this circuit, as we will discuss later. I can’t be sure if this would be the case when the chip package was new, or if the dielectric constant of the wafer/substrate had drifted higher over time (perhaps due to humidity penetrating the package over 47 years).

Figure 3 – Dissected Varitone chip. Note that the round areas on the capacitor plates are not hidden components, but rather solder blobs.

Figure 4 – Reverse side of dissected chip package.

Figure 5 – Small fragment of chip used to measure stray capacitance

Figure 6 – Stray capacitance measurement on small chip fragment (lead capacitance was zeroed out).

Measurements of Functioning Varitone Assemblies Resistance and capacitance measurements were then made on both the 1959 and 1965 assemblies. The anti-pop resistors on both measured pretty close to the correct value of 10MΩ. The filter capacitor values all measured fairly close to nominal on both too. However, capacitance measurements were then made across the 10MΩ anti-pop resistors with striking results. Stray capacitance was negligible on the 1959 assembly but significant on the 1965 circuit. And recalling the analysis from my previous paper, these resistors are what block the Varitone form affecting the signal when in position 1 bypass. And in the 1965 assembly, they are “leaky”.

position 1959 1965

2 8 708

3 8 992

4 8 1150

5 8 1300

6 8 1500

Table 1 - Capacitance across pop resistors (pF)

Figure 7 – The Varitone circuit. Note that the 10M resistors are blocking the Varitone from affecting the signal when in position 1 bypass (shown).

Chips and Dip These measured stray capacitances were then entered in my original LTspice model to see how they affected the calculated frequency response of the circuit. Measured capacitances were inserted in parallel with the 10M anti-pop resistors and the results were calculated. The result is the same as true bypass for the 1959 discrete assembly, but the 1965 chip assembly shows a pronounced 5dB dip with a 2 dB resonant peak at the leading edge of the dip.

Figure 8 – Model of Varitone circuit with leaky switching resistors. Capacitances between 0.7nF and 1.5nF were measured, which are not insignificant. In other words, the highly resistive 10Gig switching resistors in the chip package are leaky to AC signals. All Varitone capacitances are measured values, including the intended filter capacitors.

Figure 9 – LTspice model of 1959 and 1965 Varitones, using measured stray capacitances.

Frequency Sweeps of the Two Assemblies Frequency sweeps of both the 1959 and 1965 Varitones were made to compare the frequency responses at each position (and true bypass). These are actual measurements, not models, but it should be pointed out that like the models, these too might not be the “correct” frequency responses. The Varitone is a passive filter network, and passive filter networks are very dependent on the impedances of the devices hooked up to them. Simple series and shunt resistors were used to approximate input and output impedances seen by the Varitone in a guitar, but these are only approximations. The goal wasn’t here to look at absolute values, but rather compare the two different constructions to see if the response of the 1965 construction was different from the 1959 construction. Sweep test setup #1 – High input impedance test BK Precision 3020 function generator set to 0.22V sine wave 50ohm output impedance with 10K series resistor at output, fed into Varitone circuits Keithley 175A multimeter 10Mohm input impedance with 1M shunt resistor across inputs Sweeps were performed manually, with data logged and plotted in MS Excel. 40 samples per plot were taken.

-40.00

-35.00

-30.00

-25.00

-20.00

-15.00

-10.00

-5.00

0.00

10.00 100.00 1000.00 10000.00

Modeled position 1 Bypasses

1959 Pos 1 model

1965 Pos 1 model

Figure 10 – Manual sweep tests through 1959 Varitone

Figure 11 – Manual sweep tests through 1965 Varitone

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10 100 1000 10000

1959 Varitone Sweep

Position 1

Position 2

Position 3

Position 4

Position 5

Position 6

True Bypass

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10 100 1000 10000

1965 Varitone Sweep

Position 1

Position 2

Position 3

Position 4

Position 5

Position 6

True Bypass

Figure 12 - Manual sweep test bypass data (zoomed in) These measurements show some clear differences between the two Varitone assemblies. Most obviously, the 1959 assembly is more powerful when used, leading to more drastic notches than the 1965 assembly. But more importantly perhaps, the 1959 assembly exhibits perfect bypass behavior in position 1, while the 1965 assembly does not. Under the measurement conditions, in position 1 bypass the 1965 assembly exhibits nearly a 3 dB cut at 1.8KHz, which is significant. This is almost certainly the result of the stray capacitance measured across the 10M resistors, allowing the signal to leak into the filter even in position 1 bypass. Sweep test setup #2 – Medium input impedance test Automatic frequency sweeps made by Audiotester 3.0 running on a PC Sine waves were looped through the Varitone circuits using an Emu 1820M sound card 560 ohm output impedance with 10K series resistor at output, fed into Varitone circuits 10Kohm input impedance 200 samples were taken per plot, leading to higher resolution than the manual test. However, due to the impedances involved, these values are probably less accurate than those from the manual sweeps. However, the same trends were observed, where the 1965 Varitone colors the signal in position 1 bypass, while the 1959 assembly does not. This test was run through long studio cabling/patch bays. As a result, some HF roll-off is seen from cable capacitance.

-3.00

-2.50

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

10 100 1000 10000

Bypass Positions

True Bypass

1959 position 1

1965 position 1

Figure 13 – Medium impedance sweeps of 1959 assembly

Figure 14 - Medium impedance sweeps of 1965 assembly

The Varitone Pedal As fun and useful as modeling and bench testing are, it is not always the end to an argument. As shown in Figures 1 and 2, both constructions of Varitones were mounted in a foot pedal in order to do fast A/B’s of a real guitar playing through a real amp. The pedal allows for comparing the 1959 Varitone directly against the 1965 Varitone, or comparing either directly against true bypass. A battery and pair of LED’s are used in the pedal, but they are not at all connected to the audio circuit. They are hung off the extra poles of the bypass and selector switches and only serve to indicate the current state of the pedal. The pedal functions identically without the battery, but there is no way to know the status of A/B/bypass. For simplicity, only a single inductor was used (from a 1959 ES-345), and the A/B switch switches the inductor to the active Varitone unit.

Figure 15 – Schematic for the A/B/Bypass Varitone pedal. This pedal was used for my Youtube video demonstration found at http://youtu.be/HecbQnYoY5g

Conclusions

The earlier discrete Varitones are likely to have no effect on the signal in Position 1 bypass

The later orange Sprague chip-package Varitones are likely to have an upper midrange notch in Position 1 bypass

No statements can be made at this time regarding 1970’s and beyond Varitones (like the ones with the blue chip packages) since I didn’t have any to experiment with