December 1957 Popular Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
RF Cafe visitor Jim L. requested that I post
this Build Your
Own Vibrato article from the December 1957 edition of Popular Electronics. "Make like Elvis
with an 'electronic' throbbing guitar," is the pitch line. Vibrato, for the non-musically inclined,
is the 'wa-wa' sound of an instrument as it smoothly wavers in pitch about a central note. This circuit
is for use with an electric guitar, but acoustical stringed instruments like the violin and cello are
routinely played with vibrato effect by rocking the finger up and down the length of the string. In
typical 1950s style, the project is built with point-to-point wiring rather than using a printed circuit
Build Your Own Vibrato
Make like Elvis with an "electronic" throbbing guitar
By Frank H. Tooker
If you own an electronic musical instrument or a conventional instrument equipped with a pickup,
chances are that the vibrato described in this article is just what you've always wanted.
A vibrato is a device which continuously and automatically varies the amplitude of the music at a
low rate of speed, usually somewhere in the range of 6 to 15 times per second. For instance, it is a
vibrato which produces the soul-stirring throbbing, especially in the bass notes, of a pipe organ.
Construction. Layout and wiring are not especially critical. For convenience a turret
socket was used to mount the oscillator tube, V1, and its components, but a standard socket and tie
points will do as well.
Miniature coaxial jacks, of the type found on most hi-fi amplifiers, were used at J1 and J2 for the
output and input connections, respectively. If your setup requires something different, any conventional
type of jack may be employed.
Keep the power supply components well separated from the oscillator and modulator components, to
prevent hum pickup - and orient the tube sockets for reasonably short lead connections. Make sure that
the 6.3-volt heater leads are dressed snugly against the chassis and that they are well separated from
grid terminals and leads.
A miniature amplifier foundation (Bud No. CA-1754) was used for a chassis (any other setup of suitable
size can be substituted). The Bud chassis measures 5" x 7" X 2", and its over-all height with the grille
cover in place is 6". The finish is black crackle - but if some other color appeals to you, it's a simple
matter to go over the chassis with a couple of coats of plastic spray.
In some instances,
it may be possible to mount the vibrato circuit proper on the same chassis with the musical instrument
amplifier. This can be done provided that there is room, and that the power transformer in the amplifier
can supply the 0.6-ampere additional filament current demanded by the two vibrato circuit tubes. The
plate voltage requirement is approximately 175 volts.
Vibrator Circuit Point-to-Point Wiring Diagram
CI, C2, C3, C5-0.05-µfd., 200-volt, metalized paper capacitor
150-volt, dual electrolytic capacitor
C6, C7, C8-0.02-µfd., 200-volt, paper capacitor
C9a/C9b-40/40-µfd., 250-volt, dual electrolytic capacitor
CH1-3.5-henry, 50-ma. filter choke
J1, J2-Miniature phono jack
R1 R4, R7, R8, R10-220,000-ohm, 1/2-watt resistor
R3-500,000-ohm potentiometer (Rate control)
R5-10,000-ohm, 1/2-watt resistor
R9-3.9-megohm, 1/2-watt resistor
R11-500,000-ohm potentiometer (Depth control)
R13-560-ohm. 1/2-watt resistor
R14-47,000-ohm, 1/2-watt resistor
S1-S.p.s.t. switch (on Depth control)
SRI-20-ma., 130-volt selenium rectifier
T1-Miniature power transformer, 125 volts at 15 ma., 6.3 volts at 0.6 amp. (Stancor PS-8415)
V1-Type 12AX7 tube
V2-Type 12AU7A tube
1-Miniature amplifier foundation chassis or equivalent
1-Turret-type miniature 9-pin tube socket
1-Miniature 9-pin tube socket
hardware, grommets, tie points, etc.
Top and bottom views of the vibrato chassis are seen at left and below. Twist the
lugs of the can-type filter capacitors so that the filters are tight to their metal mounting plates.
Vibrato Circuit Schematic
Schematic and pictorial at left show the simplicity of the vibrato construction. If your amplifier
tends to "thump" at vibrato frequency, try a lower value capacitor for C7.
Block diagram shows correct interconnection of the three basic components of the
revised setup. Use shielded microphone cable between units to prevent electrostatic hum pickup. If there
seems to be excessive 60-cycle hum present, try reversing line plug of either amplifier or vibrato unit.
Vibrato Circuit Chassis Assembly Photo
Hooking It Up. The vibrato is intended to be inserted or connected between the musical
instrument and its amplifier. All you have to do to use it is unplug your musical instrument from its
amplifier and plug it into the input jack of the vibrato. Then connect a jumper cable of convenient
length between the output jack of the vibrato and the input jack of the amplifier.
The Rate control determines the rate or frequency of the vibrato effect. i.e., the speed at which
the rise and fall in amplitude occurs. Proper setting of this control depends upon the type of instrument
with which the vibrato is used and the type of music being played. Component values given in the parts
list permit the unit to be adjusted over the most useful range of speeds.
HOW IT WORKS
The schematic shows that the vibrato consists of two parts:
(1) a very low frequency audio oscillator, and (2) a modulator. The low-frequency oscillator is of the
phase-shift type which uses resistors and capacitors C1, C2, C3, R1, R2 plus R3, and R4 in three RC
sections to obtain feedback in the proper phase to produce oscillations. The rate or speed of the vibrato,
i.e., the frequency of the oscillator, is determined by the resistance and the capacitance used in the
RC sections. Thus, R3 is the Rate or speed control of the vibrato.
V1's second triode section serves a dual purpose: (1) it provides a low source impedance for the
RC feedback loop (taken from the cathode), and (2) it acts as a buffer to isolate the oscillator from
the connection to the modulator (taken from the plate).
The low-frequency oscillator signal is fed through the gain or Depth control, R11, to the control
grid of one section of the twin-triode modulator, V2. At the same time, the output signal from the musical
instrument is fed through to the control grid of the second section. The two signals mix in V2, with
the result that the gain or amplification of the musical signal is made to increase and decrease, smoothly
and periodically, at the rate of the low-frequency oscillations. Output from the modulator is taken
through C7 and coupled to the input of the musical instrument's conventional amplifier.
Plate current demand is very small, so a miniature power transformer, T1, and a miniature selenium
rectifier. SR1, more than meet the requirement.
The Depth or vibrato-frequency gain control determines the amplitude of the vibrato effect. The more
this control is advanced, the more pronounced the vibrato effect becomes. Proper setting depends upon
the strength of the musical signal fed to the modulator as well as on the type of instrument and the
selection being played.*
Probably the best method is to adjust the Rate and Depth controls for the most pleasing effect. You
can get some idea as to proper settings by listening to recordings in which a vibrato is used. Many
beginners tend to use too much vibrato, or to use it too frequently. Much can be learned by listening
POSSIBLE TROUBLES AND CURES
No Vibrato Effect: Check all wiring for errors. Make sure input and output plugs
are making good contact in the jacks. Look for faulty components. Check the tubes. Make sure the 12AX7
tube is in the oscillator socket, the 12AU7 in the modulator socket, and not vice-versa. Check interconnecting
cables for a poor solder joint or broken wire.
Distortion: Most, if not all, vibratos cause a certain amount of distortion. However,
the effect should not be objectionable. Reduce the setting of the Depth control slightly to see if it
improves the condition. If so, check the musical signal voltage at the input to the modulator. An input
greater than 0.7 volt r.m.s. may cause distortion. Check the value of components around the modulator.
Look for a possible defective component. And check the modulator tube.
Thumping in Speaker: A thumping noise at the vibrato rate may be due to low frequencies
pulling the voice coil or cone out of linearity. This isn't likely to happen unless you have an amplifier
with exceptionally heavy bass response. A high-pass filter cutting off at about 100 cycles inserted
between the vibrato and the amplifier may be necessary in such cases if all else fails.
Hum: Hum can come from a variety of sources such as a poor layout of components, excessively
long leads or poor lead dress, a defective modulator tube, faulty filter choke or capacitors, unshielded
interconnecting cables or cables with the shield ungrounded to chassis, using the unit close to power
wiring carrying heavy a.c. currents, etc. All of these possible troubles can be easily corrected.
* Maximum musical signal voltage should not be greater than approximately 0.7 volt,
to prevent overloading the unit. Optimum operation occurs with a musical signal input from the contact
microphone between 0.5 and 0.7 volt.
Posted January 1, 2012