Fender Amp Mod
Many Fender amplifier owners have wanted to reproduce the full-blast sound of the amp, but at lower volume levels. Adding a master volume control accomplishes this purpose, as it can lower the final sound level, allowing the original volume control to be cranked up to 10. This modification has become so popular that it is built-in on the newer Fender amps. Periodically, technically inclined guitarists ask me for schematics to Fender's updated circuit so that they can "upgrade" their amps. But when you look at the revised circuit, the design is so confusing that, at first glance, it appears they may know something that we don't. Surely, I reasoned, there must be some urgently compelling reason for using such a monstrous part as a center-tapped potentiometer into which has been grafted a pushpull DPST switch (available only from Fender by special order through an authorized retail store). And the macabre use of feedback into the reverb tank... obviously that must impart some kind of startlingly unique texture to the sound. Plus there is the mysterious connection of the master volume pot into the power amp feedback loop from the output transformer... hmm...
In spite of the unknown advantages of Fender's elaborate design, independent technicians have for years been installing master volume controls, using a shamelessly simple trick. Figure 1 shows a simplified drawing of the output and driver stages of a typical 50 Watt Fender amp (100 Watt Fenders are basically the same, but include two extra power tubes in parallel with V7 and V8. Smaller amps - except for the Champ - have similar circuits as well). V6A and V6B are connected as a differential amplifier, supplying a pair of balanced signals to the inputs of power tubes V7 and V8. The outputs of the power tubes are developed in the primary coil of the output transformer, whose secondary coil connects to the speakers. To install a master volume control, all that is necessary is to connect a 500k audio pot across the inputs of V7 and V8!
As shown in figure 1, the 500k pot connects between the two 1.5k resistors wired to the inputs of V7 and V8. These two resistors are mounted between pins 5 and 1 of the tube sockets for V7 and V8. Wires from pin 1 of each of these tube sockets can be traced to two solder grommets on the circuit board, directly behind the BASS control of the VIBRATO channel. These grommets provide us with convenient terminals for connecting the two wires from the 500k master volume pot.
The master volume pot itself is usually mounted in the hole previously occupied by the tremolo INTENSITY pot, which is then exiled to some lonely spot on the rear panel, and henceforth ignored. Some guitarists, after years of relentlessly boycotting the unique tremolo capabilities of their amps, take this opportunity to eliminate it entirely by soldering the middle terminal wire to some nearby ground connection, and ditching the pot. Others, caring less for cosmetic appearance than total access to all controls, wind up drilling a hole through the front panel nameplate, mounting the master volume pot there. True to its reputation for standing up under abuse, the Fender amp thus mutilated does not seem to suffer in resale value.
Alternatively, for those guitarists who are not sure why they wanted the master volume control in the first place, you can mount it in the same inconvenient spot on the rear panel that is usually reserved for the neglected tremolo INTENSITY pot. This will make it quite difficult to adjust during performance. Whatever you do, however, twist the master volume pot wires together in the same fashion as the tube heater wires. Omitting this crucial detail would violate a long-standing tradition - and leaves the unmistakable mark of a non-professional job.
As popular and effective as this modification has proved to be over the years, one drawback is that the master volume control distorts both the VIBRATO and NORMAL input channels at the same time. This means that the NORMAL channel can't be used, to amplify another instrument along with guitar, since serious intermodulation distortion would result. To overcome this difficulty, I have developed an alternate master volume scheme, which produces distortion in the final amplifier stage of the VIBRATO channel alone. Since the signal from the NORMAL channel does not pass through this tube, it remains undistorted and operates normally. Thus the master volume control affects the VIBRATO channel only.
Figure 2 shows the final gain stage of the VIBRATO channel in a typical Fender amp (Fender amps that don't have built-in reverb do not have this stage, and are not good candidates for this type of master volume control). At the input of tube V4B, the ordinary sound of the guitar from tube V2B is mixed with the sound of the reverb from tube V4A. The output of V4B is then mixed with the signal from the NORMAL channel (V1B), and sent to the power amp.
Figure 3 shows the same circuit modified to include a master volume control (new components indicated by *). In order to increase the signal strength at the input of tube V4B, the grounded 220k resistor at pin 7 in figure 2 has been removed. The REVERB control has also been changed from a 100k to a 500k linear taper pot, so that the ordinary signal from V2B will receive less attenuation when only small amounts of reverberation are needed. The 100k resistor at the wiper of the REVERB pot prevents the output of V4A from distorting when the control is set close to zero.
When the master volume control is rotated clockwise to 10, negative feedback through the 8.2M resistor reduces the gain of the amplifier stage so that the signal level at pin 6 is the same as it was in the original circuit, thus providing normal operation. As the master volume control is reduced, less feedback flows through the 8.2M resistor, increasing the gain of V4B, which overdrives its output when the signal level is high enough. At the same time, the output signal to the power amp is reduced, resulting in less volume. The position of the master volume control has negligible effect on the NORMAL channel signal from V1B since its impedance is only 100k.
A side effect of the use of a master volume control is lack of brightness when the original volume control is turned up close to 10. To remedy this problem, the original SPST BRIGHT switch (in the VIBRATO channel) is replaced with a DPDT switch, with one half of this switch replacing the original BRIGHT switch circuitry, and the other half switching a 1200 pF capacitor and 12k resistor across the master volume pot as shown in figure 3. This produces more brightness as the master volume control is turned down. As a final precaution against oscillation, a 5 pF 400 Volt capacitor is soldered directly to terminals 6 and 7 of the V4B tube socket.
The 100k 1/2 Watt resistor at the wiper of the new REVERB pot should be mounted on the pot itself. Figure 4 shows how this resistor can be physically stabilized by pressing the right-hand terminal of the pot down on top of the body of the resistor.
The 1200 pF capacitor and 12k 1/2 Watt resistor can be mounted on the DPDT switch as shown in figure 5. The 8.2M resistor should be mounted on the circuit board at the junction of the 3.3M and 470k resistors at the input of V4B. A tiny hole should be drilled nearby into the top circuit board layer so that the free end of the 8.2M resistor can be inserted into it. This will prevent the resistor from swivelling around. Also, I generally use shielded cable to make connections to the master volume pot, since this part of the amp is more likely to pick up hum than the section illustrated in figure 1.
Fender amps have traditionally been very popular, and have inspired much loyalty among musicians. But just wait until you add this modification; that black and silver box will really sing.
P.S. At this point, I suspect that some readers, intent on producing ever more excruciating sounds from their amps, will imagine that they can combine both master volume schemes into one amp. Unfortunately for their purposes, this will not increase the amount of distortion that can be obtained from either of these modifications alone. Guitarists who seek to explore the limits to which their Fender amps can be abused are advised to locate a service technician who is willing to wire up the NORMAL and VIBRATO channels in series... although the resulting sound can also be closely approximated with two or three inexpensive fuzz boxes.
Fender is a registered trademark of CBS Musical Instruments
Feature by Lee Powell
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