STOVER MODIFIED FENDER AMPLIFIERS
Stover-modified Fender amplifiers: vintage tone machines brought to the pinnacle of performance.
Stover modifications were intended to bring all Fender tube amplifiers up to the operating performance level of the vintage pre-CBS “blackface” amps without sacrificing any of the quality Fender sound. No amps were modified in any way that would prevent the owner from restoring the amp to stock specifications if so desired. Various circuit modifications (as described below) were made to improve the amps’ usefulness in both recording studio and live performance situations.
Al “Smokey” Stover was a gifted musician and audio engineer. During the 25 years running his “Amplifier Man” business, Smokey refined a comprehensive package of modifications for Fender tube amps that brought these classic “tone machines” up to a standard of performance previously unknown without sacrificing the sonic signatures of the vintage Fender line.
A Stover-modified Fender amp offers the ultimate in tone, flexibility, dynamic range, signal to noise ratio, and reliability.
The Stover-modified amp was a complete top-to-bottom enhancement of the 1964 pre-CBS “blackface” standard circuit design. All “silverface” (post CBS) amps were changed to pre-CBS circuit designs.
All amps received a 3-prong
grounded cable for safety purposes, and a central star grounding
point was created by soldering the brass face plated shield directly
to the chassis to eliminate to eliminate ground loop hum.
The power supply was upgraded by adding
a large choke and filter capacitor.
(This feature was found in the original
4-10 tweed Bassman amp but strangely disappeared in later models.)
This modification removes the background hum and dramatically lowers the noise floor as well as preventing a 120 hertz “ripple note” from being intermodulated with the signal – especially when the amp is driven into full clipping output.
The values of these components were carefully selected to produce desirable compression characteristics including attack and release times and compression ratio.
The rest of the power supply was re-capped or tested as good.
A variable negative bias was added to facilitate output tube changes as was a hum balance control for the filament line.
Signal Path and Pre-Amp
The stock ½ watt carbon plate resistors were a common source of thermal noise and they were replaced with 1-watt metal film precision types.
The tone circuit remained true to 1964 specifications with the exception of the middle control which was expanded in effectiveness. This helped thicken the tone of single-coil pickups. Amps without middle controls were given one on the “vibrato” (main) channel.
When signal coupling caps were replaced, orange drop mylars or polystyrenes were used.
Reverb was added to the normal channel. The system used a “send” control for each channel as well as a reverb tone control, all mounted on the rear amp panel.
Pre-amp overdrive was accomplished by disabling the vibrato and adding a gain stage. The gain structure components were carefully selected to produce a highly symmetrical wave form - free of artifacts and anomalies.
Output Section Modifications
Output section modifications included a switch for pentode or triode operation (full- or half-power), and a switch to change from fixed bias to self bias. Fixed bias is stock operation, and self bias produced a kind of instantaneous compression especially useful for recording.
A line output was created by placing a voltage divider circuit between the output transformer and the speaker, providing the true sound of the total amp circuit. It is unbalanced and produces 1.25V, (+4DB) as a nominal level for ideal interfacing with pro level audio equipment . The 1/4” female Line Out is on the rear panel.
The driver stage was set for symmetrical clipping of the output stages. Some units were equipped with driver balance controls. The balanced output stage and the balanced pre amp stages are responsible for the remarkable over drive characteristics of the Stover modified amp.
On the overdrive channel, another tone control labeled “Grunch” was sometimes installed to swing the tone more toward a Marshall amp sound.
Every solder joint was re-soldered for enhanced tone and reliability. (Smokey was a government-certified solder expert.) No amps were modified in any way that would prevent them from being restored to stock specifications.
On Stover-modified amps, the original ground switch became the power on/off switch. The original standby switch became the pentode/triode switch. The original power switch became the fixed/self bias switch.
In The Studio
The Stover-modified amp has a greatly enhanced dynamic range because of the dramatic lowering of the noise floor, and the usability of the full output power. The use of top quality low noise parts, the prudent elimination of unneeded gain, and the power supply modification all combine to produce an amp that far exceeds the original specification of the blackface line. Naturally, this makes these amps very studio friendly.
Copyright 2014 Bruce Bishop firstname.lastname@example.org