Tube Amp Modifications
A word about tubes amp mods. Mods will only be performed on an
amp that is reliably safe and running at spec'd voltages. If an
amp needs caps, tubes, screen resistors or some output tube
re-wiring, I will not and highly recommend that you do not
perform any mods. You will not observe the maximum benefit of the
mod and possibly push your amp into destruction, waste my time
and your money. In general, I will most likely discourage mods on
vintage amps. And BTW, I don't drill new holes on BF amps. On some old Valco's, Ampegs etc, it is sometimes
necessary to modify wiring or component placement for a cap job
or transformer replacement, but this is considered an evil
necessity to achieve a healthy amp. The mods below are all
reversible, they all have been proven to work and do not
sacrifice the reliability of the amp. For local folks, I usually
have an amp on hand that has one of the following mods so the
owner can "taste test" the mod before deciding.
Silver to Black Face Conversion
Buy a '70s Silverface for 2/3 the cost of a Blackface and have it "Black Faced". Satisfaction guaranteed. Silverface circuit design changes incorporated by CBS, caused the amps to become sterile and rigid sounding with less musical character. A Blackface conversion returns the amp back to the more musical amps we all grew up with.
$92.90 parts and labor.
The tremolo circuit used on Black and Silverface Fenders uses an optoisolator. An optoisolator is nothing more then a neon light and a photo sensitive cell. The photosensitive cell's impedance changes when the neon light fires. This circuit works well and is a cheaper solution then the old Tweed or Brownface style of tremolo, but the circuit scheme used to implement the photocell robs on the order of 30% of the original preamp signal. I replace the 50KRA Intensity pot with a SPST (on/off) switchable 50KRA pot. When the Intensity knob is clicked off, a 30% increase in volume and thus fidelity is observed. This is analogous to pushing in the loudness button on your home stereo. When the Intensity knob is clicked on, the tremolo circuit works the same as the stock factory setup.
$37.55 parts and labor.
Tremolo Mod Schematic
Dual Channel Reverb
Convert that never used Normal channel to reverb. This mod also puts both the Vibrato and Normal channels in phase so you can the use an external A/B box like ones from Morley for channel switching while always keeping reverb. Surfs Up! I personally have a '64 Vibroverb Clone I designed with this mod that is killer. I use a Morley A or/and B box. I play through the vibrato channel and when I want some extra punch, I add the normal channel to the Vibrato channel via the A and B switch.
$95.70 parts and labor.
(a.k.a. Feedback/Distortion Control).
This mod controls the amount of feedback that is fed back to the inverter stage. Normally a fixed value of 820 ohms is used on Fender Black/Silverface tube amps. The mod uses a 2K ohm wired in series with the fixed feedback resistor and lets you adjust the feedback. The extension speaker jack is most commonly used to insert the pot. Thus the mod is visually hidden from the front view of the amp. As the pot is turned up to 10, less feedback is allowed to return to the amp and thus the amp will breakup up more easily at lower volume The variability of this pot allows the player to adjust the distortion characteristics unique to the player's style and desire. Note, typically Fender Tweed amps did not use feedback. So this mod when the sole pot is on 10, is somewhat characteristic of a Tweed amp.
$29.00 parts and labor.
Soul Control Schematic
2/4 Output Tube Power Switch
(a.k.a. Twin and Showman's)
This mod converts the ground switch to a half power switch by allowing either all 4-output tubes to run (full power) or by allowing only 2-output tubes to run (half power). Since this mod uses the existing ground switch, the power cord will need to be replaced by a 3 prong grounded AC cord.
$71.50 parts and labor.
Pre Amp Out
There are several ways to accomplish a preamp out mod. The most common I use is to use the Normal channel as a preamp. This allows you to still use the Normal chaanel tone controls for EQ and the Normal channel volume control as a preamp output level control. The output can be routed to the Tremolo foot switch in the back of the amp or back to the number two Normal channels input 1/4 jack. This mod converts the Normal channel tone recovery tube into a low impedance cathode follower capable of driving any stomp box, mixing board or back into the amps' Vibrato channel. Maximum output level is 500mVp-p.
$90.60 parts and labor.