Fender Blackface Amp – Re-Voicing the Normal Channel
How many players have a two channel Fender blackface (or silverface) amp? How many players actually use the Normal channel? How many players would like to use the Normal channel for added versatility? If you are one of these players then this DIY article is for you. The modification to the Normal channel will give it a very different voice from the Reverb/Vibrato channel and using a quality A/B box you can transform your amp into a two channel machine. This mod applies to the older hand wired point to point amps. You can apply this mod to the newer amps with pcb’s if you locate the parts on the board and swap them accordingly. This also can be done on single channel Fender amps like the Princeton and Princeton Reverb as well.
Before we get started: WARNING: IF YOU DO NOT HAVE ANY EXPERIENCE WORKING ON TUBE AMPS, LEAVE THIS TO SOMEONE WHO DOES!!! THE WRONG MOVE IN A TUBE AMP CAN BE PHYSICALLY PAINFUL, DAMAGING, OR EVEN FATAL!
Let’s begin by identifying the part of the amp that you are going to be working on. After opening the amp look at the chassis and find the section of the circuit board all the way to the right. This is where the mod will take place. Then there is one other mod that is optional that will affect both channels. We will get to that part later in the article.
The stock values of a handful of components will be changed and if you do not like the mod it is very easy to return it to the stock values. Look at the layout below. The parts that will change are: C2, C3, C5 and R2. You can see the stock values in the chart below the layout. The new values will be:
R2= 220k – ½ watt minimum
I like to use the capacitors available from Sozo amplification and carbon composition resistors to retain the vintage sound and response. Resistors absolutely do make a difference! Metal film resistors do not affect the sound and response like carbon composition resistors. And yes, carbon composition resistors are a little noisy.
What basically happens here is that swapping the .047 and 0.1 tone stack caps tightens up the bottom end a bit and brings the mids forward. Swapping the 100k resistor for the 220k makes most of the signal pass through the 250pF treble cap. And lastly swapping the .047 cap for the 0.1 makes the sound a little fuller. It sort of restores what you are narrowing out with the other parts swaps.
In the end the Normal channel sounds a bit more tweedy and gainy (not too much) and works well for solos or rock and blues styles. The new components work well with pedals and your amp will be much more versatile.
The last mod is not for players that like the nice, clean blackface tone. This resistor swap will make the whole amp much more aggressive and will sound just a little pissed off, in a good way. Rock and blues players that don’t use the squeaky clean sound will love this. Look at the far left of the circuit board. There will be a few parts in this section called the phase inverter. When you locate the area work your way over toward the right. On the layout below there is a resistor labeled R12 and shows a value of 820R which is 820 ohms. Find this resistor on the circuit board. This resistor has colored bands: Gray, Red, Brown and Gold. This is the negative feedback resistor. If you want your amp to be much more aggressive try a 10k resistor. If it is too much cut back the value to say 4.7k. And again if it is still to aggressive cut back again until you dial in what you like. You can also replace this resistor with a pot if you like to vary the amount of negative feedback. Again this affects the whole amp not just one channel.
A few component changes can make a big difference and you can dial in a different voice to compliment the amps other channel making the amp more versatile. A good A/B box for this is made by Lehle. They are quiet and built like little tanks. Have fun tweaking and let us know how it worked out in our forums!!
Disclaimer: 300Guitars.com assumes no responsibility for any accidents occurring as a result of performing this mod.