Add a Presence knob!

 So, your amp lacks a bit of treble? Does it sound a bit "muddy"?  Here's a quick guide on how to add a "presence" knob to your tube amp!
  note: 
Always be careful when opening a tube amp as the voltages inside it can truly KILL YOU! If you get hit by ~500Vdc, you will get severely injured or even killed... So BE CAREFUL!
 
 If your amp has a push-pull power amp design(meaning it has 2 or more power tubes) , it's fairly easy to add a presence control pot to the phase inverter stage.
 The phase inverter(PI) tube stage is the last 12AX7/12AT7 preamp tube in the signal path. The PI stage, drives the power amp tubes (usually 2 or 4 power tubes) and it has a common topology pictured below:

   The above phase inverter topology is common for many push-pull classA/B amps and it's easy to spot the approx. 4,7Kohm (=4700ohm) cathode resistor that goes to ground. It's important to find this cathode resistor in your amp as we will solder our presence control across this particular resistor. 

  The presence control consists of a 5kOhm(=5000ohm) linear potentiometer (you can go up to a 25k pot) in series with a 100nF(=0,1uFarad) capacitor. Marshall uses a 22kOhm Presence pot on the JCM800 model. Try different pots and capacitors for a different tonal range. 

  The pot will be used as a variable resistor so you only got to use 2 lugs (not 3) of the pot: the center lug and one side lug(see picture below).

   You have to connect the side lug of the presence 5k pot to the 4,7k resistor (point "A" on the schematic) and the 100nF capacitor to the ground:

 Once you have connected the presence control, you're ready! Just bolt the new knob somewhere on the amp chassis and try the new sounds!

 If your amp has already a presence knob, you could modify it by using different cap and pot values!

Just remember:

 Always be careful when opening a tube amp as the voltages inside it can truly KILL YOU! If you get hit by ~500Vdc, you will get severely injured or even killed... Unplug the amp from the wall plug for more than an hour, measure the power capacitor voltages with a multimeter and WAIT until voltages are lower than 12V inside the amp before you do anything!