Changing Your Cathode Bypass Capacitor

Published on
January 21, 2021 at 4:43:35 PM PST January 21, 2021 at 4:43:35 PM PSTst, January 21, 2021 at 4:43:35 PM PST
Welcome back everyone to another installment of Make It Monday.  Today we are going to walk through a rather simple modification that can be made to plenty of circuits; changing your cathode bypass capacitor.  I'm going to be making this modification on my Tweed Deluxe style amp kit from Mojotone, but for those of you following along at home, feel free to use whatever you have available.  

Before we dive in, let's talk a bit about what exactly the cathode bypass cap does, and how changing it can affect the tone of your amplifier.  We need to begin with the concept of "cathode degeneration," which is a normal part of tube operation, but can be seen as an unwanted byproduct -- it is also a form of "negative feedback."  Negative feedback can notoriously cancel out frequencies in your signal, so in order to combat this cancellation of frequencies, a "cathode bypass capacitor" is implemented.  Take a look at Image 1A below and notice all the way to the left, the capacitor and resistor running in parallel...

The resistor is in place to hold the desired operating range of the tube itself, which in turn, creates the negative feedback and the cancellation of frequencies.  As mentioned, the capacitor is there to boost those frequencies back into the signal, and depending upon the value of the capacitor we choose to implement, we can shape the tone of our amp.  Below is a sound clip of my amplifier with the original cathode bypass capacitor.  My amp settings are TONE - 6, VOL - 3; I will insert other sound clips along the way for comparison, always using the same settings and same mic placement.

Now that we've covered the basic theory behind this operation, let's go over the short list of tools you'll need to get this done:

Screwdriver or electric drill
Needle nose pliers
Soldering iron

The first step, which you can see from the above photo I've already taken, is to remove our chassis from our amp and get it onto our workbench.  So go ahead and use your screwdriver or electric drill to remove any back panel and/or chassis screws and gently remove the chassis from the cabinet.  Once you've located your cathode bypass capacitor, the rest of the operation will be quite simple.  Go ahead and heat each of the solder joints holding the capacitor in place, and simultaneously use your needle nose pliers to pull the lead(s) out (Image 2A).

Note: Pay attention to which direction the capacitor's negative and positive ends are facing.  We will need to insert our new capacitor in with the same orientation when the time comes.  

In all likelihood, the resistor will come out along with the capacitor -- this is fine as we will reimplement the resistor at the same time as the new capacitor.  

This next part is not required for today's project, but I am going to reinsert my resistor with absolutely NO cathode bypass capacitor, just to demonstrate what the amp sounds like when the negative feedback is NOT bypassed.  Check out the sound clip below...

Notice the lack of life in the sound; there is a general flatness to the amp's tonality now so I really need to get another cathode bypass cap in there ASAP!  The original cap in my amp was a 25uF Sprague capacitor; this is a pretty "broad" cap when talking about the frequencies it allows through.  The cap I will be implementing is a .47uF cap and will be much more selective about the frequencies it allows through.  The expected (and desired) effect is to attenuate some of the lower frequencies while boosting some of the higher ones, ultimately giving my amp a brighter sound.  Below is the cap I'll be using, and as you can see, there is an arrow pointing towards the negative lead; I'll need to make sure this goes in facing the right direction.

The following may not apply to your situation, however in my case, the leads on this new capacitor are too short to connect to both eyelets on my fiberboard.  Same thing with my resistor!  To get around this (temporarily) I'm going to use a piece of solid core wire I have sitting around for the "length" I need, and then I'll wrap the leads of my capacitor and my resistor around this piece of wire.  Then, I'll apply solder to the wrapped leads before inserting the components into my fiberboard.  It's a bit of a mess, but you can see what I'm doing in Image 4A below.  


I will, of course, implement a fresh resistor with the proper length of leads if I decide to keep this modification in my amplifier.  It will work just the same, but I prefer a cleaner build!  

Finally, it's time to solder this bad boy in.  Just grab your soldering iron, heat the corresponding eyelets (one at a time) and insert the leads.  You should end up with something along the lines of Image 5A below.  


Okay so now it's the moment of truth.  What does it sound like?  Check out my sound clip below...


There is an immediate and obvious difference in this amp's tonality.  The liveliness came back to it, and it has an overall brighter sound than it did when we started out today.  I'll include a final sound clip at the very bottom, which will quickly move through all three of the above stages so you can compare more easily.  I'm actually really happy with the results of this project.  It's a super simple modification that takes hardly any time, and hardly any money.  

All that's left to do is put the chassis back inside the cabinet and keep on rocking out!  You know what to do, grab that electric drill or screwdriver and put that thing back where it belongs.  I know it's hard to believe, but we're done!  There are a ton of mods out there, especially for simple circuits like this Tweed Deluxe.  Not all mods have to be big and daunting and scary; sometimes they are nice and quick...and cheap!  Thanks again for tuning in today -- we hope to see you all next time.