The Cathode Follower

Let’s talk about another tube stage that is common in Marshall amps but isn’t a gain stage, the

cathode follower. Take a gander:

This looks familiar, but different. The guitar signal enters pin 7 of V2 and exits pin 6 as a standard gain stage. This time, the signal next enters directly into pin 2 (note there isn’t a coupling capacitor to block DC.) Pin 1 does not have a plate load resistor like other gain stages. High voltage is placed on both pins 1 and 2, forcing the signal to go through the pin 3 cathode. Why do this? This acts as a buffer and lowers the output impedance and therefore loads down the signal less in the tone stack that follows. It helps keep the tone controls to retain those gains.

The cathode follower is also an effective way to feed an effects loop! Soldano did this very thing with the SLO100 seen here:

Note the 2.2K resistor to ground. This dumps a ton of signal to ground and makes the signal small enough to feed an effects unit.

As far as its effect on the guitar signal, when the previous stage is really pushed with a lot of

gain, it creates a very unique and musical overdrive with some compression. It warms up a fizzy signal, too. My three cents is that the cathode follower’s unique tone is a big part of the reason why a cranked Fender Super Reverb will never really sound the same has a cranked up Marshall. It’s also the reason some people are surprised when their version of a Marshall JCM800 does not sound the same as other JCM800’s. At some point, Marshall removed the cathode follower on a few of their models and they really did not have the sound most players expected to get.

The first version of the Sunn Model T used an even more aggressive take on this by adding a fairly large bypass capacitor to the gain stage before the cathode follower. See here:

If you think this Sunn Model T circuit looks familiar, you’re right! It’s just a slightly tweaked version of the Marshall circuit we were just looking at.

Now might be a good time to mention the famous This is Spinal Tap “This one goes to 11” joke. Don’t get me wrong, it truly is a very well written, acted, and directed scene. The punchline is solid and really sums up Christopher Guest’s brilliantly dumb character. But here’s my problem. This joke has been told to me 509 times. And without the deadpan delivery. The deadpan delivery is what made it funny to begin with! The joke is not that the amp has controls that go to 11, it’s that Nigel doesn’t understand that 11 is just a number printed on a faceplate. If you’re going to tell me or someone else this joke, please try to have the confused dopey look on your face and get the tone correct.

Editors note: at the time of this publication, the author has been told this joke 521 times and no one has performed it well at all.