Loading... Please wait...
  • Call us at 778.655.2263
  • My Account

Tube Guitar Amp Voicing

Categories

Tube Guitar Amp Voicing

What is voicing?

The following discussion will dive into the guts of amp design.  However, I’ll embark on this voyage with a promise that you won’t need to open your calculus textbooks to understand what’s going on (you do have calculus textbooks, right?).

First some background.  Amps are filled with various components ... transformers, potentiometers, resistors, capacitors, etc.  In this article we’ll focus on how selecting specific values of resistors and capacitors at specific points in an amp’s circuit change its tone.

Resistor Capacitor 3We don’t need to delve into the details of resistors and capacitors.  All we need to know is that they are individual components that, when connected in various configurations, alter the sound of the electrical signal from your guitar as it passes through them ... basically they can roll off highs or lows or enhance or diminish mids.

Even in a simple tube amp there are multiple points in the circuit where a designer has to make a decision as to exactly what value of resistor and capacitor to use so that the signal passing through the amp will be altered to meet that particular amp’s tone objective.

Of course the tone alteration I’m talking about is not the only way that an amp creates its particular sound.  The choice of tubes, the overall circuit design, transformers, speakers, etc. all have a significant impact on the amp’s sound.  However, “voicing an amp” typically refers to the small tweaks that a designer makes to dial in the details of tone shaping.

The below block diagram depicts a common layout of a tube amp.  The guitar signal plugs into the input circuit, wends its way through the amp and the speaker is connected to the output transformer.

A few notes to review before we get going:

  1. There are a bunch of details not shown in this diagram.  However, it’s enough for our discussion.
  2. I won’t discuss tone controls in this article.  If you’re interested, you can refer to my earlier article “Tone Controls: Tone and Gain Sucking Leaches?
  3. The term “gain stage” refers to a section of an amp related to a tube or ½ of a tube (some tubes come with two tube sections in one glass bottle).  Since tubes are present to amplify the signal the word “gain” is used in the term.  Our amplifier has four gain stages: preamp gain stage #1 (½ of a 12AX7 tube); preamp gain stage #2 (the other half of the 12AX7); the phase inverter (uses both halves of a second 12AX7); and the power amp stage using two EL84 power tubes in our example.
  4. We’ll leave the inner workings of the phase inverter and power amp stages for upcoming articles.

OK, let`s have at it!

Tube amp block diagram resized 600

 

 A bulletThe first voicing opportunity is in the input circuit.  There are many input circuit configurations out there in the amp world.  Typically, they consist of one or more resistors although some include a capacitor or two.  For those resistor only designs, the input circuit resistor(s) interact with the tube’s own internal capacitance and alter the frequencies that pass through to the first stage tube.

 In its simplest form the purpose of the input circuit is to make sure that the guitar signal is not ‘loaded down’ resulting in an unacceptably weak signal entering the amp and to make sure that the signal entering the first tube stage is properly coupled to the tube input.  The other purpose is to filter out radio frequency signals that may be picked up at the guitar or on the guitar signal’s route to the amp – that’s the primary spot where we stop your local ‘easy rock’ station from playing through your amp (which would ruin it and make it forever unplayable if it were ever to happen!).

Those more complicated input circuits can be used as a first step in voicing the amp.  For example, the Ampeg B22X input circuit has a ‘normal’ input and a ‘bright’ input.  The bright input circuit has a different resistor/cap arrangement than the normal input circuit and as a result quite significantly rolls of the bass frequencies of the guitar signal hence making a bright tone.

 

 B bulletEach of the preamp stage tubes has to be biased.  This is a commonly referred to term that I won’t bother defining other than to say that it involves adding components to establish the operating conditions of the tube.  For our purposes all we need to know is that for a preamp tube the bias circuit typically consists of a resistor and maybe a capacitor.

The resistor is chosen to set the tube’s operating condition.  The voicing decision involves the capacitor.  The first choice to be made is whether to add one at all.  A premp tube without a cap in the bias circuit will have relatively less gain than one that does.  If we add a cap we increase the gain of that stage.  We can also adjust the amount of bass that the stage passes on to the next stage.  The bigger the cap the more bass frequencies we pass, the lower the cap the less bass we pass on. 

So, by adding a cap and adjusting its value we can filter more or less bass from the signal.

 

C bulletAs you can see, a cap is placed between each stage of the amp.  This is called a coupling cap – it couples one stage to the next.  Here again we can alter the frequencies that pass from one stage to the next.  The smaller the cap value the more bass frequencies we filter out of the signal and vice versa.  Often this cap is sized so that it passes through all audible bass frequencies, however it can be used to ‘tighten’ the sound going to the next stage if the designer wants to take some bass response out of the amp’s tone.

 

We usually hear about output transformers and tubes as being the determinants of amp tone.  While they certainly are significant contributors, as we’ve seen above making a single component change can alter an amp’s frequency response.  Once tubes and transformers have been selected “voicing an amp” is a critical task (and one that is painstaking and time consuming!).  However, it is necessary to REALLY dial in an amp’s character.

If you want to dive into the details of gain stage voicing I highly recommend Richard Kuehnel’s fine books “Gutiar Amplifier Preamps” and “The Fender Bassman 5F6-A”.  These are both exceptionally well written books that step through amp circuit details.  HIGHLY recommended for the amp lover that already has an electronics background.

As always I’d love to hear from you regarding this article or any amp related questions or comments.  Just send me an email!

Don Mackrill

Mack Music Systems Limited