Ask Eddie | Guitar Tone, Part 2

Mar 1, 2012 9:00 AM, By Eddie Ciletti



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photo of Eddie Ciletti

The goal of this three-part series is to help recording engineers, guitarists and DIY’ers understand some of the factors that influence guitar amp tone. The amp in question—Fender’s Hot Rod Deluxe (HRD)—was chosen because several of my students have complained of the same problem: excessive gain and insufficient tone-control range, which makes it an unruly recording amp. I felt I could offer affordable solutions to those who are cash-poor, and I am always happy to trade technical services for musicianship.

Last month, I showed where and how to tweak the preamp and overdrive circuits so that the default Tone Control settings—Bass, Mid and Treble—can start at mid-position. This month’s focus is about installing a real Master Volume Control because the factory MVC affects only the Lead/Drive channel. But before we dive in, I’ll answer a few questions from newbie tweakers.

Q1: Aren’t there dangerously high voltages in vacuum-tube amps? And if so, what can I do to keep from being electrocuted?
If the amp is powered up and down before the tubes have warmed up, and are drawing current, the power-supply capacitors can hold their charge for quite a while. The first healthy geek habit is always to unplug the amp. Then, using a voltmeter, measure the filter caps and discharge, if necessary, with a pair of insulated clip leads and a 10kΩ, 1-Watt resistor. If you can’t read the schematic enough to know what a power supply is or where to find it, just ask…or search the Net.

To get you started, here are two video links about transformers and power supplies:

Electrical transformer tutorial

Diode tutorial and AC to DC conversion

Q2: How do I learn to read a schematic?
The way I learned was to draw them; it’s kind of like practicing your letters in kindergarten. Through repetition and comparison, you will eventually be able to correlate the schematic symbols with their physical counterparts and notice circuit similarities. Vacuum-tube guitar amps have more in common than they have differences. Identical circuits can be drawn very differently.

Here is a PDF of the original factory schematic. All schematics used in this three-part series have been modified to improve clarity and to highlight modifications.

On to Part Two
Introduced in 1995, the Hot Rod Deluxe has an entirely “thermionic” signal path unless the Power Amp Input Jack (J4) is used, in which case an IC op-amp is introduced into the signal path. (The Low-Voltage IC circuitry, along with an overview of vacuum-tube options, will be explored next month.)

Preamp circuitry that relies on individual (discrete) gain stages (tubes or transistors) is Class-A, meaning that each device amplifies all 360 degrees of a sine wave, from the positive half (0 to 180 degrees) to the negative half (181 to 360 degrees). The Fender Champ, for example, has a single 6V6 power-output tube that, like its preamp tubes, is also running Class-A.

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