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:
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.
Without a Master Volume Control, the power amplifier is more likely to be overdriven first, sometimes generating a type of distortion that is not always easy to ignore with a microphone. This is often due to using negative feedback around the power amp, which can be remedied with the second of two mods in this excursion. A true Master Volume Control reduces power amp sensitivity so that one key tube in the preamp chain can be driven harder in a way that is more musically complementary, generating the more subtle even-order harmonics (octaves).
Then and Now
The original Fender Deluxe delivered about 20 watts from a pair of 6V6 output tubes, while the HRD’s 6L6 pair doubles the power. Note that power output should not be confused with sensitivity; Voltage Gain comes from the number of preamplifier stages.
Of the many Fender Deluxe variations, all consistently have two preamp stages per channel, not counting tremelo and reverb tubes. The HRD swaps ICs for tubes in the reverb drive and recovery section. The “extra” tube stages have been repurposed into the Rhythm (clean) and Lead (Drive) channels, which certainly explains the nearly uncontrollable amount of gain.
A friend of mine, Wes Kuhnley of Resonant Amplifiers, tells me that some designs include a certain wow factor. Like the smiley-faced EQ curve that sells “studio monitors,” some manufacturers employ a gimmick that allows their product to compete with the cacophony of other players who are all searching for the affordable Holy Grail of amps at a music store. Said features are not necessarily useful outside of the store.
The stock HRD has three level controls: Rhythm (Clean), Lead (Drive) and Master—the last compensates for the amount of Drive required to saturate the Lead channel, so that the difference between the rhythm and lead levels can be optimized to taste. Two switches enable three modes that are officially called Clean, Drive, and More Drive.
Master Volume Control (MVC) circuits have a few variations, but whatever the implementation, my preference is that the MVC affect both Rhythm (Clean) and Lead (Drive) levels. You can easily test this theory on the HRD by inserting a potentiometer or a volume pedal in between the preamp output (Effects Send Jack J3) and the Power amp input (Jack J4).
Link to Document
Figure-1: The Power Amp section of the Fender Hot Rod Deluxe, detailing the Master Volume Control and Negative Feedback pots.
In Fig. 1, at the top left of the schematic, resistors R40 (220kΩ) and R38 (470kΩ) combine the dry and wet signals, which on the factory schematic feed C24 (.02uF) and then pin 2 of V3A (12AX7), which is one-half of the power amp driver stage. To insert the new Master Volume Pot into the path, the junction between C24 and R38/R40 must be broken and rerouted to the top of the new Master Volume Pot (1MΩ log/audio taper). The wiper (pot output) now feeds C24.
Based on my former student and now fellow geek John Kargol’s experiments, the new Master Volume Pot lives between noon and 2 o’clock so that the Rhythm and Lead level controls can finally be turned up a bit. The Drive Control level is guitar-dependent (2 for Tele, 4 for Gretsch Electromatic), and the Drive Master moved from 1 (pre-mod) to 6.5 (post-mod). The new gain structure allows John the ability to play in the center of the sweet zone, using a lighter touch for clean and a heavier touch for more saturation. This is similar to a compressor-limiter’s soft knee.
Surprise Feedback Tweak
If you like the vintage tone of Tweed-era guitar amps, you might be interested to know that part of their charm is due to the lack of negative feedback around the power amp. Negative feedback reduces gain by injecting a bit of the output into the input, a simple process that reduces distortion and improves frequency response. This works great for hi-fi applications, but it makes overdriven power amps sound like broken glass.
Deep in the belly of the Hot Rod Deluxe—and to the far right of the schematic—the feedback source is the 4Ω secondary tap (Green/Yellow wire) of the power output transformer, T1. At the External Speaker Jack, this tap connects to a gray wire that feeds a 10:1 voltage divider (R 69 = 47kΩ, R68 = 4k7Ω), the junction of which feeds the input (pin 7) of the 12AX7 driver tube V3b via C25 (.1uF).
The new Master and Feedback controls are accessible, but not disfiguring.
The adjustable feedback mod swaps out R68 (4k7Ω) for a 5kΩ pot, the wiper of which connects to R69. When the wiper is at ground (max CW), there is no feedback so the power amp has more gain—another reason for the new Master Volume pot. Turning the wiper fully counter-clockwise returns the feedback to Stock.
You know how digital recording allows production decisions to be postponed until the very end? Well, design engineers start with more variables than end up in the final production. Some ideas aren’t necessarily “features,” but variables in the equation that must be nailed down before the product’s release. And some features can always be improved—or at least modified—to suit the shredder.
Feel free to ask any questions or share your own mods on my blog! And many thanks to John Kargol, my former student and very motivated geek brother!
Eddie Ciletti’s virtual residence is at tangible-technology.com.