Ask Eddie | Guitar Tone, Part 2

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

HOT ROD DELUXE

Polls


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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).

photo of Master and Feedback controls

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.

Feature Creep
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.






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