This Article appeared in the March'12 issue of MIX Magazine

Hot Rodding the Fender Hot Rod Deluxe: Part-2

2012 by eddie ciletti

Go to Part-1
This is Part-2
Go To Part-3

Q-1: I want to learn about audio electronics - to be able to repair, modify and build my own gear. How and where do I start?

A-1: If you're super new to audio geekdom, look for simple DIY projects that have step-by-step instructions. Take risks with 'expendable gear' and if possible, attempt to collect schematics for everything your own. (This will be easier with older gear.) Search and destroy the net in your quest for knowledge. 

I am not going to take you step-by-step, but rather introduce some concepts that will assist in the learning process. Doing repairs and modifications is how many well-known audio geeks got started before creating their own products.

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 students have complained of the same problem - excessive gain and insufficient tone control range, making it an unruly recording amp. I felt I could offer affordable solutions to those who are cash poor and am happy to trade technical services for musicianship.

In Part-1 (February), 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 (MVC) because the factory MVC affects only the Lead / Drive channel. 

Q-2: Aren't there dangerous high voltages in vacuum tube amps? And if so, what can I do to keep from being electrocuted?

A-2: If the amp is powered up and down, before the tubes have warmed up and are drawing current, the power supply capacitors (caps) can hold their charge for quite a while. The first healthy geek habit is to always unplug the amp. Then, using a voltmeter, measure the filter caps and discharge if necessary with a pair of insulated clip leads and a 10k?, 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 - that's what I'm here for - or search the net. 

PS: Here are two video links about transformers and power supplies.

Q-3: How do I learn to read a schematic?

A-3: The way I learned was to draw them - it's kinda 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's a link to the original factory schematic. ALL schematics used in this three-part series have been modified to improve clarity and to highlight modifications.

PART-2: You Are Here

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 opamp is introduced into the signal path. The Low-Voltage IC circuitry, along with an overview of vacuum tube options, will be explored in Part-3.

Preamp circuitry that relies on individual (discrete) gain stages (tubes or transistors) are Class-A, meaning that each device amplifies all 360 degrees of a sine wave, from the positive half (0 - 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. 

Higher power amplifiers have two or four output tubes that operate in Class AB "push-pull" mode, meaning that each tube in the pair amplifies a little more than half the wave. Overdriven Class-AB amplifiers will symmetrically distort the top and bottom half of the wave, generating odd-order harmonics (mostly musical 3rds, followed by lesser amounts of 5ths, 7ths, etc.).

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 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 complimentary and 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 re-purposed 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 latter 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). 

FIGURE-1: The Power Amp section of the Fender Hot Rod Deluxe detailing the Master Volume and Negative Feedback pots.

In Figure-1, at the top left of the schematic, resistors R40 (220k?) and R38 (470k?) 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 re-routed to the top of the new Master Volume pot (1M? log / audio taper). The wiper (pot output) now feeds C24. 

Based on John Kargol's experiments (see PS), the new Master 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.

Figure-2: The new Master and Feedback controls are accessible but not disfiguring...

SURPRISE FEEDBACK TWEAK

If you like 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 HiFi 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 4? 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 = 47k?, R68 = 4k7?) the junction of which feeds the input (pin-7) of the 12AX7 driver tube V3b via C25 (.1uF). 

The adjustable feedback mod swaps out R68 (4k7?) for a 5k? 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 CCW 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. For example John has consistently preferred the no feedback setting. I

Feel free to ask any questions or share your own mods on my blog! 

PS: Many thanks to John Kargol, my former student and very motivated geek brother!