This Article appeared in the March'12 issue of MIX
Hot Rodding the Fender Hot Rod Deluxe:
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
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
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
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
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
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)
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
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
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.'
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!