How a 4 track session inspired a
24 track guitar amp modification
Copyright January 2012 by Eddie
Those of us who have suffered through uncooperative
guitar rigs are especially appreciative of guitarists who not only can
play, but who intuitively know which axe to use and how to complement it
with electronics and effects. From an outsider's perspective it may seem
like black magic, but in reality, there is a scientific method to the madness,
even if it's distilled from obsessive woodshedding and successive approximation,
a.k.a. trial and error!
Matching up instruments, pickups, effects, amplifiers
and speakers can be a very spendy tweak-fest. Or is it a very tweaky spend-fest?
That said, my students are not bathing in the warm glow of green cash,
so I help with affordable guitar amp tweaks from the inside out. Because
it is important to know where to point the finger - even if overstating
the obvious - start by asking these questions...
(a flow chart comes to mind)
Q-1: Is the guitar amp consistently not in
the ballpark no matter who plays through it and no matter what axe is connected?
A-1: If yes, try connecting the amp
to another speaker cabinet. If the sound improves, it could be the speaker,
so proceed to Q-5. For combo amps, there is also the possibility that one
or more of the tubes are microphonic - that's next month's column, but
one of the reasons an alt cab might not improve the sound.
Q-2: does the instrument sound good with any other amp and speaker
A-2: If yes, proceed to Q-4. If no, proceed to Q-3. If confused
by all that, just read on...
Q-3: Is it my axe or my amp?
A-3: Assuming the axe is fun to play and capable of being in
tune - up and down the neck - there are plenty of pickup options. Back
in the day, Fender amps were intended for Fender instruments and the same
applied to Gibson's products. It was the geek version of an 'arranged marriage.'
Cross-pollinating the two heightened one's awareness of pickup and preamp
variations and while pickups are not my area of expertise, their relationships
with electronics are. (See Q-4.) If you've got a second- or third-hand
axe, have someone who knows guitar innards to inspect the wiring. For something
so simple, it is amazing how messed up some can be...
Q-4: Do instruments have special relationships with their amplifiers?
A-4: Why YES! The EQ section of guitar and bass amps - known
as the Tone Stack - can be genre and instrument specific, but the circuitry
is simple as you'll see at the HOT ROD MODS heading.
Q-5: There are so many speaker options.
How do I choose?
A-5: SPEAK EASY
Vintage and Retro speakers can be expensive, but instrument speakers
are neither high fidelity nor fancy - they act as filters and resonators
- which is why we generally prefer their 'altered' tone over a full range
HiFi speaker. Table-1 shows how speaker diameter changes two (of many)
loudspeaker parameters - Frequency Response and Free Air Resonance (Fs).
For the same raw materials - cone, surround and spider - increasing cone
diameter increases mass, which degrades high frequency response (filter)
and shifts the Free Air Resonance (Fs) down. The HiFi 'cure' would be to
reduce the woofer's mass by using 'space age materials,' like a thinner
- but equally rigid - cone material. A dark woofer is preferable when it
hides the less desirable high frequency distortion artifacts.
Put your ear close to a disconnected woofer and tap
on the cone - thump, thump, thump! That's the Free Air Resonance. (Fs is
parameter you're looking for in a sub kick woofer.) Now short out the speaker
terminals with a clip lead or a dollar coin and notice how the resonance
disappears. This exaggerated difference is similar to the way vacuum tube
and solid state amplifiers affect a speaker's personality - an effect known
as 'damping factor.' You can do a similar test with a powered monitor's
woofer. Starting in the OFF state, tap the woofer a few times to get used
to its tone and then power up, dry as a bone!
|Free Air Resonance
Table-1: Three woofers built from the
same materials, allowing comparison of the effect diameter has on two parameters
resulting in what we might perceive as warm or bright, for example. Compared
to vintage and retro designs, these three are available at a price that
won't break the bank, allowing a place to start with some wiggle room to
HOT ROD MODS
The seed for this column started with an analog recording session to
4-track half-inch tape. My class was doing a demo for a student band. Drums
and bass were mixed to track-1, the lead vocalist went direct to track-2
and two guitars got their own space on tracks 3 and 4. The lead guitarist
had his parts and tone, but the rhythm guitar needed nuance on multiple
levels - production, tone and performance and his Hot Rod Deluxe amp wasn't
What we gleaned from the demo experience was applied to the 24-track
session. We found sweeter cymbals, replaced the snare head and swapped
out the Hot Rod Deluxe for my own customized studio amp - a Fender Pro
Junior (with the 10-inch speaker in Table-1, MCM part number 55-2951).
We also swapped out the electric (rhythm) guitar for an acoustic with a
After the session my assistant, John Kargol, told me that his Hot Rod
Deluxe had similar problems. A quick web search yielded a schematic. During
our tweak session, John adjusted his amp based on hours of experience and
I listened, tried to dial in a better tone and concurred with his analysis
- too much gain and too much bottom.
Figure-1: Simplified Fender Hot Rod Deluxe preamp schematic. Capacitor
C1 shown in Reverse Video is the key component change.
HOT ROD 101: TONE MOD
From the outside, a Hot Rod Deluxe chassis has a vintage, tweed-era
look. But modern amps are expected to toggle between Rhythm and Lead (Drive)
settings - via footswitch or front panel - and to do that requires relays
(K) and switches (S). The letters relate to 'part designations' on the
schematic (See Figure-1).
Like most modern amps, the printed circuit board (PCB) construction
does not lend itself to tweaks as compared to older turret board designs.
This "restriction" guided us to choose the 'low hanging fruit' approach
- we clipped out the component in question and tacked in a decade box to
audition alternate values. Afterwards, the PCB was removed and the new
component properly soldered in.
While a guitar amp's EQ - a.k.a. the Tone Stack -
has no 'unity' setting, the Tone controls 'should' be able to live in their
center region to allow boost and cut. This was not the case with
the Hot Rod Deluxe and so it became our goal. Rather than attack the Tone
Stack, we started at the First Gain Stage, which is one-half of a 12AX7
(V1a). There, Capacitor C1 is in parallel with cathode resistor R5 (1k5?).
NOTE: “uF” = micro-farads.