Groove Tubes Soul-o Series Guitar Amplifier

The Soul-oTM

Operators Guide30TMA 30 watt Class A Dual 6L6 tube amp with PREBIAS control and tube reverb
The Preamp Stage

PREBIAS control options:

The S30 preamp stage design uses fivedual triodes in a Class A design, eachtube performing multiple functions in thecircuitr y. The basic preamp controls are
PREBIAS, Gain, Pull-Gain +10dB boost,Bass, Middle, Treble, Presence, Reverband Volume.

LO offers the most clean headroom, aflatter, more accurate tone and response.
Great for calming down those high output
Humbucker type pickups.
MED increases the voltage, adding gain,fattening tone, and softening attack
dynamics. It provides more compression
as the the player picks harder.
HIGH gives the most voltage for the V1
and V2 preamp gain stages and reallymakes the overdrive character take off.
Thus, more touch, gain and compression.
Great for most lead applications or aheavily distorted, yet tight, Metal rhythm
tone. Usually this setting will be desired
for the lower output, single coil pickups.
These 3 very different PREBIAS settings
are actually doubled by the possibility to“pull boost” regardless of which PREBIAS
setting you are in. This +10dB boost canbe achieved by simply pulling the Gainknob or with the optional footswitch.

Our exclusive and innovative PREBIAS
control is located before the first Gain
control, and allows you to literally modify
the bias voltage for both the V1 and V2
preamp tube stages. These are the mostinfluential stages to the overall tone andfeel of your amp. In all other amps, thisvoltage is fixed by the designer.
S30-15C 15” Combo

Thank you for choosing a Groove
Tubes amplifier, and welcome to thegrowing list of artists and friends who share
our lust for true tube tone.
We’ve more than 25 years acquiring theknowledge to design and build your S30
amp....so please just take a few minutes toread this guide to it’s operation and learn
the theory behind the unique S30 design.

Construction Details
1 - All components are hand soldered,with over 100 hand soldered connections
on the tubes sockets alone.
2 - All transformers are custom designed
and made exclusively for Groove Tubes.
3 - GT S30 amps are all tube, Class A
circuitry designed specifically for use withthe 6L6/KT66 family of power tubes.
They come with our own GT6L6GE USAtubes unless other wise specified.
4 - We use only double sided, double
thick PC boards with double thick traces.
We use chassis mount type power tube
sockets hand bolted, hand soldered, andthen hand wired to the PCB. PCB andpoint to point construction, best of bothworlds!
5 - Each amp is hand built to order.
There are more than 18 hours of skilled
hand labor in each S30 amp we build!
6 - There is no particle board in any ofour cabinets, only solid core USA
plywood.
Of course all these details cost more, but
you can really hear the difference!

Players have been paying 100’s ofdollars to “mod” their preamp stage
voltages. Now, this “mod” feature is builtin to the S30 preamp. A lower PREBIAS
voltage setting will give the preamp stage
less output for the same given input
signal. Hence, more dynamic headroom
for a cleaner tone. The LO PREBIAS
setting can also be used to compensate
for hotter guitar pickups, making the amp
less “touch sensitive” which is better
suited for pounding rhythm guitar where“overloading” is not usually desirable.
Likewise, for lower output single coilpickups, switching to a MED or HI
PREBIAS position increases gain,expands the frequency bandwidth (morelows and highs) as well as making it more
touch sensitive so as to produce a“hotter” lead tone with noticeably moregain. YOU can shape the pramp frothe
guitar pickups and style of playing as theneed arrises.. an instant “hot-rod mod!”NOTE: The V4 and V5 preamp tubes can be altered
between using a 12AT7 or 12AX7 tube.
Using a 12AT7 in V5 (the power tube drive section)
will give a cleaner drive with more dynamics andsparkle. A 12AX7 in V5 adds more drive, a bit fatterand will be louder with more distortion.
Much the same attributes and differences can beachieved in V4 by alternating between the 12AX7 and
12AT7 dual triode tubes. The S30 leaves the factorywith a 12AX7 in both V4 and V5 for the maximum
overdrive and compression.
You can safely
experiment by trying a 12AT7 in either V4 or V5position. Your S30 has a 9 pin straightener attachedto the side of your cab, which can hold an extra tube
and should also be used when changing preamp
tubes. This keeps your pins straight and clean andyour sockets tight! See tube layout for exact locationof V1 thru V5 preamp tubes.
NOTE: there is also a “secret” gain trim pot locatedunderneath the chassis near the transformer. This
screwdriver adjusted internal pot sets the preamp
gain for the S30 in the non-boost mode (will notchange the Boost position).
This can balance the preamp gain to your guitar
pickup (more gain for the single coils and less for thehumbuckers). We usually set this control in the
middle from the factory. If you choose to adjust it
further, please turn of f the amp and also use an
insulated screwdriver for safe operation.

“Passive” Tone Controls
The S30 tone controls are the Passive
type, similar to those used in theBlackface era Fender amps.
Many amps made today use Active
type tone controls, which have aseparate gain section for each tone
band which literally “suck tone”
from the preamp. That’s why we
don’t use them. Passive controls
use a common tube gain stage toboost the entire frequency range,
then divide the Bass, Middle, andTreble with passive filters to shape
your tone. Passive tone controls
are more musical, but are alsointeractive with one another.
To get the most out of passive tonecontrols, star t listening with all
S30-15 Top + EX212 Cab

Soul-o 30 Amplifier Operators Guidecontrols at the 12 o’clock position, wherewe “pretune” your amp here at thefactory. If you want more bass, either: a)
Crank up the Bass control; or b) Lower theTreble control
then compensate thevolume loss (or gain) by increasing (ordecreasing) Gain and/or Volume controls.
Passive controls share a common gainstage, so turning any one of them fully up(or down!) diminishes the effect of theothers. Should you want more “push” or“drive” from the preamp, try setting all 3
tone controls at the 3 o’clock position andadjust the EQ from there. This adds more
touch sensitivity to the preamp. Find adrier tone and feel by setting the tone
section to 9 o’clock and adding back tone
as needed.

adjustments needed!) allows you tocustomize your amp, dramatically changing thepower, tone and response.
Unless ordered other wise, your S30 amp
will come equipped with our ver y best
6L6, a matched Duet of the GT6L6GE
tubes, our USA made Power Tetrode.
NOTE: Always turn your amp off, and let the tubes cool
down, before changing tubes.

Speaker Impedance Selector
and Speaker Output Jacks
The S30 custom wound
outputtransformer has 3 secondary windings forselection between 4, 8, and 16 ohms
speaker loads.
Always try to exactly
match the speaker load with theimpedance selector for maximum power.
Use the #1 speaker jack for just one
speaker (or as a combo amp). The #2 jackis for adding a second speaker.

NOTE: The S30 preamp design is like the FenderBlackface era amps and more linear than the Marshallor earlier Fender Tweed designs. We follow this withan early USA style two 6L6 Class A output designcommon to all amps from the 50’s. We added an
optional high gain preamp stage typical of the “Metal“
amps via the selectable PREBIAS control and PullBoost feature. To our knowledge, this is the first timethese designs have been combined in one amp.

NOTE: The #1 jack has shorting circuitry which turns
the amp off if there is no speaker connected.

GT’s Parallel FX Loop w/ Mix
This is the most tonally transparent FX
loop possible. When properly set, our loop
allows you to use any digital or analog FX
unit mixed in parallel with your original
preamp signal with no sacrifice of tone!
Most oops are the Series type, so the FX
unit is placed is between the preamp andpower stage...so the sweet, fat, tube
preamp signal is changed into a thin,wimpy transistor signal. We call this “thetone suckingtransistor FX loop
syndrome”, and the GT’s parallel loopwith Mix control is the cure!

The Presence Control
This control interacts with the S30 poweramp stage, changing the high frequency
response. It alters the feedback loop of
the power tubes. It adds highs to thepower stage, making it noticeably brighter.
Use more Presence for an acousticallydead room, and less for live rooms.

The Reverb Control
Our tube driven long spring Accutronics
Reverb is deep and harmonically rich andruns in parallel to the signal path so it
doesn’t sap highs as you crank it up.

To Operate:
Set the FX unit 100% Wet and 0% Dry.
Now, you can use the Mix control on theS30 rear panel to adjust the desired levelof the FX box. Usually, best results
happen with a Dry:Wet ratio of 98%:2%.
Another cool trick is to insert an ordinary
foot controlled volume pedal in the signal
path returning from the FX to the “INPUT”
of your S30 FX loop. This allows you togradually increase or decrease the FX level
without changing the amps volume!

Rear Panel Features
The Power Amp Stage
Your Soul-o 30 Class A design is “selfbiasing” and can use any of 8 different
matched Duets of 6L6, KT66 or 5881
Power Pentode (or Tetrode) tube types
currently available from our GT catalog,and in any of the 1 to 10 power ratings.
Changing power tubes (no bias

OUTPUT
XFMR

6L6

V1 - 12AX7
V2 - 12AX7
V3 - 12AU7
V4 - 12AX7 or 12AT7
V5 - 12AX7 or 12AT7
V6 & V7 - 6L6/KT66

V3

SECRET GAIN
TRIM POT
for non boost
mode only

6L6POWER
XFMR

Soul-o Speaker Cabinets
GT cabinets are “tuned” for guitar playing.
They are fundamentally different than mostother guitar cabinets on the market today.
We use 3/4” solid core plywood for thetop, bottom, back and sides but alightweight 5/8” plywood speaker baffleb o a rd. We slant the baffle board upwards
for better projection, and “tune” it’sresonance to optimize the tone for guitar.
This design produces tighter bass notes
avoiding the common “Big E String Mush”
syndrome.
GT guitar cabs come with our exclusive“Classic Clone”, USA made speakers.
These are made to our specifications,inspired by our favorite 60’s era speakers.

Technical Specs:
Input: 1/4 “ phone jackInput Level: -10 dB nominally
FX Input level: +4dB dB nominally

Preamp Tube Section
V1 - 12AX7
V2 - 12AX7
V3 - 12AU7
V4 - 12AX7 or 12AT7
V5 - 12AX7 or 12AT7
V6 & V7 - 6L6/KT66

Power Section:
30 watts RMS Class A type @ 4, 8. 16 Ohms
Optional 6L6 Family types:
GT5881, GT6L6R, GT6L6R2, GT6L6C, GT6L6CB,
GT6L6GE, GT6L6S, GTKT66HP and GTKT66C
AC requirement: 117 VAC Standard
100 VAC (special order), 230 VAC (special order)
H.T Fuse: 1 Amp Slo-Blo
AC Fuse: 100-120VAC@3A Slo-Blo
230 VAC = 1.5A Slo-Blo
Weight: S30-12C Combo 42 lbs.
S30-15C Combo 44 lbs.
S30 Top 32 lbs.
Options:
Footswitch (Boost & Reverb) with 25’ cord
Slip Cover with pocket for footswitch and cables

Speaker Cabinet Options

V4

V5

TUBE LAYOUT

www.groovetubes.com

V2

V1

SIGNALPATHFRONT PANEL

S30-12C - 1 x 12” speaker
S30-15C - 1 x 15” speaker
EX112 - 1 x 12” speaker
EX212 - 2 x 12” speakers
EX410 - 4 x 10” speakers
EX115 - 1 x 15” speaker

INPUT

Groove Tubes LLC - Custom Shop Products
1543 Truman Street, San Fernando, California 91340 USA, Tel 818-361-4500 Fax 818-365- 9884

www.groovetubes.com©2003 Groove Tubes LLC. All rights reserved. Groove Tubes, the “GT-in-a-circle” logomark, Groove Tubes Custom Shop, Groove Tubes Custom Shop Products, Soul-0 and S30 are trademarks andservice marks created and owned by Groove Tubes LLC. All other marks are the property of their respective owners. All specifications subject to change without notice.