Groove Tubes Soul-o 50 Operator's Manual

A 50 watt Class A/B Dual EL34/6CA7 tube amp with PREBIAS control and tube reverb

Thank you

for choosing a Groove

Tubes amplifier, and w

elcome to the
growing list of artists and friends who share
our lust for true tube tone.

We’ve spent 25 years acquiring the
knowledge to design and build your S50

amp....so please take a few minutes to

read this guide and learn about it’s
operation and the theor y behind the S50.

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 S50 amps are all tube, Class A/B
circuitry designed specifically for use with
the EL34/6CA7 family of power tubes.
They come with either our exclusive
GTE4Ls or the new GT6CA7GE tubes
unless otherwise 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, and
then hand wired to the PCB. PCB and
point to point construction, the best of
both worlds!

5 - Each amp is hand built to order.
There are more than 18 hours of skilled
hand labor in each S50 amp we build!

6 - There is no particle board in any of
our cabinets, only solid cor e USA
plywood.

Of course all these details cost more, but
you can really hear the difference!

The Preamp Stage

The S50 preamp stage design uses five
dual triodes in Class A, each per forming
multiple functions in the circuitr y. The
preamp controls are PREBIAS, Gain, Pull­Gain +10dB boost, Bass, Middle, Treble,
Presence, Reverb and Volume.

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 most
influential stages to the overall tone and
feel of your amp. In all other amps, this
voltage is fixed by the designer.

Players have been paying 100’s of
dollars to “mod” their preamp stage
voltages. Now, this “mod” feature is built­in to the S50 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 coil
pickups, switching to a MED or HI
PREBIAS position increases gain,
expands the frequency bandwidth (more
lows and highs) as well as making it more
touch sensitive so as to produce a
“hotter” lead tone with noticeably more
gain. YOU can shape the preamp from
the guitar pickups and style of playing as
the need arrises.. an instant “hot-rod
mod!”

PREBIAS control options:

LO offers the most clean headroom, a
flatter, 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 really
makes the overdrive character take off.
Thus, more touch, gain and compression.
Great for most lead applications or a
heavily distor ted, yet tight, Metal rhythm
tone. Usually this setting will be desired
for the lower output, single coil pickups.
These 3 ver y different PREBIAS settings
are actually doubled by the possibility to
“pull boost” regardless of which PREBIAS
setting you are in. This +10dB boost can
be achieved by simply pulling the Gain
knob or with the optional footswitch.

NOTE: Your S50 has a 9 pin straightener attached to
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 and
your sockets tight! See tube layout for exact location
of V1 thru V5 preamp tubes.

NOTE: there is also a “secret” gain trim pot located
underneath the chassis near the transformer. This
screwdriver adjusted internal pot sets the preamp
gain for the S50 in the non-boost mode (will not
change the Boost position).
This can balance the preamp gain to your guitar
pickup (more gain for the single coils and less for the
humbuckers). We usually set this control in the
middle from the factory. If you choose to adjust it
further, please turn off the amp and also use an
insulated screwdriver for safe operation.

“Passive” Tone Controls

The S50 tone controls are the Passive
type, similar to those used in the
Blackface era Fender amps.
Many amps made today use Active type
tone controls, which have a separate 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 to boost the
entire frequency range, then divide the
Bass, Middle, and Treble with passive

filters to shape your tone. Passive

tone controls are more musical,
but are also interactive with one

another.
To get the most out of passive tone
c o n t rols, start listening with all
controls at the 12 o’clock position,

where we “pretune” your amp here
at the factory. If you want more
bass, either: a) Crank up the Bass
c o n t r ol; or b) Lower the Tre b l e
c o n t r ol then compensate the
volume loss (or gain) by increasing
(or decreasing) Gain and/or Volume

controls. Passive controls share a
common gain stage, so turning any

Groove Tubes Soul-o Series Guitar Amplifier Operators Guide

The

Soul-o 50

TM

TM

S50-15C 15” Combo

S50-15 Top + EX212 Cab

one of them fully up (or down!) diminishes
the effect of the others. Should you want
more “push” or “drive” from the preamp,
try setting all 3 tone controls at the 3
o’clock position and adjust the EQ from
there. This adds more touch sensitivity to
the preamp. Find a drier tone and feel by
setting the tone section to 9 o’clock and
adding back tone as needed.

NOTE: The S50 preamp design is much like the Fender
Blackface era amps; more linear than the Marshall or
earlier Fender Tweed designs. We added some gain
options including a second high gain preamp stage
typical of the “Metal“ amps via the Pull-Boost feature
(footswitchable) and also the in ovative PREBIAS
control which changes the overall gain structure of the
preamps stage by increasing the supply voltages.
The power amp stage is a unique output design we
‘borrowed” from Aspen’s famous Black Plexi paneled
Red Park 50 amp made in the mid 60’s during
Marshall experimental period. The Park’s unique
output transformer is a complex design with many
interleafings and simila r to Hi Fi designs.. .ver y
interactive to the players touch. To our knowledge,
this is the first time these designs have been
combined in one amp.

The Presence Control

This control interacts with the S50 power
amp stage, changing the high frequency
response. It alters the feedback loop of
the power tubes. It adds highs to the
power stage, making it noticeably brighter.
Use more Presence for an acoustically
dead room, and less for live rooms.

The Reverb Control

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

The Power Amp Stage

Your Soul-o 50 Class A/B design can use
any one of nine different matched Duets
from our EL34/6CA7 and 6550/KT88
family of 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 will require
a new bias adjustment unless you are
replacing tubes of the same tube type with
the same rating. A change in the type

and/or rating of your power tube can make
a dramatic difference in your tone and
power.
For example, the S50 output power can
range from about 50 watts RMS with a low
number EL34R up to almost 80 watts
R M S with our KT88sv! Rebiasing is
required with all tube changes unless you
are usg the same type and rating in the GT
system. Unless ordered other wise, your
S50 amp will come equipped with a Duet
of our very best, the GT34Ls @ 60+ RMS.

Speaker Impedance Selector
and Speaker Output Jacks

The S50 custom wound output
transformer has 3 secondar y windings for
selection between 4, 8, and 16 ohms
speaker loads. Always try to exactly
match the speaker load with the
impedance selector for maximum power.
Use the #1 speaker jack for just one
speaker (or as a combo amp). The #2 jack
is for adding a second speaker.

NOTE: The #1 jack has shor ting circuitr y which turns
the amp off if ther e is no speaker connected so you
will hear NO sound from jack #2 if #1 is not in use.

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 loops are the Series type, so the FX
unit is placed is between the preamp and
power stage...so the sweet, fat, tube
preamp signal is changed into a thin,
wimpy transistor signal. We call this “the

tone sucking transistor FX l o o p
syndrome”, and the GT’s parallel loop

with Mix control is the cure!

To Operate:

Set the FX unit 100% Wet and 0% Dry.

Now, you can use the Mix control on the
S50 rear panel to adjust the desired level
of 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 S50 FX loop. This allows you to
gradually increase or decrease the FX l e v e l
without changing the amps volume!

Soul-o Speaker Cabinets

GT cabinets are “tuned” for guitar playing.
They are fundamentally diff e r ent than most
other guitar cabinets on the market today.
We use 3/4” solid core plywood for the
top, bottom, back and sides but a
lightweight 5/8” plywood speaker baff l e
b o a rd. We slant the baffle board upwards
for better projection, and “tune” it’s
resonance 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.

www.groovetubes.com

Groove Tubes

LLC

Custom Shop Products

1543 Truman Street, San Fernando, California 91340 USA, Tel [phone] Fax [phone]

www.groovetubes.com

©2004 Groove Tubes LLC. All rights reser ved. Groove Tubes, the “GT-in-a-circle” logomark, Groove Tubes Custom Shop, Groove Tubes Custom Shop Products, Soul-0 and S50 are trademarks and
service marks cr eated and owned by Groove Tubes LLC. All other marks are the property of their respective owners. All specifications subject to change without notice.

S50 Technical Specs:

Input(s): 1/4 “ phone jack front and rear
Input Level: -10 dB nominally
FX Input level: +4dB dB nominally

Preamp Tube Section

V1 - 12AX7
V2 - 12AX7
V3 - 12AU7
V4 - 12AT7
V5 - 12AX7 or 12AT7
V6 & V7 - EL34/6CA7 or 6550/KT88

Power Section:

50 watts RMS Class A/B type @ 4, 8. 16 Ohms
Optional EL34 Family types:

GT6CA7GE, GTE34Ls, GTEL34R, GTE34R2,
GT6550C, GT6550R, GT6550R2, GTKT88SV,
GTKT88C and GTKT88C2

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: S50-12C Combo 42 lbs.
S50-15C Combo 44 lbs.
S50 Top 32 lbs.

Options:

Footswitch (Boost & Reverb) with 25’ cord
Slip Cover with pocket for footswitch and cables

Soul-o 50 Amplifier Operators Guide

SIGNAL

PATH

EL34
6CA7
6550

EL34
6CA7
6550

V5

V8

V7

V4

V2

V1

V3

FRONT PANEL

INPUT

OUTPUT

XFMR

POWER

XFMR

SECRET GAIN

TRIM POT

for non boost

mode only

TUBE LAYOUT

V1 - 12AX7
V2 - 12AX7
V3 - 12AU7
V4 - 12AT7

V5 - 12AX7 or 12AT7

V6-V7

EL34/6CA7/6550

Speaker Cabinet Options

S50-12C - 1 x 12” speaker

S50-15C - 1 x 15” speaker

EX112 - 1 x 12” speaker

EX210 - 2 x 10” speakers

EX212 - 2 x 12” speakers

EX410 - 4 x 10” speakers

EX115 - 1 x 15” speaker

Rear Panel Features