Groove Tubes Soul-o Series Guitar Amplifier

The Soul-oTM

Operators Guide50TM

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

PREBIAS control options:

The S50 preamp stage design uses fivedual 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.

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.
S50-15C 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 spent 25 years acquiring theknowledge to design and build your S50
amp....so please take a few minutes toread this guide and learn about it’soperation and the theory 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 withthe 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, andthen hand wired to the PCB. PCB andpoint to point construction, the best ofboth 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 ofour cabinets, only solid core USAplywood.
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 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 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 preamp fromthe guitar pickups and style of playing as
the need arrises.. an instant “hot-rodmod!”NOTE: Your S50 has a 9 pin straightener attached tothe 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 S50 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 themiddle 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 S50 tone controls are the Passive
type, similar to those used in theBlackface era Fender amps.
Many amps made today use Active typetone 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 usea common tube gain stage to boost theentire frequency range, then divide theBass, 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 tonec o n t rols, start listening with allcontrols at the 12 o’clock position,
where we “pretune” your amp here
at the factor y. If you want morebass, either: a) Crank up the Bass
control; or b) Lower the Tre b l econtrolthen compensate thevolume loss (or gain) by increasing(or decreasing) Gain and/or Volumecontrols. Passive controls share acommon gain stage, so turning any

S50-15 Top + EX212 Cab

Soul-o 50 Amplifier Operators Guide
one of them fully up (or down!) diminishes
the effect of the others. Should you wantmore “push” or “drive” from the preamp,
try setting all 3 tone controls at the 3o’clock position and adjust the EQ fromthere. This adds more touch sensitivity tothe preamp. Find a drier tone and feel by
setting the tone section to 9 o’clock andadding back tone as needed.

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and/or rating of your power tube can makea dramatic difference in your tone andpower.
For example, the S50 output power canrange from about 50 watts RMS with a low
number EL34R up to almost 80 watts
RMS with our KT88sv! Rebiasing isrequired with all tube changes unless you
are usg the same type and rating in the GTsystem. Unless ordered otherwise, yourS50 amp will come equipped with a Duetof our ver y best, the GT34Ls @ 60+ RMS.

NOTE: The S50 preamp design is much like the FenderBlackface era amps; more linear than the Marshall orearlier Fender Tweed designs. We added some gainoptions including a second high gain preamp stage
typical of the “Metal“ amps via the Pull-Boost feature(footswitchable) and also the inovative PREBIAS
control which changes the overall gain structure of thepreamps 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 similar to Hi Fi designs...ver yinteractive to the players touch. To our knowledge,
this is the first time these designs have beencombined in one amp.

Speaker Impedance Selector
and Speaker Output Jacks
The S50 custom wound output
transformer 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.

The Presence Control
This control interacts with the S50 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.

of your S50 FX loop. This allows you togradually increase or decrease the FX level
without changing the amps volume!

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.

S50 Technical Specs:

NOTE: The #1 jack has shorting circuitry which turns
the amp off if there is no speaker connected so you
will hear NO sound from jack #2 if #1 is not in use.

Input(s): 1/4 “ phone jack front and rear
Input Level: -10 dB nominally
FX Input level: +4dB dB nominallyGT’s Parallel FX Loop w/ Mix

Preamp Tube Section

Power Section:

The Power Amp Stage

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 andpower stage...so the sweet, fat, tube
preamp signal is changed into a thin,wimpy transistor signal. We call this “thetone sucking
transistor FX loopsyndrome”, and the GT’s parallel loopwith Mix control is the cure!

Your Soul-o 50 Class A/B design can use
any one of nine different matched Duetsfrom 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 withthe same rating. A change in the type

Set the FX unit 100% Wet and 0% Dry.
Now, you can use the Mix control on theS50 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”

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.

Rear Panel Features

OUTPUT
XFMR

To Operate:

V7V8EL346CA76550

EL346CA76550

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

SECRET GAIN
TRIM POT
for non boost
mode only

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

POWER
XFMR

S50-12C - 1 x 12” speaker
S50-15C - 1 x 15” speaker

V4V5Speaker Cabinet Options

EX112 - 1 x 12” speaker
V3V2FRONT PANEL

V1EX210 - 2 x 10” speakers
SIGNALPATH

EX212 - 2 x 12” speakers

INPUT

EX115 - 1 x 15” speaker

EX410 - 4 x 10” speakers

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©2004 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 S50 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.