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 welcome 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 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, 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 par ticle board in any of
our cabinets, only solid core 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, Pull-
Gain +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 built-
in 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 factor y. 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 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 factor y. If you want more
bass, either: a) Crank up the Bass
c o n t rol; or b) Lower the Tre b l e
c o n t rol 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
