THD Electronics Flexi-50 User Manual

THD FLEXI-50 INSTRUCTION MANUAL 1

THD Flexi-50

Inst r u c tion M a n u a l

Thank you for your purchase of the THD Flexi-50 amplifier! The Flexi-50 is a precision
hand-built 50-watt Class-AB amplifier with foot-switchable overdrive/boost, foot­switchable master volume control and the ability to use almost any preamp and power
tubes in any combination, including 6L6, EL-34, 6V6, 6CA7, 8417, 6550, KT-66, KT-77,
KT-88, KT-90, KT-100, EL-84 (with Yellow Jacket adaptor), 6K6, 6F6, 12AX7, 12AY7,
12AU7, 12AT7, 12AZ7, 12DW7, 12BH7, ECC-83, ECC-82, ECC-81 and many more,
giving the user a huge pallet of available sounds simply by changing tubes. The amplifier
has a fat, clear, full clean sound reminiscent of late 1950s to early 1960s American
combos, and overdrive to rival the best British heads. Even at the highest overdrive
settings, the amp still demonstrates dramatic touch sensitivity, cleaning up very well
when the volume on the guitar is reduced. Additional features include external bias test
points and individual bias controls, permitting the user to quickly and accurately set the
output tube bias with any digital voltmeter. The dual bias controls permit the user to
correctly bias even mismatched tubes, eliminating the need for matched power tubes.

The unique combination of our touch-sensitive input circuit and the wonderfully active
and reactive tone control section (that we slaved a year to perfect) make for a front-end
that really responds to subtle and not-so-subtle changes in the signal being fed into the
amplifier either by the guitar or any effect that may be between the guitar and the
amplifier.

THD FLEXI-50 INSTRUCTION MANUAL 2

TABL E OF C O NTE N TS

Instruction Manual .................................................................................................. 1

For People Who Hate Manuals—Quick Start Setting................................................. 2

Hot Rod Flexi ............................................................................................................. 2

Flexi-50 Controls........................................................................................................... 3

Front Panel.................................................................................................................. 3

Back Panel .................................................................................................................. 5

Example Settings........................................................................................................... 7

Dark and Smooth ........................................................................................................ 7

Less Compressed ........................................................................................................ 7

Heavy Bass ................................................................................................................. 8

Smooth Upper Mids .................................................................................................... 8

Vintage Chime ............................................................................................................ 8

Biasing the Flexi-50 For Different Tubes..................................................................... 9

Power Tubes ............................................................................................................... 9

Preamp Tubes ........................................................................................................... 10

For People Who Hate Manuals—Quick Start Setting

We highly recommend you at least scan through the manual first to learn how the Flexi
works. If you are itching to get started now and will read it later, here’s a basic amp
setting that will get you up and running. Keep in mind the following conventions:

• There are no numbers on the dials, so knob positions are expressed in “clock face”
notation.

• “Full” means that the knob is turned clockwise to its limit. (Sometimes referred to as
“dimed” since it represents a value of “10” on numbered dials).

Hot Rod Flexi

Input: Hi Bright: ON Master: OFF
Volume: 1:00 O’clock Boost: ON Master Vol: N/A
Treble: Full Gain: Full Cut: Full
Middle: Full Tone: Full Power Mode: 50W
Bass: 10:00 O’clock

Notes: Use your guitar’s volume knob to control amp breakup. For example, to clean up

the sound, roll back your guitar’s volume knob. To get a thick, hot-rodded British
distortion sound, turn your guitar’s volume knob completely up.

THD FLEXI-50 INSTRUCTION MANUAL 3

Flexi-50 Controls

Front Panel

First, let’s go through the various control options on the front panel:

1. Input: This jack fits a standard 1/4" mono instrument cable. If you use a stereo cable,
the sleeve signal will be ignored. Do not use speaker cable into this input.

2. Input Lo/Hi switch: This switch toggles the Input gain from low to high. With the
input on Lo (sometimes called "normal") the input signal is not boosted. You may prefer
this setting for higher output pickups or less high gain tones. Switching the input gain to
High raises the input gain, which boosts the volume of the pickup slightly. You may
prefer this setting for lower output pickups, or to drive the Flexi input slightly harder.
The yellow LED above the Input jack will light when High gain mode is engaged.

3. Volume: This knob controls the input volume into the preamp tubes.

4. Treble: This knob can be used to boost or cut the high frequency response of the Flexi.

5. Bright Switch: If the bright switch is on, an upper high frequency boost will be added

to the signal. This boost is independent of your Treble knob setting, so you could, for
example set the treble knob to roll of the high end for a darker overall sound, but turn on
the Bright switch to add some upper brilliance.

6. Middle: The middle control offers a very musical mid-range adjustment over a narrow
frequency range.

7. Bass: This knob can be used to boost or cut the bass frequency response of the Flexi.
You’ll find that the Flexi has bass to spare!

THD FLEXI-50 INSTRUCTION MANUAL 4

8. Boost Gain: A special feature of the Flexi is that the boost circuit is voiced differently
than the main channel, so when the Boost circuit is engaged, this control adds both an
additional gain stage and a more “British” EQ voicing, with thicker low and lower­midrange frequencies.

9. Boost Tone: A unique Flexi feature is its full-frequency sweeping Tone knob. This
control can completely change the texture and tone of the boost from dark and woody to
searing and open.

10. Boost ON/OFF: This switch engages or disengages Boost circuit. The red LED
above the Boost controls will light when the Boost is engaged. The Boost can also be
toggled with the included footswitch.

11. Master: The Master knob controls the overall output volume by limiting the amount
of signal to the power amp tubes. Some classic amplifiers did not have a master volume,
and their volume and gain were both a function of the single input volume. The Flexi
allows you to switch the Master completely out of the circuit to better emulate the signal
path of those amplifiers.

12. Master Switch: This switch engages or disengages the Master control. If the green
LED above Master control is lit, the Master is engaged. The Master can also be toggled
with the included footswitch.

13. Cut: This control functions much like a presence control in other amplifiers, but with
a significant difference. A Presence control uses negative feedback from the amplifier to
simultaneously adjust both gain and brightness. The Cut knob on the Flexi adjusts the
overall brightness, but not the gain.

14. 50W/20W switch: The 50W/20W switch toggles the Flexi’s plate voltage to the
power tubes from 475v (50W) to 325v (25W). Lowering the Flexi’s plate voltage both
reduces the volume of the amp slightly and changes its character. In 50W mode, the Flexi
is slightly tighter and more aggressive; in 20W mode it is a bit warmer and smoother.

15. Standby Switch: Standby mode keeps the power tubes powered and the amplifier
itself off. Always power up in Standby mode and allow the tubes to heat up for at least
two minutes before switching to Play.

16. Power On/Off: The Power switch turns the amplifier on or off.