THD Electronics Flexi 50 Amp Head

"Wait a minute, this amp isn’t a clone of a plexi Marshall?!"

THD Flexi-50That was my initial thought when plugging into this amp for the very first time. And that’s actually a good thing for many users who will enjoy the added flexibility of the many tonal options presented by THD Electronic’s new Flexi 50 amplifier. Though my initial idea really was to pit a ’68 50 watt Marshall against the THD Flexi-50 amplifier head, it was clear from both looking at and listening to the Flexi-50 that this would have been an apples to oranges type of comparison. The Flexi-50 offers quite a bit of tonal and feature-set variety and it would be inappropriate to simply compare it to one single amplifier.

Perhaps I subconsciously took the name "Flexi" and associated it mostly with a 1960’s plexi-era Marshall. In any case, Andy Marshall, THD’s President explained it best: "It’s named the Flexi-50 because of its flexibility in being able to capture vintage British and American tones."

Features, Features, and more Features…

The Flexi-50 is indeed very versatile and has more options and abilities than I had time to actually experiment with. Though the layout on the front panel is simple enough to use with controls for Volume, Treble, Middle Bass and Cut control, the Flexi-50 also adds a switchable Boost stage with Gain and Tone controls in addition to a switchable Master Volume.

So how do these all work together? Well, that’s where the flexibility of the controls really comes into play. First, the Flexi-50 can be operated as a non-master volume vintage style amp since the master volume can be completely switched out of the circuit (either via the front panel or by using the included footswitch). The Boost stage is also foot-switchable for a variety of additional tones. The Cut control is similar to a presence circuit in some respects, except it’s not based on negative feedback. As THD describes it, it can best be thought of as a voicing control similar to a Vox AC-30 Top Boost’s own Cut control. It controls some of the high frequencies but is not tuned like a Presence control that will add more "sizzle".

The Flexi-50 has a single input, but is switchable between low or high sensitivities. On the back panel, there is an interesting additional switch (labeled "switch input with boost") that enables a change from low to high input simultaneously when engaging the boost.

THD Flexi-50 (Cover Removed)In addition to these controls, the Flexi-50 can operate using virtually any power and preamp tube type and also has a 50w to 20w power reduction switch next to its Standby and Power switches. The Flexi-50 allows mixing of different power tube types as well so you can run an EL34 in one socket and a 6550 in the other to harness unique tones (it comes equipped stock with a pair of EL34s). It can do this because the bias for each power tube is independently adjustable and only requires some reading time in the very-extensive manual and a Volt/Ohm multimeter. Jacks on the back of the Flexi-50 for a Multimeter are provided so pulling the chassis is not necessary. The tubes when placed in the circuit operate in class AB mode, a.k.a. "push-pull" and provides maximum efficiency and power output versus Class A designs.

You’d think that all these features would be more than enough in this amp, but there’s actually a little bit more… an effects loop, a direct out jack with level control and switchable 2, 4, 8 or 16 ohm speaker outputs that can run two cabinets round out this versatile amplifier. The Flexi-50 is handsomely housed in a steel "roll cage" attached to the top of the chassis. The cage top has indents on the side for easy lifting and transport of the amplifier and is also easily removed using four knurled thumb-screws.

Though the Flexi-50 is packed with a lot of features and options, it’s certainly not intimidating and is very user friendly and even those players that like their amps "simple" can easily get accustomed to the Flexi-50’s layout.

Design and Construction

The components inside of the chassis are very cleanly layed out on a nicely designed circuit board. Quality components such as metal-film resistors are used throughout.

THD Flexi-50 (Cover Removed)All mechanical parts and tube sockets are mounted directly to the chassis rather than the circuit board. This is the best choice since heat from the tubes as well as movement of mechanical components can weaken solder connections and the board over time. THD’s layout is an ideal combination of balancing the need to have reliable construction with providing cost efficiency and maximum value to players.

When discussing the transformers used in the Flexi-50, THD’s Andy Marshall explained that he had a number of requirements for the design. The output transformer in particular was selected to emphasize fidelity while still having some saturation and coloring capability in its tone. While there are some output transformers such as those used in early Hiwatt amps that offer only very clean fidelity, other designs in early Marshalls and Fenders tended to be noteworthy for having much more saturation and color. Andy Marshall emphasized that he didn’t model after any one particular amplifier, but that his choice of transformer tends to sit comfortably in between the cleaner hi-fi types and those that provide saturation when driven.

In the Listening Room…

A Gibson Les Paul with Duncan Antiquity PAFs and a ’69 Fender CS Stratocaster with stock pickups were the axes of choice. We ran the Flexi-50 through two different 4 x 12 Marshall cabinets, the first loaded with vintage "pre-Rola" 25 watt G12M "Greenbacks," the second with the more common Celestion G12T-75 watt speakers.

With the Master Volume control turned off, I began with clean tones at reasonable volumes. The Flexi-50 was able to cover a range of tones with both guitars and cabinets. From the fatter-voiced bottom heavy tones to those with chimey top end reminiscent of an AC-30.

As I played with the tone controls for Bass, Middle, and Treble, they seemed somehow to feel "different" as they were obviously very focused on particular frequencies and also had a larger sweep range than what I was accustomed to. Curiosity was what ultimately lead me to call THD to inquire about this circuit.

It turns out that Andy’s tone control circuit is indeed unique in the world of today’s guitar amplifiers and is based on what is called the Baxandall tone circuit. Most tube guitar amplifiers today use sloping tone controls where changing one control also impacts the other controls. The Baxandall works independently to allow fine-tuning of particular bass and treble frequencies that the designer chooses. Each control, just as I had heard them, works independently and won’t affect the other frequencies when making adjustments to them. Very interesting stuff! THD noted that some early Ampeg and Gibson designs also deployed aspects of the Baxandall-type circuit.

The Flexi-50 did a great job of retaining the tonal characteristics of my guitars – Both Strats and Pauls retained their character attributes and tones. Definitely a plus.

Turning up the volume control (still not using the Master), the benefits of power tube compression came into the mix. This is where the Flexi-50 likes to be – at volume appropriate for playing with others. With the Boost off, the Flexi-50 can crunch in a fashion reminiscent to ‘70s JMP Marshalls with a bit of AC-30 sparkle mixed in but is definitely still aggressive. The voicing of the tone network of the Flexi-50 is different though – you can’t say it’s a clone of a Marshall as the midrange and treble controls seem to be emphasizing different frequencies and it’s not the intention of the amp anyway. But when using EL34s, a lot of the Marshall "feel" is there as is the amount of gain. Classic rock players will likely feel right at home with the Flexi-50 and a Les Paul as it’s a mix of both familiar and new sonic territory at the same time.

Using the Strat in the bridge pickup position, one can get a really nice drive push as well, but ultimately my personal preference was using the Boost mode with the Strat. The Boost circuit is voiced for gain but also has a bit of a darker tone overall so the end result is a high gain that comes from more of the Clapton/Cream or Eric Johnson "smooth-gain" school depending on the guitar you use. It’s a thicker voice, not a buzzy one, that lovers of classic rock and blues will likely enjoy.

With both guitars, touch response and dynamics of the Flexi-50 was excellent. This is really critically important with any amp that will be used for classic rock and blues.

It’s interesting to note that I had success using both Celestion cabinets. With my Marshall Super Lead, I don’t like to play it through the 75 watt speakers/cab as it’s just too bright on the top end because of how a Marshall circuit is tuned. The Flexi-50’s highs, though still present with all you can want really, are tuned to lower frequencies than a Marshall Super Lead’s, which makes the Flexi-50 work better with the G12T-75 loaded cabinet. All said, I still preferred the greenback-loaded cabinet with the Flexi-50 (and of course my own Marshall Super Lead), but it was nice to see the Flexi-50 actually be able to function well in both environments. The vintage Marshall cabinet with greenback 25’s in my opinion is really the best for amps like these that are designed for classic rock and blues. The speakers and cabinet (and let’s not forget the basketweave grill cloth material) together work to taper off the high frequencies, push up the upper-bass frequencies, and accents the mids in a very smooth, musical way.

Activating the Master Volume control, which is a post phase inverter design, I ran the gamut of volume experimentation from bedroom quiet to near full output. One very cool application with the Master Volume circuit is to use it for what I refer to as "volume ducking" purposes. For example, if you set the Master Volume to full, it gives a nice cut of volume but there’s still plenty of power tube interaction happening. For passages of music where you want to reduce volume, switching on the Master Volume for this purpose can be very handy. Conversely, keeping the Master Volume on during rhythms and turning it off during solos to increase level output can be extremely useful for many players.

After playing with the Flexi-50 in "stock" form, it was time to experiment with some tube swapping. It is quite a fascinating and educational thing to actually go through and play with the various power tube and preamp tube combinations. Ed DeGenaro THD’s Sales Manager, stated that his favorite power tube combination was blending a 6550 with an EL34 tube. I tried that as well and though the tube types are almost polar opposites (the 6550’s hard vacuum versus the easy compression of the EL34), it was a combination that I found really worked well too. With the ability to adjust the bias simply from plugging in my multi-meter, I ran through a range of other tubes I had on hand including 6v6s, 6L6s, big-bottle 6CA7’s and KT88s.

The preamp tubes in the Flexi-50 are also interchangeable, and this offers more predictable results. The preamp tubes I have on hand are all variations of the 12A series, including 12AX7, 12AT7, 12AU7 tubes which essentially are all similar except they result in different levels of gain.

Final Thoughts

Players that are interested in gaining a wide range of unique and classic-based tones will find the Flexi-50 does indeed live up to its name as there’s a lot of variety from clean to crunch to be had. Further, those that want to "tweak" their amp’s sound and experiment with different tube types to create something more unique and personal will love playing with the Flexi-50.

However, if you’re a player that is looking for a clone of a particular British or U.S. vintage amp with the "most exact" tones represented therein, then the Flexi-50 is probably not for you and may provide disappointing results. The THD Flexi-50 really does have its own voice that is quite frankly a unique blend of these amp types and doesn’t have a dominant Marshall, Vox, or Fender sound as its primary tonal base. It should go without saying that the Flexi-50 is also definitely not a metal-player’s amp as it’s not voiced for the preamp high-gain distortion and buzz attack that metal players these days tend to favor.

However, if you’re interests are in an amp that can roll through a range of tones that carry a definite influence and nod to those British and U.S.-made greats, while at the same time offering something new in both application/settings and tonal versatility, then the THD Flexi-50 may be just the perfect amp you’ve been looking for.

Leave a Reply