That 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.
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.
Thd Flexi 50 Class A/b 50 Watt Tube Guitar Head
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.
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.
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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.
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.
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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.
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.
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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.
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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.
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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
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