Pros: Ultimate versatility in tube configuration, built like a tank, feature-rich, amazing line out
Cons: No effects loop
The THD Univalve is a class A, single-ended , single-channel tube amplifier capable of running a laundry list of different preamp and power amp tubes without having to rebias the amp. The amp is advertised at 15 watts, but the actual output wattage will vary based on the type of output tube used (more on that below).
The Univalve has two preamp tubes (9-pin) and one output tube (8-pin).
Just about any 9-pin dual triode tube will work in either of the preamp tube sockets. This includes the usual suspects: 12AX7, 12AT7, 12AU7, 12AY7, 5751, and about 10 other part numbers you likely haven't heard of if you were born after about 1965. The first preamp tube governs the gain and tone of the preamp, while the second preamp tube directly drives the power tube. So by varying the gain of these two tubes, you can influence the mix of preamp distortion and power amp distortion, or run a decently clean tone at higher volume by reducing the gain of both.
Almost all 8-pin (octal) power tubes you can find these days will work in the Univalve. No rebiasing necessary, just pull one tube and put the new one in it's place (but wait until it cools off!!). This is the real selling point of the amp, in my opinion. Anything from EL43, 6V6, 6L6, KT66 / 77 / 88, 6550, which is the tubes you typically see still being produced, and some more obscure types like 6G6 and 6K6 (which are no longer being made but are cheap since basically no guitar amps use them). There is also an adapter (sold separately, of course) that will allow the use of 9-pin power tubes, such as the EL84/6BQ5.
With 6L6 or EL84, the amp puts out about 20 watts at max. With a 6V6 you get about 6 watts. With a 6G6, output is about 2.5 watts. So just by changing tubes, you can dramatically increase or decrease clean headroom. For the bedroom player (like myself), having the option of a saturated power tube at non-stadium volume is very nice.
The Univalve has quite a few knobs, switches, and options.
Along the front, there are two inputs: Rock and Roll. Rock is the higher gain input. Roll is lower gain, and it bypasses the first triode gain stage. This is critical for getting good clean tones at decent volume.
First is Volume, which on a single-ended non-master amp like this one, is the same as a gain control. It controls how much signal is fed from second triode stage to the third.
The next two are High and Low, which controls the Baxandall-style tone stack. Pretty simple.
Then there's Attitude, which I'll let the company describe: "The Attitude control determines how the driver stage responds to signal and how it drives the power tube." The general idea is that when turned fully counter-clockwise, it's more a Fender-like response, and fully clockwise is more like Marshall. THD says it's not a presence control, but the effect is in the same ballpark, in my opinion.
The Hot Plate knob (which activated via the Full Power switch) controls the overall output volume. It's based on THD's popular Hot Plate line of attenuators. In a nutshell, this allows you to fully crank the amp to get the power tube to saturate/break up and yet keep the volume at landlord-friendly levels. A very nice feature.
Noise Reduction. This engages a very simple high-cut type noise reduction that dampens hiss quite effectively. There is a lamp just above the switch that lights when you play (and dims when you don't) to show that the noise reduction is in effect.
Full Power. This is the attenuator switch as mentioned above. It toggles the Hot Plate attenuator in and out of the circuit.
Hi V / Low V. This adjusts the overall voltage, which is critical for using certain power tubes. The big tubes can run either high or low, but the small tubes should only be run on low voltage. For the big tubes (6L6, EL34, etc), more volts = more headroom, generally speaking.
Standby. This allows for a gentle power-on to extend tube life. When in standby, only the tube's heaters are given direct power. This allows them to warm up before getting hit with full voltage to the plates.
Power. I'm sure you know what this does. ;)
There are speaker output jacks for anything ranging from 2-ohms up to 16-ohms.
There is also a fantastic isolated line out jack and control knob. You can actually use the Univalve as a preamp for other amps, or you can run it like stompbox in your signal chain. It also works great for direct recording. There is also a switch to toggle between instrument-level and line-level output. Its probably the best line-out feature set you'll see on a tube amp.
The bottom line is that this amp is built like a tank. It's heavy, made entirely of thick metal (steel?), and can take a beating. You can tell that a lot of thought and a lot of engineering when into the overall design.
What's it sound like?
The Univalve has it's own distinct tone, especially when the volume is cranked and the tubes are all pushed into breakup. While it certainly doesn't sound exactly like any other brand out there, I'd say it's more on the Marshall end of things than on Fender. However, with the right tubes in the right spots, you can pull off Fender-like clean tones. It just takes more thought and tweaking.
It does a nice warm clean at lower volumes, but gets crunchy and above medium volume. With the preamp tubes at max gain (12ax7) and the power tube saturated, it does a very nice hard-rock (or older metal) tone. With an EL34, it can get close to the tone on Iron Maiden's Piece of Mind album. :)
What doesn't it have?
For one, it doesn't have an effects loop. That's about the only thing I really wish could be added. This may or may not be a big deal to you, depending on your gear setup.
Too Expensive for You?
New, the Univalve goes for about $1000. While that's not exactly "high priced" in the world of boutique amplifiers, it's still a lot of cash for the average guitar player. If the price tag is something you can't swing, I'd recommend searching out a used one. I got mine for $550 shipped and including the optional THD gig bag, and my unit arrived in immaculate condition. I would've thought it was new had I been told so. Also, a lot of used Univalves also come with a small collection of power tubes, so there's at least a good chance you'll get a lot of value out of the deal.
For all sorts of technical details and user ratings and advice (especially regarding the extensive list of usable tubes), check out the Univalve.net forum, which was made by and for Univalve owners.
Also, be sure to see the product page at Univalve's Web site: