[Listening test] Super Tube Rectifier STR 104 mkII by Space-Tech Laboratory

Space-Tech Lab Super Tube Rectifier

I got it straight and I am not looking back

Product name: Super Tube Rectifier STR-104-MK2
Manufacturer: Space-Tech Lab - Canada
Approx. price: $784 USD
Reviewer: Henry Venema - TNT Canada
Reviewed: May, 2023

Space-Tech Lab is a Vancouver based company that specializes in high quality audio products without the fancy case work that would double or triple the cost, but more exotic case work is available at additional cost. The Super Rectifier (here after referred to as SR) is a standalone unit 14L x 6W x 4H inches. With the AC power plug on the rear panel and a three-foot detachable cable that goes to the rectifier socket of your piece of equipment you are upgrading. On the top panel are 2 high quality 4 pin sockets and a toggle switch for 2.5 or 5 volts tubes. The front panel has a power switch and a standby and operate switch.

Like all audio components the SR is system dependent, and my system is unique. I store all my music on a PC hard drive with a Lynx sound card that feeds my highly modified Line Magnetic LM 502 CA DAC (new clock, power regulators, and discreet op-amps, coupling caps with bi-pass caps), the signal is then fed into a 6SN7 preamp, then into a Pass Labs analogue active crossover splitting the frequency at 360 Hz. The bass frequency is rooted to a AudioMat Duo power amp, and the higher frequencies to a pair of Serious Stereo 2A3 mono blocks which use Sophia Electric 300B 2.5 volt tubes. My speakers are DIY and weigh 260 lbs. The upper frequencies are reproduced by AER 3 drivers and the lower frequencies by AER StudioBass drivers.

The SR can be used as a replacement for 5U4G, 5RU4 rectifier tubes, and it can also be used to replace 6X4 tubes. In my case I have been testing the SR as a replacement for the single 6X4 tube in my Line Magnetic DAC. Simply pull the tube and insert the connecting cord and turn on the SR, wait 5 mins and flip the switch from standby to operate and you're good to go, just make sure the toggle switch is set to the correct voltage (2.5 or 5 volts), failure to use the right voltage will burn out tubes and if left on long enough burn out the transformer.

It's a dangerous move to use the SR, once you hear music with it you will never be able to go back. Every parameter expands. The sound is more dynamic, soundstage is wider, detail galore, rhythm and pacing are more coherent. I'll never listen to music in my system without it again.

However, tube selection is paramount and makes a huge difference. Because my system is 102 dB efficient, some tubes were too dynamic with a very forward soundstage, others matched perfectly. As you can see from the line up of tubes I've tested, perhaps too many tubes, this just the sort of thing I love to do, after all I am a self-confessed audiophile.

An unfortunate event occurred. During the testing my AudioMat Duo decided to blow a couple of tubes and I had to send the amp out for repair. However, during this time I burnt in several pairs of rectifier tubes. To my surprise rectifier tubes need a couple hundred hours of burn in time to start to sound their best.

For the purpose of this review, I've used two categories of tubes: 1) 2A3 power tubes, 2) Mercury vapor rectifiers tubes which turned out to be best of the bunch.

I compared a set of Sophia Electric, AVVT, and JJ 2A3 tubes. This grouping had one thing in common: the soundstage was wide and very deep. The most expensive of these, the AVVT, generated too much hum, and was the least musical, the JJ tubes were more musical with better flow. The JJ's had a naturalness particularly noticeable on piano, and a very precise 3D sound stage. The Sophia Electric tubes had less bloom and were cleaner but too dry, it also sounded too aggressive and bright. So, in this category of tubes the JJ 2A3 were the hands down winner. I should mention that when used in my 2A3 mono blocks these tubes sound great and provide their own unique soundscape.
The mercury vapor tubes need a word of caution: break a tube and you have a deadly poisonous vaper in the room. However, this class of tubes is glorious. At 5 volts I tested a pair of 80 and 83. The 80's sounded somewhat tired, but the 83 was smooth and produced a very beautiful sound with one exception, the bass was rolled back and softer, almost as if there was a gentle fading of the bass the deeper it would go. Nevertheless, still very musical without listing fatigue.
At 2.5 volts I tested several 866A tubes. At first I thought the Westinghouse 866A's were a waste of money, way too bright and aggressive, but after a couple hundred hours of burn in time they sound very similar to all the other 866A I tested, except it they had a sweeter top end and a deeper sound stage. The next pair, Mullard 866AX tubes, were typical of the Mullard sound. Images were rounder and sweeter but with too much glow. They lost control with peak outbursts of volume and were less dynamic than all the tubes in this round of testing. Still, they were pleasant to listen to. I followed with a pair of GE 866A tubes. These tubes had a very forward sound stage, very transparent but dry. Sharp attack with too much pluck on guitar strings yet they sounded with excellent instrument separation.
Then to my surprise a brand I had never heard of before, Brown & Boveri DQ2 was significantly better than the rest except for the RCA tubes. The B&B tubes were warmer with an expansive sound stage completely behind the speakers, musical with slightly less bass than the RCA tubes. My only complaint with these tubes is that they are slightly more congested with over tones in complex instrumental music, nevertheless completely non fatiguing.
After that I tried two different 3B28 tubes, a pair of EE Jan-CDZ 3B28s, and a pair of GE tubes. Of the two the EE were the best and very near the top of the bunch: detailed, lots of upper end extension, almost too much, very wide soundstage. Whereas the GE had too many overtones that smeared the clarity of images. With violins the GE tubes sounded a bit aggressive. The EE tubes had an extended top end with great clarity and precision and upward tilt of the high frequencies, and violins sounded more coherent and smoother.
The RCA tubes on the whole are the best of all the tubes I tested, but require hundreds of hours of burning time. I bought two used pairs and one NOS pair. The used pairs sounded great but still needed burn in time. The NOS pair has about 300 hours on them and have risen to the top of the tubes. But with a few caveats. On audiophile recording they are detailed, transparent with solidity of image, huge soundstage layered front to back, and that “I'm in the room with the musicians” presence. However, they are verge of being too bright side, with a more forward soundstage than the BB and the 2A3 power tubes. Yet the soundstage remains behind the speakers. The RCAs ruthlessly reveal the truth of recording quality. Great recordings sound amazing, and if one is bright to start with, I switch them out and replace it with the BB or the Westinghouse tubes.

This has been a very interesting exploration of such a wide variety of tubes with a product capable of extraordinary musicality and soundstage. The SR is a wonderful upgrade to my DAC. I heartly recommend the SR, and it is now a permanent component in my audio system. All the tubes that I tested did something similar with the main difference of top end brightness. Now to buy a pair of SRs for my 2A3 Mono blocks.

Test Music
Johnny Cash, "The first time I ever saw your face," American Song Book IV.
Johnny Cash, "Can't help but Wonder where I'm Bound,' American Song Book VI.
Bruce Cockburn, "Isn't that what friends are for?" Breakfast in New Orleans.
Bob Dylan, "Most of the time," Oh Mercy.
Bod Dylan, "Man in the Long Black Coat," Oh Mercy.
Patricia Barber, "Mourning Grace," Café Blue.
Patricia Barber, "Romanesque," Café Blue.
Patricia Barber, "Nardis," Café Blue.
Fred Eaglesmith, "Ship," Dusty.
Stevie Ray Vaughan, "Riviera Paradise," In Step.
Diana Krall, "The Look of Love," The Look of Love.
Ray Lamontagne, "Empty," Till the Sun turns Black.
Ray Lamontagne, "You can bring me Flowers," Till the Sun turns Black.

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