AliExpress Amorphous SE OPTs + Jean Hiraga and the SE-DHT Amp Revival

C-core amorphous 3.5K SE output transformer

In late 2023, this pair of SE output transformers from AliEx were sent to me by my buddy Joe Roberts for evaluation. They bear a striking physical similarity to the 6.5K:600Ω line transformers I reviewed in this blog upload.

The C-core pair of OPTs was substituted for a pair of KA-6625STs fitted to my Radiotron SE2A3 amp chassis using 6J7 input tubes (see below). Don't be encouraged by the clean-looking 100 Hz, 1kHz and 10 kHz square waves, as pictured above. The amp starts clipping at a measly 1.6 W per channel with the 8Ω taps loaded with 8Ω non-inductive 10W resistors. These OPTs made the amp sound very veiled and practically devoid of dynamics!

After a week or so, I changed the driver section to a wider bandwidth choke capacitance coupled 6C8G (triode sections connected in parallel). In spite of a visibly faster 10 kHz rise time, the amp still struggled to make 2W per channel and worst of all, it sounded just as veiled and compressed.

Since the two other pairs of Japanese OPTs tested below easily produced 3Wpc, I can only conclude that this is a flawed design. I wasn't compelled to shoot more square waves or record a video of the amp fitted with these OPTs in action.😔

Kasuga Wireless KA6625ST

In stark contrast, the original configuration of the Radiotron SE2A3 amp with 6J7 driver tubes + KA6625ST OPTs easily delivered clean 3Wpc into 8Ω with both channels driven. There was no amusical veiling or compression of dynamics and sound quality wise, second only to the Finemet FM6WS OPTs below.

Finemet FM6WS

Here’s how my Lilliputian Noguchi/G-Trans Finemet FM6WS-equipped Radiotron SE2A3 amp with 6C6 input tubes perform on the test bench.

To my ears, my anachronistic Radiotron SE2A3 amp with FM6WS OPTs presents music with a sense of ease and realism I normally associate with a live music event! 

4C double C-core amorphous SE output transformer

I gambled $130 for this pair, shipped from China through AliExpress.

The OPTs were duly installed on the aforementioned SE2A3 chassis using plate choke loaded paralleled 6C8G input tubes. I had no problems getting clean 3 Wpc sine waves from them. It may not be obvious from the Bill Evans/Toot Thielemans video that micro dynamics and detail are OK but macro dynamics are compressed. Extended listening revealed that instead of actual detail, the top end was gritty and grainy. These OPTs made the amp sound labored and tiring to listen to!

I brought the amp back to my test bench and injected square waves. The 10 kHz square wave looks similar to the pair of amorphous 6.5K:600Ω line output transformers I tested and rejected last year. For comparison, click here for square waves of good sounding OPTs I tested using the choke-capacitance coupled topology - none of them exhibited ringing of this magnitude at the top edge of a 10 kHz square wave.

That said, as I showed at the beginning of this article, clean square waves don't automatically translate to high quality. I use test equipment to facilitate testing and troubleshooting. However, I still rely mainly on my ears as the final arbiter of sound quality! 

So far I've tested and listened to two pairs of AliEx-sourced 3.5K amorphous SE output transformers, a potentially defective pair and the other, sonically fatiguing! I need a break.😫

Let's do a wrist check...😉

I had been looking for a British military-style mechanical watch I could afford for quite a while. When I saw this Baltany watch, I just had to add it to my AliEx cart along with the 4C OPTs. I'm happy to report that more than a year later, this Seagull ST1701 powered automatic is still keeping good time!😊👍

A hi-mu triode driver stage

Robin and Lipman 6SF5-DC-2A3 + Noguchi/G-Trans PMF10WS
Faster rise time at 10 kHz (more vertical)
Frequency response @ 1W = +/- 1 dB 20 Hz - 15 kHz

As 2024 drew to a close, I built another SE2A3 amp derived from the Robin and Lipman direct coupled SE6B4G schematic + my pair of PMF10WS OPTs. This musical-sounding circuit deserves an article of its own. I gave the 4C amorphous OPTs another chance in this chassis, which made them more tolerable...but there was something else I wanted to look into.

Robin and Lipman 6SF5-DC-2A3 + 4C amorphous OPT
Slower rise time @ 10 kHz

Frequency response @ 1W = +/- 1dB 30 Hz - 10 kHz

-1.5dB @ 15 kHz

When I cranked my audio generator beyond 20 kHz, the Noguchi/G-Trans PMF10WS gracefully rolled off beyond 35 kHz as specified. The 4C amorphous started rolling off even earlier than the PMF10WS. Curiously, despite the relatively narrow bandwidth of this hi-mu triode driver stage, the top end started rising again (albeit at a lower level) around 85 kHz before hitting a brick wall around 110 kHz. Could that be the ringing evident in the 10 kHz square wave of the wider bandwidth choke capacitance coupled circuit above?

Hmmm...I've seen a similar scenario before, but worse.

In the late 90s, Steve of Angela Instruments asked me to test a pair of the then newly introduced Hammond SE OPTs weighing over 10 lbs each! The 10 kHz square wave hardly resembled one because of excessive ringing. Instead of the smooth roll off I had observed from Japanese SE OPTs, the top end response started to fall off jaggedly beyond 15 kHz before it started spiking upwards again around 40 kHz. Steve agreed that they sounded dreadful compared to the Audio Note UK, Bartolucci, Tamura and Tango OPTs he was importing then. 

However, these Hammonds sold better because they were cheap. Sales were also bolstered by the emphasis on "BIG iron, high primary inductance = MORE BASS" that had been permeating through audio circles for decades, perhaps due to a misguided interpretation of the Vacuum Tube Valley Issue 3 SE output transformer article. Eventually, the high frequency anomaly was exposed by experts and only then did Hammond supposedly rectify the problem.

Being a formally trained classical musician, I always refer to the wide range of acoustic musical instruments, the human voice and the harmonic overtones they produce in evaluating the sound of reproduced music. Aside from the Hammond mentioned above, in the past decades, I've encountered a couple more output irons highlighting high inductance specs but with irregular high frequency performance. IME, initially, these may sound bold and impressive especially in the low frequencies but prolonged listening reveals a lack of tonal definition throughout the audio band because of their inability to reproduce proper overtones. 

Therefore in my empirical mind, the most logical design brief for a high fidelity output transformer should be as stated in the paragraph above, which came from the last page of a November 14, 1934  RCA technical bulletin. 

A couple of years ago, I was reminiscing about the Sound Practices days with Herb Reichert at an intimate gathering of veteran audio DIYers. When we got to the topic of output transformers, I discovered that we shared a similar view that it's not just the extension at the frequency extremes which matters, but more importantly, the roll off behavior has to be gentle and smooth to achieve a musically satisfying sound!

Jean Hiraga

Jean Hiraga from his book Rörförstärkare

If we look into the history of Single Ended Directly Heated Triode amplifiers, it was the esteemed Jean Hiraga who reintroduced western audio hobbyists to the joys of this type of amplification in the 1970s. Being part Japanese, he got exposed to custom made SE-DHT amps driving high efficiency theatre type horn high fidelity setups during his extended visits to Japan. He built, listened, tweaked and wrote about them for French audio magazines.

One can only deduce that Hiraga, along with the Japanese audio technicians and experimenters he credited in his book, analyzed and reverse engineered the 1930s Western Electric 171A single-ended output transformer from the WE91 theater amplifier. 

It's quite ironic that while Japan was exporting Kenwood, JVC, Onkyo, Pioneer, Sansui, Sony, Technics + other audio equipment all over the world in the 1970s, perfectionist Japanese hi-fi enthusiasts had been importing, restoring, tinkering with and listening to unloved pre-WWII surplus studio and theater equipment made in the USA by Western Electric, which American exporters were just so eager to get rid of. 

My 90s reissues of Volume 1 (1972) and Volume 2 (1975) of
The Fascinating History, Design and Construction of Vacuum Tube Amplifiers by Isamu Asano

As all of this was happening, a South Philly street urchin named Joe Roberts was reviewing for his ham radio license exam while listening to Sun Ra - wondering if he would become a TV repairman or get accepted into an Ivy league school to pursue his interest in archeology. Unbeknownst to him, two decades later with degrees from UPenn and Yale, he'd be the publisher and editor of Sound Practices, hanging out with Jean Hiraga at hi-fi shows!

Noguchi/G-Trans PMF-15WS + Japanese Leica Thread Mount Lenses from the 1950s
L to R: Super Rokkor 45mm f2.8, W-Nikkor 3.5cm f2.5 and Canon 50mm f1.8

Since photography is another hobby I pursue, it's worth noting that the Japanese camera industry started dominating the world market after renowned photojournalists like David Douglas Duncan, as well as darkroom technicians at press agencies, discovered the phenomenal resolution of Nikkor lenses during the Korean war. All of these evolved from their innovations after careful study of German optical schematics from Leitz, Zeiss, etc. 

Using this as an example of Japanese work ethic in combination with their two-decade head start in reviving and innovating upon SE-DHT amplifier technology, not to mention their passion for classical music and jazz, Tamura and Tango iron were no brainer acquisitions by the time I wrote my Sound Practices Homebrewer article at the end of the 20th century.

AliEx 4C amorphous 3.5K double C-core OPT

I was hoping that by the second decade of the 21st century, these affordable Chinese OPTs would at least be serious challengers to entry-level Japanese offerings. Alas, I'm very disappointed! 

Although it might not be quite evident in this video, this direct coupled circuit was a bit more forgiving of the 4C amorphous output transformer's sonic flaws. However, it also allows the refinement of a competently designed and wound output transformer to shine through.

Hence, re-installing the Noguchi/G-Trans PMF10WS output transformers was a totally refreshing experience!