While I was relishing a pair of Altec 32C horns in the attic during the late aughts, unbeknownst to me, my stateside audio comrades, Dave V. and Joe Roberts, were praising the same horn's virtues in audio forums. When these veteran audio hobbyists consider the Western Electric 124 one of the finest-sounding amps they've ever heard, I listen.
See Dave's (aka Salectric) WE124 amp project at Hifi Haven.
 |
| 6F6s and 6V6s |
Due to my uncontrollable addiction to building SE-DHT amps, I unloaded my stash of 6L6s/5881s/7851s along with a pair of classic Williamson Heathkit W4Ms I restored in the early 90s. However, as 2A3, 45, 46, 10 and 71A prices soared through the years, I started collecting 6F6s and 6V6s because of their directly heated triode (DHT)-like sonic attributes in triode mode.
The 6F6 was Western Electric's recommended substitute for the WE349A output pentode tube in the lower powered WE 131A and 133A monitor amps, which shares the same circuit topology and iron as the WE 124. Dave V. was just as impressed with a modern build of the WE133A using the original iron - 618B for the input/phase inverter + 171C output transformer. Likewise, J-Rob held those lower powered WE amps in high esteem such that he boxed up parts for the amp two decades ago.
 |
| Various types of 6J7s + a Ken-Rad 6C6 |
In the metrics-obsessed sector of this hobby, the 6C6/6J7/6SJ7 family of pentodes and their Western Electric counterparts don't get much respect because gurus disdain their low transconductance. But I don't listen to gurus, I listen to music! 😊
These pentodes have given me musical pleasure for decades. I'm even more delighted that WE deemed the affordable 6J7 as a plug 'n play alternative to their 348A for the 124(x), 131A and 133A amplifiers.👍
I originally planned to build a WE131A/133A-derived stereo amp as a winter project. Unfortunately, the unusually bitter cold and snowy winter prevented me from doing chassis work in our unheated garage. Since I was anxious to hear this amp, a mono version was more manageable on my test/work bench. Using a hand held drill, step and drill bits, a wrench + Greenlee punches, I went to town with an 11" x 7" x 2" steel chassis.
Western Electric-style amp built using working-class parts
 |
| Altec 15356A into a 600Ω load |
Years of collecting junk paid off because I had all the parts in my bin to build this amp. For the input transformer/phase inverter, I chose the tried and tested Altec 15356A 600Ω:600Ω line transformer. To make sure it would perform up to its original specs, I injected 100Hz/1kHz/10kHz square waves and swept it with my audio generator + scope.
I got tired of finding a mate for my Grommes 10PG8 mono integrated amp so I pulled out the output transformer. It measured ~ 8.6KΩ at the primary, close enough to the 10kΩ WE171C. The power transformer came from a PP6V6 amp chassis that may have been part of an Emerson radio phonograph.
 |
| ready for testing |
I was running low on carbon composition resistors, so I used carbon films in the signal path. As always, paper in oil coupling caps drive the output tube grids. The power supply is patterned after the Western Electric 18A, albeit, with a proletariat's 5Y3 rectifier, surplus 2.5H/150mA choke + generic electrolytic filter caps. I had hum/noise issues initially due to the unshielded power transformer. But thanks to helpful tips from J-Rob, that was mitigated.
 |
| McMartin MT-4 (schematic) |
As-is, my mono preamp didn't have enough juice to drive the 600Ω transformer phase inverter. So I wired a spare McMartin MT-4 microphone transformer as a 10KΩ:600Ω. The 10kΩ side is easily driven by the paralleled 6SN7 cathode follower + 1µf output cap in my line stage. Voila! 600Ω output capability just like my stereo line preamps.🙂
Back to my roots...
 |
| My refurbished Eico HF87 ca. early 90s |
Building a push-pull tube amp is a trip back to my early days in DIY audio. As I mentioned in my Simple Triode 6V6 upload, the very first amp I built from scratch in the late 80s was a PP6V6 using this para-phase circuit published in both the Acrosound and Dynaco transformer catalog. It's similar to Dave's WE124 amp, except for the driver tubes and feedback configuration.
That PP6V6 amp project was the deciding factor to unload my ST70 and pair of MKIVs I modified in the mid 80s with the GSI kit after reading a glowing review in The Absolute Sound. I was initially thrilled by the GSI dual-differential cascoded and constant current source loaded 6922 driver board with Wonder Caps + low-Z solid-state rectified and photoflash-capped power supply, but it wore me out quickly. In hindsight, the stock 7199 cathodyne (aka split load) phase inverter driver boards of the ST70/MKIV + GZ34 rectified power supply played nicer tunes. Then I became fascinated by the cathode-biased Eico HF87 with its Mullard-style 6SN7 long-tailed pair driver tubes.
 |
| I also treasured and studied these classic PP EL84 amps Clockwise from top left: Acrosound 20/20 = 12AX7 direct coupled (Loftin-White), dynakit stereo 35 = 7247 cathodyne, Leak 20 = 12AX7 Mullard LTP and Pilot SA232 = 12AU7/12AX7 para-phase |
My directly heated triode journey commenced ca. '92 with the PP2A3 mono-block amp chassis pair below. The first circuit I tried in them was the 6SL7 para-phase/6SN7 driver/PP6B4G schematic found in the Acrosound Transformer Catalog. This was followed by the Loesch CCS loaded 6CG7>6SN7 dual-differential featured in Sound Practices Issue 3. Although better than the GSI, I became convinced that mixing solid state devices with vacuum tubes wasn't my cup of tea. I finally settled with what I learned from the Eico HF87- Mullard-style driver circuit with 6SN7s sans feedback.
Ever since I was captivated by the life-like midrange and low level detail produced by an SE amp, I slowly gravitated away from PP amps. Yes, PP amps typically produce more bass grunt but not necessarily better tonal articulation.
By the time Herb Reichert wrote about the Feral Eye PP2A3 amp in Sound Practices Issue 14, I had practically unloaded all of my pentode and tetrode output tubes, classic PP amps, iron and the PP2A3 mono blocks above. I reinvested the proceeds on DHTs + air-gapped Tamura and Tango iron. However, my busy performing and teaching schedule kept my Weller soldering station cold. So I posted the idea of the transformer being the ideal phase splitter from Herb's article in the back of my mind.
Before going further, I'd like to discuss how the phase inverter is implemented in the WE 131A/133A vs. the Feral Eye PP2A3. In the Feral Eye current flows through the primary of the Tango NC-14 interstage/phase inverter transformer whereas in the WE131A/133A the input transformer/phase inverter is completely passive just like an MC step up transformer. Simpler!
In my recent Bonsai SE amp upload, I test drove the Western Electric-style plate to driver tube cathode local feedback and got surprisingly better sound. The WE131A/133A as well as the RCA SP10 employ the same local feedback principle of isolating the output transformer from the feedback loop. This approach probably avoids the adverse sonic effects of the industry standard global negative feedback that set the %THD race in motion and still has traction in certain circles.
It's interesting to note that MJ Audio Technology recently featured an article about an amp employing this topology and called it a "double push-pull." Perhaps that's due to the elegance of its symmetry!🍻
!!!WARNING!!!
The voltages in this circuit are potentially lethal! Build at your own risk!
Inasmuch as I wanted to follow the 133A schematic closely, I had to use the modern equivalent values of resistors and caps marked in red above. I skipped the additional resistors that were necessary to create test points in the cathodes of the input and output tubes because they were redundant for my purpose. The voltages I measured in my amp build are very close to those indicated in the above schematic.
The moment I fired it up in my mono hifi, I knew something was very special about this circuit. But first, I had to do something about its sensitivity. Despite the fact that the input transformer had zero gain and merely functioned as a phase inverter, the amp only needed ~ 0.5 V rms to full output. Removing the cathode bypass caps from the 6J7s cathodes brought sensitivity down to ~ 2.5 V rms.
I could've stopped right here except when I reinstated my plate choke coupled 2C22 driven SE2A3/H203S amp in the mono system, my immediate reaction was, "I still miss some of that SE-DHT magic." In an effort to make sure I leave no stones unturned, I had to check what the simpler WE 131A circuit could do.
 |
| Click here for the WE131A factory brochure + schematic |
Here's my concoction so far...and subject to change.😉 The 6J7 driver stage plate resistor values are from the 133A while the cathodes are un-bypassed for lower gain like the 131A. I applied the 2:1 proportion between plate and screen voltages by using a 690k 1W (paralleled 0.5W 1M+2.2M) dropping resistor to the 6J7 screens from my Radiotron SE2A3 build from two decades ago. No need for matched output tubes since they are cathode biased independently just like the 133A. I also dispensed with the 0.47µf/400V blocking capacitor in the plate to cathode feedback and replaced the C/R combo in the 133A with a 320k, 1W resistor (paralleled 0.5W 470k + 1M) ala 131A. Whether it was the 131A, 133A or the above recipe, bass definition is better via the Altec 414A, 756B, 755A/C and the GE A1-400 using 0.047µf PIO coupling caps rather than 0.1µf.
 |
| Input = 1.95 V rms Output = 5W into 8Ω 100 Hz/1 kHz/10 kHz square waves |
Plugging 6V6s adds another 0.5W or so in output, tighter bass, less warmth in the mids and more energy in the higher frequencies. Audiophiles weaned on high end equipment will probably find this more appealing.
I'll save further raves until I've lived with the stereo version. 😉
Audio connoisseurs in Asia have been collecting Western Electric audio gear for decades because of their sound quality. As the supply chain dries up, not much is left for American audio enthusiasts who are just beginning to see the light. As the saying goes, "no one is a prophet in their own land."
No comments:
Post a Comment