!!!WARNING!!!
The voltages in the following schematics are potentially lethal! Build at your own risk!
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| Acrosound/Dynaco floating para-phase inverter |
As I had mentioned in my Western Electric 131A/133A amp article, the very first amp I built from scratch was the above floating para-phase schematic, using transformers I scavenged from a rusty Dynakit SCA35. That was a nice-sounding amp that I've always wanted to revisit despite some trepidation. I was held back by the global negative feedback (1k + 0.001ยตf) from the output transformer's secondary back to the input tube's cathode.
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| Western Electric 124 |
After many decades of DIY'ing tube preamps and amps, by far the only type of feedback I've encountered, which didn't get in the way of my musical pleasure has been the local feedback illustrated in the above WE124 amp schematic: C3/R13 from plate to cathode and C6/R14 from plate to grid. This local feedback topology can be found in various preamp and amp models produced by Western Electric. Being a floating para-phase inverter circuit using two pentode 6J7 tubes was another incentive to build this instead!
See the HiFi Haven WE124 amp and WECo 133A threads.
Basically, I combined the floating para-phase 6J7 driver stage from the WE124 + the output section of the WE131A/133A. I omitted C7, C11, C12 and R12 depicted in the WE124 schematic because I don't have those values in my parts bin. Besides, I don't know if they're relevant to this concoction. The lack of a low Z transformer input/phase inverter makes this amp an easy load for typical tube preamps. I highly recommend using shielded cables to connect the grid caps. Don't ask me how I know. ๐
I'm using an output transformer pulled out of an Eico ST70 integrated amp. Its primary impedance measured closer to 6600ฮฉ rather than the 10kฮฉ of the original WE171C. I would've preferred not to see ringing in the 10 kHz square wave (see below). IME, servicing, restoring and listening to classic push-pull tube amps in the 90s, that's fairly typical of an uncorrected BIG iron. Creative tuning of a global negative feedback network from the secondary of the output transformer to input tube cathode would've obscured it from the prying eyes of Hirsch-Houck Labs. But as mentioned earlier, we avoid that at JE Labs.๐
Using 6V6 beam tetrode output tubes, I measured 8 watts into 8ฮฉ before clipping with 0.93 V rms input. Frequency response @ 2.83V rms/8ฮฉ output was flat from 30-15,000 Hz. Surprisingly, when I inserted 6F6 pentodes, the amp barely made 4W. Could it be that the 100k grid leak resistors are optimized for beam tetrodes? ๐ค
Typically, I see grid leak resistor values that are at least two to three times greater than the plate resistor (see the 100k/270k in the Acro/Dyna schematic above). It's the other way around with the WE124. But then, who am I to question Western Electric engineers?
In my WE131A/133A derived amp, the sonic difference between the pentode 6F6 and 6V6 beam tetrode wasn't as pronounced. However, in this amp, the 6F6 sounded rather unexceptional. I surmise that the 6.6kฮฉ primary Z of the Eico output transformer may also be a factor. That said, I must've done something right because the amp looks decent on the scope and more importantly, I like the ๐ถ sounds I'm hearing from the 6V6s!
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| WE124-style PP6V6 amp on a GepettoCoLLC 12" stool ๐ |
The tonal balance of this amp is reminiscent of my Radiotron SE2A3 whereas the WE131A/133A derivative is akin to my nickel plate choke loaded SE2A3 amps. Both circuits allow the harmonic richness of the 6J7 pentode to shine through, so I don't think it'll be an easy choice. Since I've only had this amp running for barely a couple of weeks, these are just my initial impressions. I look forward to going through my LP and CD collections to further familiarize myself with this amp.
It would've been ideal to compare these two WE-inspired amps using the same output transformer. Anybody willing to donate a pair of Dyna Z565s? ๐
You'll know which route I decided to take when I upload the stereo amp build.๐
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