Absolutely beautiful. For some reason, Head-Fi stopped sending me updates for ANY threads in the past day or three. VERY glad I popped over to see some good news! ENJOY IT. Well earned.Holy crap...
Okay first things first, got the transformers in the amp. Below is the secondary wiring, 470ohm resistor across the output to protect the transformer from open circuits.
Had a few other things to finish up, nothing major. Here is the final ciruict.
After I finished wiring, I eyeballed the whole circuit, reflowed a few solder joints, then brought the circuit up on a variac and checked voltages. Here is what I got for the two channels using the GEC U18/20:
MH4 cathode: 2.44V / 2.47V
MH4 plate to cathode: 174.4V / 184.4V
B+ top of OPT: 312V / 312V
6A5G cathode: 44.6V / 45.5V
6A5G Plate to cathode: 247.6V / 251.5V
All of these values are within 1-2% of my targets, I was absoltuely dumbfounded that I nailed them so accurately. That crazy Georg Ohm knew a thing or two. Difference between the MH4 plate voltages is tube variation I am sure.
Well then there was only one more thing to do...have a listen? NO! Take measurements of course. Here is the frequency response, down ~2dB at 20kHz, might investigate that, but essentially flat from 20Hz - 20kHz
I am witholding FFT measurements until the amp settles down and have some time to mess around with different tube combinations. Here are some beauty shots of the final product:
So how does it sound...with all of the bias suppression I can muster, it sounds incredible. I braced myself for bad sound (expect the worst, hope for the best kind of thing), but it didn't come. The soundstage is absolutely expansive, the bass is deep, full, and tight, treble is detailed, airy, and not the least bit strident. Maybe I shouldn't be, but I am still a bit shocked it all worked out! It is a keeper.
I have been listening for about an hour straight, the panel with some of the dropping resistors on it does get warm, but so far I think it is manageable, haven't taken any surface temperature measurements yet. Will be adding some ventilation in the near future.
I guess that is all for now, just going to keep on listeninghave quite a few different 4V drivers to try out (REN904, MH4, E424N, A4110, NR52, K1694), a few different rectifiers to try, and different makes of the 6A5G, but probably won't get to it tonight, might try one other combination. At some point I have to get this baby in my stereo, that will be its final destination, but enjoying headphone listening for now.
So overall a rousing success I would say. I will have more impressions and measurements to come I am sure (need to get a square wave generator...).
Big thank yous to @Tjj226 Angel , @2359glenn , and @A2029 for the input along the way. This is the hardest DIY project I have ever taken on, learned an absolute ton in the process. I'd imagine the first one is the hardest, should be smoother sailing from now on.
have to wonder if any of the Visseaux 6A5G do have cathode sleeves, but I am not likely to try again to find out.
I've been duped by Visseaux.
While working on this amp, I purchased a pair of French Visseaux 6A5G. This is the only other tube manufacturer to make the 6A5G other than Sylvania, likely under their license. My past experience with Visseaux tubes has been very positive, so I bit the bullet and paid a pretty penny for a NOS pair of these beauties.....
But while listening, there is an obvious audible hum, unlike the Sylvanias which are dead silent, and I immediately suspected they were not in fact indirectly heated, which was the major feature of the 6A5G over the 6B4G.
Took some measurements, and sure enough, there are 60Hz and 120Hz peaks down at something like -45dBFS!!! I checked continuity between the heaters and cathode to see if they were center-tapped (the other difference between 6B4G and 6A5G), and they certainly are. Very close inspection of the internals and I cannot see any obvious cathode sleeves...
So, Visseaux appears to have made a sort of hybrid between a 6B4G and 6A5G despite the labeling. It is basically a 6B4G with center-tapped filaments, or a 6A5G without cathode sleeves. What a bother! It is too bad because they are excellent sounding tubes. They certainly aren't unusable, the hum is only very intrusive with quiet music. I'm hopeful it will be less obvious or even undetectable when listening with speakers. C'mon guys! What are you doing. Maybe they missed the memo (although the cathode is clearly in the Sylvania datasheet written in French...).
http://www.nj7p.org/Tubes/PDFs/Frank/121-Sylvania/6A5G.pdf
Thomas Mayer over on VinylSavor did mention in his article back in December that addition of these cathode sleeves likely made assembly very difficult, so maybe they thought center-tapping the filaments was good enough. Hey, some people swear by AC on DHT tubes, maybe it is a feature not a defect
http://vinylsavor.blogspot.com/2019/12/tube-of-month-6a5.html
In other news - I placed my order for the mains and parafeed transformers from Sowter for my 45 build. Timeline to receive them is up in the air given the ever-changing environment we are living in, we will see, I am not in any rush this time around. Exchanging emails with Brian Sowter, an interesting subject came up. It appears that Sowter has been acquired by Carnhill, and while he did not say it explicitly, it sounds as though Brian is planning to step away from the company after the transition and things are running "smoothly". I have no experience with them, being new to the game, but Brian Sowter is known for being one of the premier transformer designers working today. While I highly doubt the quality will suffer when he leaves, I certainly feel some sort of pressure to make a nice amp given these may be some of the last transformers made during his tenure with the company! So hopefully this parafeed build is successful.
Otherwise, still listening and enjoying the 6A5G amp. I am really blown away by the sound, I had hoped for it to be a "contender" with other tube amps I have heard, but it has truly exceeded my expectations, I hope I have a chance to share it with others in the near future.
I performed a series of experiments today.
As I mentioned in the build log, I placed a 9.1ohm resistor in parallel with the headphone output of my 6A5G SET amp. This gives the OPT an approximate 8ohm secondary impedance, which in turn gives the output tube its intended 3300ohm load.
Now there is no reason we have to use a 9.1ohm resistor, we can essentially adjust the primary load by adjusting the value of this resistor. Along the way, I purchased some other values for the express purpose of this experiment.
So today, I soldered some alligator clips onto a pair of 9.1ohm resistors and a pair of 16ohm resistors. I wanted to see the effect on the sound with an approximate 8ohm secondary, 16ohm secondary, and no parallel resistance. With no parallel resistance, the full 300ohm impedance of my headphone is reflected across the transformer, giving the output tube a nearly flat AC load line.
Measurements with the three setups showed no effect on frequency response. However, it did affect distortion. The below numbers are all at ~0.5V RMS output, which I have found is close to my listening volume.
8ohm: 0.33% THD, H2 at -54dBFS, H3 -80dBFS
16ohm: 0.17% THD, H2 at -60dBFS, H3 -80dBFS
No resistance: 0.077% THD -65dBFS, H3 -80dBFS
As expected, second harmonic varied with changes in parallel resistance while the higher order harmonics remained relatively constant. So obviously the "no resistance" config sounded best since it had the least amount of second harmonic distortion, right? Nope! Subjective listening told a different story.
I listened to 15-20s loops of very familiar music and was able to quickly clip in and out different resistances for comparison. For the most dramatic difference - and to get a grasp of the audible changes - I compared the 8ohm and "no resistance" setups first. Removing the resistance entirely and reducing the amount of H2 resulted in a more pronounced center image and reduction in perceived lateral space and layering. Instrument separation was lessened in this configuration, while the center image was closer and more pronounced. There was a relative increase harshness as well. The reduction in the sense of lateral space made several instruments in sound field lie on top of one another rather than a more spacious, wide distribution, which resulted in a perceived decrease in clarity.
Comparison of the 8ohm and 16ohm secondary configurations told a similar albeit less pronounced story - smaller soundstage, less instrument separation, more pronounced center image, a bit more harsh. The higher H2 provides a more well-distrubted, layered, and airy sound field. So after all of that, I ended up sticking with my original 8ohm secondary configuration LOL but hey, I wanted to see the effect.
Once again, this shows that good measurements do not beget good sound. As Bob Katz discussed in his Innerfidelity article "Adventures in Distortion", what I suspect is happening is the increased H2 is masking the higher order harmonics, which are much more detrimental to sound.
https://www.innerfidelity.com/content/katzs-corner-episode-25-adventures-distortion
Katz determined -60dBFS to be the optimal amount of H2 for a pleasing listen, however, I felt the -54dBFS config still held an advantage. Just goes to show, we all hear it differently
Obligatory YMMV, my subjective listening impressions, etc. etc. etc.
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