Similar to 5U4G but slightly different, this one is made by Brimar. This tube is similar in many ways to the GZ37 tubes in that its quite warm. On the WA6-SE, this was almost too much, extremely syrupy and tubey midrange, very laid-back and lush. It did however, work well with bright headphones due to the treble roll-off. If you enjoy a warm and euphonic presentation, this tube is really nice. Compared to the GZ37's, I think it stands between the Cossor GZ37 and the Mullard GZ37. It's much better than the skinny bottle GZ37 offering more transparency (zero grain!) and a richer tonality, as well as better soundstage and imaging (rivaling that of the best tubes in depth and dimensionality). Bass is tighter and cleaner as well, though not as nice as the Cossor GZ37, its slightly less impactful and not as layered, but better than the regular GZ37 and with much better texture. While this tube lacks speed and PRaT, it does have a very natural decay and timbre, and is one of my favorite tubes for vocals and slower music. Overall, this tube is extremely good for the price, offering much of the performance of the more expensive tubes, having similar transparency and richness in tone. What's even better is the fact that this tube is still easily obtained and fairly cheap with prices ranging from $45-$100. Personally, this is a steal since I think this tube is better than the regular Mullard GZ37 and rivals the more expensive and rarer tubes out there. It is a little too slow for my taste, but if vocals, jazz, and classical are a large part of your collection, this tube is definitely a must have!