Nest Theater Part Sixteen - Rollin' The 300B Power Tubes & The Rectifier 03May21

Well almost six months have passed without a hardware update to the Nest Theater ~ so it's time. :) Everything is working very well; the NestT sounds spectacular - as attested by me and others who've listened. Especially strong reproduction is heard with acoustic and vocal material - the Fostex drivers and 300B SET amp shine there. Maybe that's the no feedback, 8.5w of pure amplification from the power tubes or my back loaded horns. Whether playing high rez files from the JRiver Win10 PC or low rez from YouTube, it sounds great. Even my Harmony remote control has been dialed in nicely and does what I want. I guess my only complaint is that the amp and dac are not remotely controllable. I have to manually/mechanically turn on the 300B SET amp and the Denafrips DAC. These are not big deals but remote control would be nice. Others might say my lack of use of vinyl is a drawback of my system, but that was intentional and for convenience and does not detract for me one iota. Before we get on with the big show, here's the amp before changes...

Let's first talk about the rectifier...I've never believed the rectifier tube choice would make much difference so I hadn't swapped it to date. However, a number of believers swear by one particular tube that can make a smoother delivery. I have acquired that tube, a 1957 NOS Sylvania CHS5931. This is a premium 5U4 design and has replaced my "stock" 5U4, a currently manufactured Shuguang 5Z3PJ. The swap does sound smoother and seems a smidge more dynamic, but who knows exactly - it made little difference on the measured SPL response curve or on impulse response measurements. It's in there and I like it.

Let's get on to the main topic of this part's mod ~ tube rolling the 300B power tubes. I currently use the musical Russian Gold Lion 300Bs and am satisfied. But when is good enough, good enough in the audiophile realm? So, rather than plunking down $10K for a pair of 1950's Western Electric 300Bs new from old stock (NOS), I sought the better of currently manufactured 300Bs. Fortunately 300Bs are still part of many fine audiophile amps and therefore manufacturing is reasonably strong - but the companies are making bank on these things. There are even higher wattage versions now but I sought the classic 8.5w tubes given my highly efficient DRC BLH 94dB speakers. Here is a pretty long list of contenders (priced by matched pair)...

Brand	Model	Price/Pair	Country
EAT	300B	$1,700	Czech Republic
Tatasuki	TA-300B	$1,600	Japan
Western Electric	300B (reissue)	$1,500	USA
Elrog	ER300B	$1,325	Germany
Emission Labs	300B Mesh	$950	Czech Republic
KR Audio	300B	$800	Czech Republic
Psvane	Acme Series 300B	$830	China
Linlai	Global E-300B	$630	China
Shuguang	300B-Z Black Treasure	$540	China
TJ Full Music	300B Mesh Plate	$380	China
Genelex	Gold Lion PX-300B	$280	Russia

I dove on many reviews of these tubes and sought the flat and great sounding, but not whackily priced, and chose some more durable tubes. Unfortunately I could not trial them with my specific amp and speakers - and match to circuit & system is important - but these tubes have the correct specs for my amp build - NP. Since I have Russian Gold Lions already (warm and musical),

I sought tubes built in other countries but read the Western Electric reissue from the US was not up to snuff. I chose to get both a Chinese pair and a Czech pair as they supposedly would allow contrast of a classic WE 300B design/sound with a more modern 300B design/sound. The Chinese TJ Full Music is the same as the Sophia Princess mostly and they're the Chinese originals, heard to produce the classic mid-range sound. Pretty underplayed packaging.

 

The Czech KR Audio is a modern design with heavier and taller glass, heard to be wider, flatter and more dynamic; it was regularly lauded for pure sound throughout the spectrum and it's robust build, and I liked the solid plate (vs another "mesh"). The KRA easily had the best unpacking experience with a nice box, foam insert and included a test measured output for the matching pair.

 

The Japanese Tatasuki or German Elrog would've been there, but I couldn't hack the prices. Too, EAT is a marketing ploy. So, on with the show... I turned off the supertweeters and sub and just drove and measured the BLHs driven unadulterated by the amp/dac. I used REW to take frequency sweep SPL measurements of each of the tubes.. And of course I audited some favorite tracks with the three tubes. The Gold Lion was warm and comfortable with a great mid-range response. The TJ Full Music about the same dynamically and frequency range but much smoother resulting in a larger rendered soundstage to my ear. The KR Audio had a tighter bass and better extension at high end. The differences are more subtle than I'd expected and the similarity in shape of the SPL frequency curves shows how the speakers really dominate the sound reproduction. I preferred the relatively bigger sound of the KRA on most tracks and they remain in the amp for now. They are definitely more fun for me on electric rock tunes - the Killers new album spoke to me but I didn't want for subtleties when playing Molly Tuttle or Reina del Cid.

To assure I apply the best filters to the "new" system with the Sylvania rectifier and the KRA 300B tubes I restarted the sub and supertweeters and remeasured the complete system using Dirac Live and set up new filters in the miniDSP DDRC-22D. It was a smidge time consuming to take 13 measurements but the resulting sound is worth it. Green sweep is the new corrected response vs the old corrected response in blue - better fit to my target home curve.

Steve helped audition the new tube setup. There's more listening and perhaps tweaking to be done but the upgrades, while somewhat subtle, are present. Finally, here's the "new" amp in with the rest of the gear in the "rack" - I need to do something about hiding some cables in back. Kala will also be pleased to "hear" that I really don't think I can do better; by "hear" I mean the explanation and that I probably won't take the NestT further into audiophilia and not the actual "hearing" right beneath our master bedroom.

 

UPDATE 06Jun21 - While in Summit I took some time to re-roll the new 300B tubes and perform complete equalization and re-measurement of the system. Below, the only variant is the 300B power tube: green is Gold Lion, orange is TJ Full Music and blue is KR Audio. I also re-listened to key tracks from The Killers, Molly Tuttle, Reina del Cid, Phoebe Bridgers and Dire Straits to cover the gamut. My conclusions are similar to my initial assessment but I must say the TJFM tubes are very smooth and render acoustic pieces phenomenally well. The Gold Lions too are not slouchy, but are clearly eclipsed for me by the others, especially on dynamics. Again the KRAs are best, they really deliver tighter overall bass and yield great punch in rock pieces without suffering on the quieter voice and acoustic stuff. They are also quieter than TJFMs, but that's not audible at playing volumes anyway. Again I am struck by the relative similarity of these SPL vs frequency curves - indicating how clearly the Fostex driver and DRC BLH cabinet design dominate performance. I've set up filters on the Dirac such that if I change tubes I can simply select a different filter set which is matched to that power tube - but the KRAs will see the bulk of the duties.