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Posts: 3063
Location: Eastern
Joined: October 4, 2006
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These "broadcast studio grade" amps have unique semi-regulated, sliding fixed bias, just like the Brook 30W 10C amps. More unique is their B+ regulation. Their output trannies were not superior products; nor were their interstage trannies. Finding or custom making equivalents will be tough, but not unobtainium. So, building a pair is not impossible, not cheap, but won't cost too many thousands either.
Also, keep in mind that these were available with two different fixed bias arrangement versions, as well as available with line output taps, only. These two are allegedly a sweet pair ready for voice coil speakers. Even if perfect, these compete with Western Electric 86 and 46 amps; but at insane pricing with either camp. The WE triode amps use interstage trannies and no overall negative feedback loops; plus they have always been the most in-demand, most expensive triode amps ever. I have heard about a pair of 30W Brooks getting $15K privately, within the last five years. A pair of WE 86 or 46 amps can fetch above $50K in Asia.
Having heard excellent 30W 2A3 Brook amps here in home on my 16 ohm VOT speakers, in direct competition with my PP triode amps using interstage trannies without any feedback, I can confirm that the Brooks can sound f*%#!^ incredible. 30W Brooks can sound like they have better frequency extension and control, a bonus of their negative feedback, but they cannot image or sound like they have the speed of the IT coupled amps. The Brooks use a PP-PP interstage but split phase with a split load inverter and drive the opt grids with direct coupled cathode followers...Yes, the large Brooks have a huge wall of sound, with super sweet sound and impressive bandwidth. But, for the most natural, palpable sonics I have heard, I stick with my IT coupled PP 45 or 2A3 type amps.
The 30W Capehart amps are allegedly by Brook as well. Has anyone heard a pair of those ? R-C coupled amps cannot throw that serious holographic imagery or yield the truly amazing speed of the sound that IT coupled amps are known to easily accomplish. I can say this because of my history with triode amps. In my past experiences, at one time, I had two pair of 10W Brooks(some prefer these to the even more unobtainium 30W Brooks), the 12A monoblocks and a hot rodded pair of 22 integrateds. At the same time, I also had two pair of Sun Radio CR-10 amps; one with Thordarson output trannies and the other pair with Peerless S-240Q opts ! The Brook 22 integrateds were my reference for over half a decade. Having these four pair of amps at my avail was quite a collecting feat; even in the mid 1980s. But, soon after, I procured my first pair of UTC tranny based fixed bias PP 2A3 amps known as the Summit Sound Systems RA-15. These pre WWII, deco looking stunners were even more sonically stunning, after simplifying and hot rodding them. Inspired by their incredible speed and the absolute hugest soundstage my wife or I ever heard in that era, I proceeded to build a pair of UTC Linear Standard tranny based fixed bias triode amps, to hear the best bandwidth available with this IT coupled, no neg feedback topology. I still kept the Summits around for over a decade after, because of their superior "sweetness" selling or trading off my other triode amps. Only after building up my current choice of amps, using a globe 45 output tube as the driver tube, thru a hefty 1:1 IT, driving 45s or 2A3s, did I allow the Summit 2A3 amps to leave here.
Looking back, the 10W 12A Brooks were very fine. I have had more than two pairs thru the deacdes. The Brook 22 integrateds were superior with their wider bandwidth opts, miniature tube front end and lower level of negative feedback. The Sun Radio CR-10 amps used a very fast sounding paraphase inverter R-C coupling without any overall negative feedback loop. These Suns were even more open sounding than the Brooks; a virtue of their no feedback, certainly. But, once you hear the speed and true holography possible with IT coupled non-fdbk PP triode amps, combined with dynamics that no SE amps can yield that I have heard, you will be hooked...
By the early 2000s, I have restored and built the Peerless A100A PP fixed bias 6A5 or 6B4 amps of 1950. These used CT choke loaded cathode follower drivers, combined with low negative feedback, to yield 18W amps. Mimicking the Brook 30W amps, these do not have the sweet sonic character of the Brook 30 Watters. The Peerless A100 amp also featured the famous S240Q opt; although I built a pair with a vintage pair of Japan sourced opts to try and sweeten this topology in order to mimic the Brook 30W stunners. The Brooks still won, though. Interestingly, Lincoln Walsh, CE of Brook, actually published a high fidelity PP 2A3 triode amp/AM receiver in 1939 that used a 6A3 as the driver tube, thru an IT, to drive the 2A3s. This effort was not the first seen to use an output tube as the driver tube. That historical first must be rewarded to Silver-Marshall in their 1929-1930 #692 amp using a 45 to drive PP 50s in their super rare, second theater amp attempt. Later, in 1935, you might be lucky enough to view the Morel amp, that boasted 35W from a PP50 design using a 50 as a driver tube thru an IT. So, inspiration for triode amps is definitely not new...
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