[Analog Addict]

Originally Posted by decooney

While I was decades behind on getting away from solid state into tube amps, and only familiar with trying 50s/60s tubes in my amps, I find it quite amazing how you guys are reaching back to tubes made in 1936!. Fun to read, interesting.

I am proud to say that has been one of my contributions to this endeavor. When I first met Dennis, he poo poo'ed old tubes, since as a "volume manufacturer" at Cary, he would always buy new manufacture tubes, since this was the only way he could ensure supply. I was able to eventually convince him that old tubes have a magic uniquely their own. Part of my personal journey of discovery has been the walk back in time from 2A3s to 45s to now finally the 71A.

From A Tiny History of High Fidelity, Part 1


The first widely used vacuum tube was the direct-heated (usually battery powered) was the RCA '01A direct-heated triode in 1922. This became the general-purpose tube of the 1920's - in the late Twenties, specialized tubes appeared, starting with the indirect-heated RCA '27, followed by the closely related 37, 56, and 76. These low-level tubes made AC-powered radios possible, since they didn't require battery power for hum-free operation.

In 1926, '71A direct-heated power triode was introduced, followed by the '50 in 1928, the '45 and PX4 in 1929, the PX25 in 1930, the 2A3 in 1932, the 300A in 1936, and the 300B in 1938. The power race started in earnest with the 6L6 and KT66 pentode in 1936, followed by the ubiquitous EL34 in 1951, the EL84 in 1953, the 6550 and KT88 in 1954, and last in the series, the 7591 in 1959 and the 8417 in 1963. (The 6L6 has been in continuous production since 1936, a record unmatched by any other electronic device - we all owe the electric-guitar players a big thank you for keeping the tube factories open.)

Seventy years later, vacuum tubes, and especially triodes, continue to be the lowest distortion amplifying elements ever made. No germanium or silicon transistor, JFET, or MOSFET has ever approached the distortion performance of the direct-heated triodes, with indirect-heated triodes following closely behind. In addition to low distortion in the absolute sense, the distortion spectra of triodes is favorable, with a rapid fall-off of the upper harmonics. (This is less true for beam tetrodes, pentodes, or solid-state devices, which are intrinsically less linear and have higher-order distortion curves.)

Originally Posted by decooney

Is there a more modern tube most are familiar with that might sound close to a 6F6G tube? Does it kinda sound like a 2A3 or 300B or something? Curious.

The 6F6 is an interesting tube. Unlike the more common 6V6 the 6F6 is not a beam power tube but an actual pentode. It precedes RCA’s 6L6, the first popular beam power tube, by a year.

Metal tubes were introduced by RCA in a Technical Bulletin in 1935. The first metals were: 5Z4 rectifier, 6A8 pentagrid converter, 6C5 triode, 6F5 hi-mu triode, 6F6 power pentode, 6H6 dual diode, 6J7 pentode, 6K7 remote cut-off pentode (called a super control pentode in the bulletin). Metal tubes were also the first vacuum tubes to have an octal base.

So, given the above, it may be impossible to compare the 6F6 tube to any of the beam power tetrodes, since they are different in construction. I haven't heard this new amp either. I suppose the best you can do is wait for Bear City's review....