RCA T8-14

About the RCA T8-14

T8-14 was introduced on July 18, 1935. The list price back then was $79.95 - the equivalent of over $1,000 in today's dollars. Accordingly, total production was a fairly low 16,502. (Thanks to Alan Douglas for providing this information.)

It was part of RCA's pioneering 1936 radio line, the first to use metal tubes; and it was one of RCA's better table sets that year. It was not the top of the line, though - the T10-1, RCA's best table set in the early part of the year, had two more tubes with push-pull 6F6 audio and an additional bass control. Both were replaced later in the model year with similar sets that added the 6E5 "Magic Eye" tuning-eye tube.

This scan out of the company's advertising brochure (generously provided by John Pelham of the amazing Radiophile Web site) discusses the radio's features, including the new "Magic Brain" front end.

Elsewhere in the brochure, RCA writes with breathless enthusiasm about the Magic Brain:

"Just as the original Magic Brain set new standards of shortwave reception, so does the new, improved Magic Brain achieve still higher standards. It utilizes three tubes instead of two. And they're the new metal tubes!...The R.F. tube - the "Watchman" - even more efficiently guards the program you want, supercharging it for clarity and freedom from noise...The Hexode Pentagrid Converter increases sensitivity five times on the short waves...The individual coil system functions still more efficiently with a new type of adjustment for precise action."

As more sedately stated in the radio's service notes, the first detector stage (converter-mixer), the biggest change in the new Magic Brain, "has unusually good high frequency mixing efficiency. The tube used, an RCA 6L7, is a new hexode type...The arrangement of the grids prevents degenerative difficulties."

Anyway, these sets were excellent home receivers for their time, especially on shortwave. Unfortunately, human factors didn't keep up with engineering - with the T8-14's small airplane dial it was still difficult for people to tune in shortwave stations. That problem wasn't solved until later years when radios were made with slide-rule dials and spread bands. But to make up for it this radio has a nice feature, two-speed vernier tuning. With the tuning knob pushed in a gear is engaged that lets the knob spin the tuner at a rate of 10:1. Since the needle only goes halfway around the circumference of the dial, that means that five full turns of the knob will traverse the entire tuning range from end to end, which is pretty fast. However, with the tuning knob pulled out, the gear is disengaged and changes the spin rate to 50:1, five times slower. So a user would leave the knob pushed in for coarse tuning, then pull it out to fine-tune the station in accurately. This is a big help when trying to tune stations close together on the dial.

The T8-14 used 8 tubes, seven of which were the new metal types.

6K7RF amplifier
6L7Converter-Mixer
6J7Oscillator
6K7IF amplifier
6H6Detector
6F51st audio
6F6Power amplifier
5Z3Rectifier (glass)

RCA was having early production difficulties with the new 5Z4 metal rectifier tube and didn't have it available for use in this set. However, since RCA was touting their new metal tubes and apparently didn't want any old-fashioned glass tubes visible, the 5Z3 was hidden inside a big black can.

About metal tubes

General Electric began development of metal tubes in 1933, then turned over manufacturing to RCA in 1934. The advantages of metal tubes were very simply stated in the service notes for the T8-14 radio: "This receiver uses the new metal tubes which are much smaller in size than the corresponding glass types. The high frequency efficiency of these metal tubes in greater, because of the shorter lengths of leads, lower interelectrode capacitance and the more complete shielding of the metallic envelopes. Their rugged construction prevents breakage and reduces microphonic tendencies. The bases and sockets of all types have a standardized arrangement of connecting prongs."

In the summer of 1935, both companies introduced their new 1936 radio lines featuring the new metal tubes. The companies had developed a line of nine tubes - enough to meet the design requirements for almost any consumer radio - with the new standardized octal base.

New Metal TypeSimilar Pre-Octal TypeDescription
6A86A7Pentagrid converter-oscillator
6L7No predecessorHexode converter
6C576Medium-mu triode
6J777, 6C6Sharp-cutoff RF pentode
6K778, 6D6Remote-cutoff RF pentode
6H675 (diode section)Dual diode (detector)
6F575 (triode section)High-mu triode (1st audio)
6F642Power amplifier pentode
5Z480Power rectifier

Widespread introduction of the 5Z4 was delayed by nearly a year when flaws in its original design became evident shortly after the tube was released, and it had to be completely redesigned. The 5Z4 was a heater-cathode type rectifier, unlike the 80 and its octal equivalent 5Y3, which were filament types.

More new metal tube types appeared shortly after these:
The new beam power pentode 6L6 and its lower-powered partner 6V6, which replaced 6F6 in newer designs
The 6N7 twin power triode, the metal octal replacement for the old 6A6 used in class B amplifiers and to drive push-pull 6L6, but few radios use it.
The 6Q7, a duodiode-triode which recombined the functions of the 6H6 and 6F5
The 6R7, a duodiode-triode that had a medium-mu triode (like a 6C5) instead of a high-mu triode like the 6F5.
The 6J5, an improved triode which supplanted the 6C5 in later designs
Two new rectifiers, the lower-powered 5W4 for smaller sets and the higher-powered 5T4 (similar to the old glass 4-pin 5Z3) for larger sets. Both of these were filament types, unlike the 5Z4.

Interestingly, the original lineup didn't leave an easy way to make a popular 5-tube radio. The tube that made that possible, type 75, did not have a single-tube equivalent until late 1935 when RCA finally brought out the 6Q7. That tube didn't appear in new radios until the 1937 model year. Nonetheless, RCA and GE needed 5-tube radios in their lineups for 1936. RCA built their T5-2 just using the old glass tube lineup of 6A7-78-75-42-80, while GE made an all-metal-tube set, the A53, by using a 6J7 as a biased detector, the resulting lineup being 6A8-6K7-6J7-6F6-5Z4. The biased detector was an old technique for creating enough audio gain for comfortable listening levels, but it tended to introduce distortion and was not generally used once duodiode-triode tubes became available.

Metal tubes are not found only in GE or RCA radios. Some other radio manufacturers such as Belmont, Stromberg-Carlson and Grunow used metal RF tubes in their radios because of the various advantages they offered over the traditional glass tubes. However, the metal power output tubes and rectifiers were unpopular as they ran hot enough to burn careless fingers and offered no operational advantages over equivalent glass tubes.

This line of tubes was mostly obsolete by 1940. RCA radios of that model year discontinued metal power output tubes and rectifiers in favor of glass (GE had done so some years earlier), and replaced the tubes that had top caps with new "single-ended" versions where all connections came out through the base. The 6SA7 replaced the 6A8 (and, effectively, the 6L7), 6SJ7 replaced 6J7, 6SK7 replaced 6K7, 6SQ7 replaced 6Q7 and 6SF5 replaced 6F5. Other types were introduced through the years, until the miniature tube took over for much new production in the late 1940s.

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Copyright 2004 Paul Dietenberger. All rights reserved.

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