Sunday, March 29, 2015

The 6V6 Vacuum Tube: For Guitarists Only?

Have you ever wondered why there are no commercially produced hi-fi audio power amplifiers using the 6V6 output vacuum tubes despite its widespread use in electric guitar amplifiers? 

By: Ringo Bones 

Despite its widespread use in the “guitar world”, the 6V6 vacuum tube is probably the only widely available vacuum tube – there’s even an excellent sounding version in current production by Electro-Harmonix – that no commercially produced hi-fi audio power amplifier uses as its output power tube. While its closest equivalent – the older venerable 6L6 is widely used in both electric guitar and hi-fi amps, the 6V6 seems to have stuck in the “guitar world” despite the existence of DIY hi-fi literature dating from the 1950s - i.e. 1950s era RCA application notes - with circuit diagrams of audio power amps using the 6V6 vacuum tube. 

The 6V6 is a beam power tetrode that was introduced by RCA back in mid 1937 and still in current use, especially in electric guitar amplifiers. Similar to its predecessor – the 6L6 which was introduced back in 1936 – the 6V6 was far more widely used during its heyday – i.e. from 1945 to the early 1970s when solid-state transistor became reliable and commercially viable enough for household audio amplification purposes. In comparison, the 6L6 produced more output power than required by general use consumer electronic devices right after World War II with corresponding power and circuit requirements. 

The lower powered 6V6 was better suited for the average home use and became common in the audio output stage of “farmhouse” table top radios (Stromberg Carlson?) where power pentodes such as the 6F6 had previously been used. The 6V6 requires less heater power and produced less distortion than the 6F6 while yielding a higher output in both single-ended and push-pull configurations. Additionally, before the transistor era, the 6V6 has applications in the automotive and portable radio market. In an audio output stage, a single 6V6 can be used to produce 5-watts continuous power and a push-pull pair for about 14-watts with the higher output requiring a larger, more expensive output power transformer with grain oriented core laminations for better efficiency.  

The 6V6 was first introduced in both metal and shouldered glass tubes. RCA was promoting the superiority of their metal tube designs during the second half of the 1930s and this tube having been introduced during that period was produced in large quantities in this format. Other vacuum tube manufacturers also produced the 6V6 in glass tubes which were commonly found in radios not made by RCA. By 1940, the 6V6 was mostly being produced in smaller “GT” glass envelope and later the 6V6GTA was introduced which has a controlled warm-up period. 

Generally speaking, 6V6 output power vacuum tubes are sturdy and can be operated beyond their published specifications. The 6P6S – which has poor tolerance for out-of-spec operation versus most American and Western European made 6V6 variants – is an exception. Because of this, the 6V became very popular for use in consumer market musical instrument amplifiers particularly combo style electric guitar amps such as the Fender Champ which uses the 6V6 in a single-ended configuration. While the Gibson GA-40 and the Fender Princeton Reverb and the Fender Deluxe Reverb amps which run a push-pull pair of 6V6s at 410-VDC plus on the plates. This market allows Mainland Chinese, Slovak and Russian vacuum tube factories to keep the 6V6 in production to this day. Because of the relative similarity in voltage and other characteristics between the 6V6 and the "more popular" EL84 / 6BQ5 power pentode vacuum tube, several electronics and musical instrument companies have developed adapters to allow an amplifier with 6V6 octal sockets to accept the miniature noval pinned EL84. However no reverse adapter has been made or developed to be commercially available to allow an EL84 tubed amp to accept 6V6 output power vacuum tubes.