The 6BQ5 and the higher voltage 7189 & 7189A are very popular tubes in both guitar and home entertainment amplifiers. The European 6BQ5 equivelent number is EL84.

The EL84 was first introduced in Europe by Philips in 1954, the tube caught on in the US as well a few years later. The 6BQ5 is a fast dynamic tube and has a certain musical charm that made it so popular.

Many manufactures offered the 6BQ5 including Philips, Mullard, Telefunken, Tungsram, Amperex, RCA, GE, Sylvania, Raytheon and others. At times one manufacture would sell their tubes to another manufacture. Thus, I have seen RCA that are manufactured by GE. The GE tubes usually have an etched dot matrix on the tube glass. At times the EIA manufactures code are printed on the tube. For example, 312 is the Sylvania code. One can google 'eia codes' for a list on the internet. 

The 6BQ5 is rated up to 300 volts, however I have use the 6BQ5 to 350 volts without any issues. The 7189 are rated 400 volts and were available from many manufactures. The 7189A are rated for 440 volts and GE was the only manufacture. I believe the first years was 1958 or 1959 for the 7189A.

Early US manufacture of the 6BQ5 family of tubes made use of dissipators on the screen posts at the top of the tube. These dissipators are a small piece of metal viewed at the top of the tube above the mica insulator. It is either two small square pieces or a 'U' shaped metal dissipator. I believe the GE used these dissipators thru 1962 and Sylvania thru 1964. Other manufactures continued to use the dissipators throughout their production run. Seeking early Sylvania or GE are easy to date code with the dissipators. I do not believe the grid dissipators make for much difference in performance, but the original manufacturing tooling was newer and the tubes may possibly have better quality such as more time on the vacuum drawdown phase of the manufacturing process. In any event I never experienced any problems with GE or Sylvania irregardless of the year of manufacture.

Modern manufactures are JJ Tesla, Sovtek Russian tubes and some brands from China. The JJ are very good sounding tubes in guitar amplifiers with I believe the best sonics for the price. The Russian 6BQ5s are very rugged tubes if a bit more bland sounding, but super reliable with lowest price available. Two Russian versions exist being the standard 6BQ5 and 7189. Numbers are 6N1P (6BQ5) and 6P14P-eb (7189). New old stock (NOS) Mullard EL84 are the tube of choice in guitar amps, but quite expensive. Two new manufactured 6BQ5 tubes sold as a reissue are Genalex & Mullard. I have auditioned these tubes, but been advised the Genalex sound good.

Two top performing tubes in home entertainment amplifiers are the GE 6BQ5 grey oval plate and Sylvania 6BQ5 black plate. I like the GE 6BQ5, 7189 or 7189A in push-pull (PP) amplifiers and the Sylvania 6BQ5 or 7189 black plate in single-ended (SE) amplifiers as top performing tubes. I suspect Sylvania 6BQ5 grey plate sound as good as the black plate, but never have compared the two versions. Both tubes are accurate and musical without being sterile sounding. For some reason the Sylvania have a superior midrange in SE amplifiers vs the GE better overall in PP amplifiers. It is likely the difference of PP amps having self-cancelling even order harmonics of the fundimental frequency that does not occur by design in SE type amplifiers.

7189 & 7189A can substitute for 6BQ5, but not exactly the other way arround. It truely depends upon how high a voltage is placed on the tubes. I never had an issue with 350 volts on 6BQ5. Guitar amplifiers as used for live entertainment are usually at maximum volume and the overdrive can stress 6BQ5 tubes vs a home entertainment amplifier. Guitar amplifiers can also place a lot of voltage on the tubes. 375 to 400 volts are quite common in vintage amplifiers. I would seek the 7189 or 7189A for the higher voltage guitar amplifiers. Be aware a pin out difference at the bottom of the 7189A tube exists as compared to the 6BQ5 or 7189 tubes. Not all amplifiers that use 6BQ5 and 7189 can accept the 7189A. The normally unused 6BQ5 or 7189 tube socket pins can be used as a tie-point for other circuitry as the unused pins were rather convenient during the manufactures assembly process. Thus, ask your local tech if your 6BQ5 or 7189 based amplifier can accept the 7189A tube.

A tip on purchasing GE is to perform a search as General Electric. Another is 7189 or 7189A that 'fly under the radar' at good prices. Most eBay shoppers just search for 6BQ5 and at times sellers do not include 6BQ5 and 7189 or 7189A together as the label on the box does not ever include both numbers. Also, perform a search as EL84. I have seen tubes listed 5BQ5/EL84.