Classic output tubes like the 6BQ5/EL84 and the 12AX7 (which are the natural choices for this design) had not been manufactured since the early 80’s. They have become rare and expensive because of the ongoing interest in tube audio. Most other tube types (there are thousands) are still widely available and cheap. There are still tube factories in China and Russia and they have stepped in to make reproductions of many popular audio tubes, but audio aficionados say that they just don’t sound the same as the older types that were made in now defunct factories in the US and Europe. So the NOS tubes sell for hundreds of dollars a pair, even in used condition.
Fortunately, there is another alternative. The number at the beginning of the tube type designator almost always represents an approximation of the tube’s rated filament voltage. The filament is just a heater that warms the cathode so that it can emit electrons. It makes the orange glow inside the tube but takes no other part in the operation. Thus a 6BQ5 has a 6.3 Volt filament and a 12AX7 has a 12.6 Volt filament (actually two 6.3V filaments in series so it can be used at either 6.3 or 12.6V). The overwhelming majority of tube types had 6.3 Volt filaments. Bear with me here.
In the days of tube TV sets, the heaviest things in the set were the CRT and the power transformer. In portable TV sets, designers found ways to eliminate the power transformer to make the sets lighter. In order to run the tube filaments, though, they had to put them in series like Christmas tree lights and run the string with the 120 Volts from the power line. It didn’t always work out that the sum of all the filament voltages of the tubes in a set added up to 120 Volts, so tube manufacturers started to make tubes identical to the 6.3 Volt types but with other filament voltage ratings. So millions of tubes with oddball filament voltages were manufactured for use in portable TV sets and their only application today is in repair of those sets. They sit on the shelf because they are not a “plug in” replacement for the popular tubes used in musical instrument and stereo amps.
For example, there were 8BQ5’s with 8.0V filaments and 10BQ5’s with 10.6V filaments. The only other version of the 12AX7 ever produced was the 6AX7 but it was not very common. There is, however, the 20EZ7 which is identical to the 12AX7 in internal construction but has a 20V filament and a different pinout. Both the 10BQ5 and the 20EZ7 are widely available from NOS tube suppliers at $5 to $6 each.
Since this amp is a new design, it does not matter what the tube’s filament voltages are. One just has to select the right power transformer for the tubes used. This amp uses a pair of 10BQ5’s driven by a 20EZ7 in each channel. This gives the benefit of true original tubes manufactured to the original specs without the exorbitant prices.