Thinking about a tube swap? Here are four factors to consider.
In the early ’90s, I bought a 1964 Deluxe Reverb for $450. It seemed like a lot at the time, but that amp sounded so good and was just the right volume for what I was up to. A local tech retubed the 6V6s for me, but I wondered … what about EL34s or 6L6s? What would my Deluxe sound like with those? It’s a fun question, and we hear it concerning different Carr amp models, too. When is it safe to try another tube type? What does one need to know? Let’s look at some general power-tube parameters with an eye for substitution and get this can of worms open!
Pin arrangement. Here’s what to consider: Which pins in a replacement tube’s base correspond to the parts inside the tube, and is that arrangement simpatico with the tube your amp was designed for? Luckily, the power tube short list contains types that all have the same pinout with one exception. The 6V6, 6L6, KT66, KT88, and 6550 tubes are all beam tetrodes (with four internal parts) and have identical pinouts. The EL34 is a real pentode (with five internal parts) and has a slight difference that’s often accounted for inside modern amps. The EL34’s cathode (pin 8) and suppressor grid (pin 1, which is the EL34’s fifth element) must be connected together to work properly. So newer amps usually connect pin 1 and 8 on the tube socket even if the amp is made for 6L6s.
To check your amp for tube fit, you can look at your amp’s socket wiring, contact the manufacturer, or check with a multimeter for zero resistance between these pins at the socket. Of course, in the latter case, make sure the amp is off and drained of all voltage. Otherwise you’re in mortal danger!
Filament current. Each of our familiar power tube types uses 6.3 volts to light up the filament while warming the cathode to the point where it freely yields electrons. Hence, the glowing glass. That said, different types of tubes require different amounts of current to do their work.
Generally, an amplifier designer specs how much current the filament supply for each tube will use, and then the transformer manufacturer uses this to size wire and design an acceptable temperature rise. More current to achieve a given voltage means more power—and a costlier transformer. Here’s the rub: My old Deluxe was designed for 6V6 power tubes. Those tubes each use .45 amps at 6.3 volts to light. The power transformer has some extra grunt … but not enough to make it really costly. An EL34 requires 1.5 amps at 6.3 volts to work. That’s three times more per power tube!
The Deluxe has two power tubes, so two times 1.5 equals 3 amps for EL34s and just .9 amps for a pair of 6V6s. In fact, when you add up all the tubes’ filament current in a Deluxe (not counting the rectifier tube, which has a separate 5V filament supply), the total current is the result of six 12A-type tubes at .3 amps each, for a total of 1.8 amps. Then add on .9 amps for the 6V6 pair, for a total of 2.7 amps. So, the entire Deluxe filament supply uses less current than one pair of EL34s! That means replacing 6V6s with EL34s in a Deluxe is not a good idea. They could overheat the power transformer and burn it out. Going the other way filament-wise—installing 6L6s (rated .9 amps per tube) in an EL34 amp—is a breeze, since the current requirement is less.
Plate voltage. If you look up the data on a power tube, it will include the maximum voltage the tube can withstand. There are other factors that effect plate dissipation and maximum power, but voltage is a great place to start. You will see, for example, that 6L6s max out at 500 volts while EL34s can run higher. But a 6V6 has a maximum plate voltage of 350 volts. That is a big difference and will generally mean a higher-power 6L6 or EL34 amp will not be a good candidate for using 6V6s. There is simply too much voltage—although 6L6s and EL34s have a similar enough voltage to possibly substitute for each other. There are some vintage amps that really push tube-voltage limits. These are not good candidates for any substitutions. They are tough on modern tubes and were designed when tubes were cutting-edge technology and much more robust.
Bias. If your amp’s bias range is designed around EL34s, it may not be able to adjust to 6L6s without being modified. Such modification takes understanding and experience to perform.There are amps that have lower plate voltages with extended-bias-adjustment or self-bias arrangements that can accommodate a wide range of power-tube types easily. We make a few, and they were designed from the ground up with substitutions in mind.
Finally, if after reading this you’re shying away from the potential missteps of power-tube substitutions, there are still other ways to conjure the sounds you’re looking for—like a new pedal. Keep your ears and mind open to all the possibilities.