I’ve bought a couple of valves for my new Rift Amp. I’ve played it extensively and although I don’t doubt the ones it came with will last a while, it’s always sensible to be prepared for the inevitable day on which they will fail. The amp contains a 6V6GT-STR power valve and a 12AX7 pre-amp valve. Thanks to multiple viewings of Uncle Doug’s YouTube channel, I now have a better understanding of how these little marvels work and how they’re classified.
Here’s a shot of a Tube Amp Doctor 12AX7, which is the most prevalent pre-amp valve used in guitar amps. It’s a duo-triode valve which means that it contains in one glass vacuum tube, two separate triode valves, each comprising a cathode, a control grid and a plate. You can see the gap between the two triode’s dark plates in the middle of the valve. The round “halo” at the top of the valve is the getter which collects waste products and ionized gases omitted when the valve is operating, the purpose being to keep the vacuum as clean as possible.
The 12AX7 is used as a pre-amp valve because of it’s low distortion, high quality sound reproduction, comparatively high amplification levels and production of 2nd order harmonics which are pleasing to the ear. In other words it sounds good. Many amplifiers use more than one 12AX7 in the pre-amp section to provide phase inversion, basically splitting negative and positive halves of the input signal and sending each half to a distinct power amp valve, thereby increasing the overall amplification when the circuit recombines the signal and sends it to the speaker.
Next up is a 6V6GT-STR, the power amp tube.
The shot below shows the connections from the valve’s electrodes to the pins set in the base below the glass tube, which in turn are plugged into sockets which connects them to the amplifier’s circuits. Thanks again to Uncle Doug, I now know that the 6V6GT is a beam tetrode valve. These were designed in 1930s American in part to bypass a UK patent created by a company called Mullard who had produced Pentode valves which successfully solved the problem of secondary emission. This is where the electron stream bounces off the valve’s plate, bringing about undesirable effects such as noise and oscillations.
The diagram below shows how the charged electrons flow from the heated cathode through the control grid and are bent by the beam forming plate so they pass through to the anode, in a beam tetrode.
Obviously you’ll remember your Ancient Greek and spot that a Pentode has five electrodes and a Tetrode has four; being the cathode, the control grid, the screen grid and the anode. Pentodes use a fifth element called a suppressor grid to prevent secondary emissions, while the tetrode uses the beam forming plates. The replacement of the suppressor grid, which was the patented element, with the beam plates was another successful method of removing secondary emissions. Why these are not actually counted as a fifth electrode, I’m not sure but it may have been due to the need to distinguish the valve from the copyrighted Pentode.
Beam tetrode valves were first introduced way back in 1933 and but are still widely used as power tubes in guitar amplifiers. The 6V6, as used in my Tweed 5 amp, was introduced by RCA (Radio Corporation of America) in 1937 and is used primarily because, like the 12AX7 produces a good clean sound which when pushed distorts pleasantly. My amp based on a Fender Champ circuit with its single 6V6 produces about 5W – easily enough for jamming sessions with other musicians and recording in studios and more than enough for practice for us flat-dwellers. Amps with two 6V6s used as a pair, usually push out about 15W.
Traditionally Fender used the US designed 6V6 valves, while in the 60s Marshall and Vox used the equivalent but Pentode based EL84 valves in their power amp stages. This gave rise to the still applicable main distinction for general amp sounds, into American or British sounds. US Fenders excel at bright clean tones, while Marshall have a much more woody basic tone. However it’s not an exclusive or fixed arrangement, for example my Fender Blues Junior has two EL84s which like the paired 6V6s produce about 15W.
There’s bound to be errors in the above but I find it fascinating to read about and try to work out the physics and science which enables the “art” of producing music from some wood, magnets, wound steel, resistors, capacitors, valves and controlling the electricity which binds it all together.