Re: Tube heads explained?
Each 12AX7, 12AT, 12Au... type tube is actually two miniature tubes in one bottle. There are triodes A and B in each bottle. How these are used and arranged can yield many different combinations and circuit designs.
For example, a Black Face Fender is a very different design from a Tweed Fender even if they use the same number of tubes. On a BF Bassmann the normal and treble channels will each have there own 12AX7. The signal path will be triode A>>tone stack (eq section)>>Triode B for each channel. Then the channels are mixed and on to the Phase Inverter.
The Phase inverter splits the signal so that the top half of the wave form goes to one (or a pair) of power tubes and the bottom 1/2 goes to the other power tube (or pair). Triode A handles 1/2 and triode B the other. The phase inverter is actually part of the power amp.
On a Tweed Bassman it's different. Triode A of the first (12AY7) handles one channel and triode B handles the other. The channels are then mixed and the signal goes to the 2nd tube which is a 12AX7. Triode A of V2 is a voltage amplifier and triode B drives the tone stack. The feed for the tone stack is taken from the cathode of triode B. Then it's on to the phase inverter.
Marshall plexis use the same topology as the Tweed Bassman. 2203/2204 amps often called JCM800s modified this arrangement slightly so that triode A cascades into triode B at the first tube and there is a master volume installed between the tone stack and the phase inverter.
The Soldano SLO took the 2203 topology and added another 12AX7 tube for two additional gain stages. Yet another 12AX7 may be used on a Soldano for driving an FX loop. Mesa Rectifiers, and 5150s are essentially copies of the Soldano.
A Mesa Boogie MK series is based on the topology of the Black Face instead of the Tweed Bassman, but instead of mixing the two channels, they cascaded one channel into the other in series, and placed a master volume in front of the phase inverter.
The original, not the modern, Orange preamps are very different from either the Tweed/Marshall or the Black Face topologies.
A Hiwatt circuit is similar to the Tweed/Marshall topology, but with and extra tube, not used to create high gain. In the case of the Hiwatt it has four inputs but the treble and normal channels are each more like a 2203 than a plexi. Hence the extra tube. The feed for the tone stack is taken from the plate of the final triode in the preamp instead of from the cathode on a Hiwatt.
