Hi All.
As threatened earlier, I'm posting a thread on the three basic styles of phase inverters. I'm going to be semi-technical and, unfortunately, will be devoid of visual props; you'll just have to imagine the circuits in your mind (kinda like listening to old tyme radio dramas), or refer to printed matter that I'll try to provide at the end of this first post. I would encourage specific questions and responses--I'm going to try to avoid math in this overview, but some queries can only be amswered mathematically.
There are four postulates that need to be recognized when dealing with PIs:
POSTULATE #1: COMMON CATHODE VOLTAGE AMPS INVERT PHASE 180°
POSTULATE #2: COMMON GRID AND PLATE VOLTAGE AMPS DON'T
POSTULATE #3: CAPACITORS SHIFT PHASE 90°
POSTULATE #4: PIs, ALMOST WITHOUT EXCEPTION, HAVE 2 STAGES
PI #1: Voltage divider. The first stage is a standard common cathode circuit with the plate being capacitively coupled to one of the output tubes' grid. However, there is a voltage divider consisting of two resistors that's also connected between the first stage plate and ground. The junction of the two resistors is connected to the grid of the second stage, also a common cathode circuit, and the values of the resistors are chosen so the signal voltage on the second stage grid is equal to the signal voltage on the first stage grid. Because the phase is inverted (see #1), the output of the second stage will be inverted again, thus putting the phase in its original form, which is then fed to the second output tube's grid. Simple.
The problem with this scheme is that coupling caps have to be used to isolate the various grids from the plate voltages of the previous stages. #3 says we're going to have serious phase issues. I've seen variations on this circuit to eliminate the coupling cap on the 2nd stage grid, but, overall, this approach to phase inversion doesn't lend itself to high fidelity.
PI #2: The Cathodyne phase splitter, AKA Concertina, Hot cathode, and other aliases. This circuit can be regarded as a half-assed cathode follower. The first stage has a large amount of gain, usually provided by a pentode, and is, again, usually directly coupled to a triode second stage. The plate and cathode resistors of the second stage are either of equal value (cap coupled) or of equivalent value (direct coupled) so that the signal voltage is equal, but out of of phase. The output tubes' grids are capacitively coupled to the cathode and plate of the phase splitter. Again, simple and quite effective.
The main problem with this scheme is getting enough gain in the first stage to overcome the inherent laziness of the splitter: the spltter wants to be a common cathode amp and will thus work one output tube as if it were a single ended stage. It's only after you almost overwhelm the splitter with grid drive that it will start behaving properly. I've read reports of DIYers having problems with parasitic oscillation using the Cathodyne circuit, but I attribute said problems to constructor ignorance.
PI #3: The long tail. When Ed and I were young'n's in the '50s, there was a fad known as "Mommy, Mommy" jokes. Sick, perverse humor at the time. The one that stuck with me was "Mommy, Mommy, I'm getting dizzy!//Shut up or I'll nail your other foot to the floor." Weellll, the long tail PI is basically a differential amplifier with one foot nailed to the floor. More specifically, one grid is nailed to ground, as far as AC signal is concerned. Again, we have a first stage coupled (usually directly) to one grid of a pair of triodes that have a common cathode resistor. The other grid of the pair is capacitively connected to ground, which clamps it to no signal voltage swing. So, if the cathodes are tied together, one grid's nailed to the floor, and the other grid's being fed signal, what does the plate of the grounded grid stage do? Well, hell, it has to follow the cathode current, but because the cathodes are tied together, it can only do the reciprocal of what the "normal" half of the PI is doing. Hence, phase inversion. Complex to the point of being mind boggling, but...
The long tail is, with the exception of a balanced differential amp, the phase inverter of choice, IMHO. The key to a good LTPI is that grid-grounding cap. If it's not prime grade, you're gonna have an inferior PI, and, by extension, an inferior amplifier.
Let the discussion begin...