12AU7 Hybrid power amp - Evil_Food

[teemuk]

The tube works on the relationship between grid and cathode, not necessarily those elements and ground.  Hold cathode at ground and wiggle the grid, you get signal at the plate.  But hold the grid to ground and wiggle the cathode, the same thing happens.

Pretty much, but there are some charactistic differences between these two circuit types. One difference is, fo example, that "grounded grid" amplifier does not invert its output signal. Basically you can consider grid of the tube as inverting input and cathode as non-inverting input.

In practice, the amp examples I cited are running closer to class B than something like a fender twin Reverb would be.

This. They are also heavily influenced by anything driving the cathode (e.g. bipolar transistor). In so much that Peavey went on to patent a circuit (VTX series amps) which provides "softer" crossovering characteristics to their circuit (because in plain format, like that of Music Man amps, this kind of cascode arrangement has "hard" and quite bias-sensitive crossovering characteristics of a bipolar transistor circuit).

...Speaking of Music Man amps. They are known for their high plate voltage vs. reliability.... but few seldom mention that they also operated at exceptionally low (on generic guitar amp standards) screen voltage. The same little detail was also the key difference of prototype Ampeg SVTs (that failed catastrophically even with extremely reliable and rugged output tubes) and later SVTs that didn't (which no longer had tubes of equal ruggedness but simply much lower screen voltage). Something to ponder at. 

[blackcorvo]

...Speaking of Music Man amps. They are known for their high plate voltage vs. reliability.... but few seldom mention that they also operated at exceptionally low (on generic guitar amp standards) screen voltage. The same little detail was also the key difference of prototype Ampeg SVTs (that failed catastrophically even with extremely reliable and rugged output tubes) and later SVTs that didn't (which no longer had tubes of equal ruggedness but simply much lower screen voltage). Something to ponder at. 

Yeah, I noticed those voltages were like 1/2 B+ right?
If I understood correctly, this is to keep plate-screen grid voltage within maximum limits. With the help of the cathode transistor keeping the cathode at a rather large, positive voltage (negative relative to the grid, which is fixed-biased), so the plate-cathode and control-grid voltages are within limits...

Yeah, that's it! This is a fixed-bias stage that's driven from the cathode instead of the grid.
Would then the transistor be simply a common-emitter stage, with the tube working as it's "collector resistor" and at the same time, the transistor working as the tube's "cathode resistor"?

If that's it, it's SO much easier to understand!

[phatt]

The same little detail was also the key difference of prototype Ampeg SVTs (that failed catastrophically even with extremely reliable and rugged output tubes) and later SVTs that didn't (which no longer had tubes of equal ruggedness but simply much lower screen voltage). Something to ponder at. 

Yes and Big brand names still churn out Valve rigs with screen way above what the hand books state as safe.  Just recently restored a M valve rig with EL34's running at 470VDC,,,, while data sheet states 425VDC as MAX screen voltage.  
Phil.

[teemuk]

Yeah, that's it! This is a fixed-bias stage that's driven from the cathode instead of the grid.
Would then the transistor be simply a common-emitter stage, with the tube working as it's "collector resistor" and at the same time, the transistor working as the tube's "cathode resistor"?

It's a "cascode" so it's simply direct coupling two amplifiers. "Up", we have common grid amplifier with speaker load reflected to its plate via transformer (as usual). Since grid of a common grid amp is held in "common" potential the amplifier is effectively driven by its cathode current/voltage. Instead of modulating grid voltage with AC signal (and shunting AC at cathode to common), cathode voltage is modulated (while AC at grid is shunted to common). The tube naturally just cares about grid-cathode voltage difference, which really makes the whole arrangement sensible (insofar how upper section of the cascode is both driven and biased). At the "bottom" we have a plain common emitter amp, which effectively controls the cathode current/voltage. Voltage drop across the BJT also establishes bias point for the upper device (along with DC voltage potential provided at the grids).

What's "load" in this type of circuit anyway. It's something to ensure proper current flow and proper voltage potentials. One can use plain resistors, or substitute them with current sources, or ampliers (gain stages) acting very much like such. What is a transistor anyway? It's shortened from "Trans-Resistor" so think of it as nothing but a variable resistor controlled by base-emitter current.