How does the output section work?

[Axtman]

I am working on a Peavey MX VTX series guitar combo amp. It has a solid state preamp and tube output.

I am confused on how it works. I have worked on other Peavey hybrid amps where the phase inverter transistors act just like tubes and put a signal on the gridsof the output tubes. On this amp it looks like the output tube grids are held at a constant voltage and the phase inverter transistors vary the voltage on the output tube cathodes.

BTW, the amp sounds very distorted, not very loud (for a four 6L6s) and the output tubes only get warm to the touch (not scorching hot).

Peavey-MX-VTX-Series-Schematic.pdf

[g1]

The cathode drive circuit works the same as the later Music Man amps. Do you have good voltage at pin 3 and 4 of power tubes? Cathode voltages (and currents) are stated in the notes of the schematic, are they good?

[Helmholtz]

Tube grid potential is fixed. They are driven by cathode voltage. The series arrangement of tubes and transistors (Q5/Q7) constitutes a (hybrid) cascode circuit (often seen with Music Man amps).
Phase inversion is provided by the inverting opamps which drive the transistors.

Check voltages according to schematic notes.

Edit: Missed post above.

[teemuk]

Indeed. The output is a "cascode" of a common emitter and common base amplifiers. Common grid amp has its grid grounded, as far as AC signals are concerned, and the cathode controls the amplification. It is non-inverting.
The NPN common emitter is equal to the usual power tube; it's loaded by the common grid and reflected load impedance. The transistor forces the power tube to follow its collector-emitter current.
In this scheme a fairly low voltage transistor can control amplification of higher voltage tube and tube is also rather immune to device invariancy because its on the tight leash of the transistor. This was a particularly important feature in times when tube manufacturing was fading out in large and quality control had degenerated significantly.

The phase inversion is handled by a unity-gain inverting opamp that inverts the output signal of the driver opamp. The driver opamp also receives the global negative feedback.

Peavey later dropped the tubes and went "full QSC" with complementary transistors and more powerful transistors that now were a commonplace.

Being under tight leash of opamp driver in global negative feedback loop and BJT-controlled load current amplification this "hybrid" circuit architecture is so "solid-state" that Peavey actually added a "tube emulation" to it achieved by a patented diode steering arrangement. This is one major feature that sets the Peavey design apart from the similar "Music Man" circuit. Peavey also added a VI-limiter for the driver transistors that limits excessive currents and significantly improves the ruggedness and reliability of the design.