Power Amplifiers

Mystere pa21 Stereo Tube Power Amplifier


The Design

The pa21 has two gain stages, with the 6SN7s providing the input stage and the KT88's at the output stage. You can substitute EL34's for the KT88's, and switch the toggle on the side to give the proper bias voltage for the EL34's vs. the KT88's.

The KT88, introduced in 1956, is a beam tetrode, and a schematic with pin-outs is shown below. The cathode (Pin 8) is indirectly heated by a filament (Pins 2 and 7, h). The control grid (Pin 5, g1) and screen grid (Pin 4, g2) are between the cathode and anode (Pin 3, a). (© Genalex)


Here is a diagrammatic representation of a beam tetrode. The light green lines represent the "beam" of electrons that flows from the cathode towards the anode in the direction indicated by the arrows.


The EL34 is a pentode rather than a beam tetrode, and was developed by Philips in 1953. The KT88, developed by MO-Valve, could not be an actual pentode, because Philips held the patent. So, the pentode design was modified such that it became a beam tetrode. The first beam tetrode was the 6L6, which is still very popular in guitar amplifiers.

A generic diagram of a pentode is illustrated below. The numbers are not the pin socket numbers, but are just for identifying the structures in the diagram.

It has a cathode (2) which is indirectly heated by a filament (1), three grids (4,5, and 6), and the anode, also called the plate (3). It can be operated as a triode by connecting the grids together.


The 6SN7 is a dual triode, originating in 1939 from the US (the ECC32 is the European version). It was used as the driver stage in audio amplifiers, and is electrically similar to the 12AU7. It is still used quite often, such as in preamplifiers. Here is a schematic with the actual pin-out arrangement, as viewed from the bottom of the tube. Pins 7 and 8 are the heater for the two cathodes (6 and 7), pins 1 and 4 are the two grids, and pins 2 and 5 are the anodes (plates).


So, with the pa21, for each channel, there are two triodes in the input stage and two triodes in the driver stage (each channel has two 6SN7's and there are two triodes in each tube), and two pentodes in the output stage. The input stage delivers the voltage, and the output stage delivers the current.

The chassis is gloss black enamel, and the cover for the tubes can be removed to aid in dissipating heat. But if you have small children, leave the cover on.

The rear panel has five-way speaker binding posts, with 8 ohm and 4 ohm "taps" (these are connected to different positions on the output transformer). The power on/off toggle is on one side and the KT88/EL34 toggle is on the other side. The AC power cord is grounded and detachable.