P134B Single-Ended EL-34

The P134 has a very unique circuit, developed over two years of research and development to reduce the intermodulation (IMD) distortion so prevalent in other single-ended (SE) amplifiers. It should be noted that we have accomplished this without the use of Negative Feedback (NFB) that will lower IMD, but will also compress the signal. The design consists of a small twin triode (12AT7), of which only one side is used per output tube, this drives the EL-34 to 11 watts, measured at standard, moderate clipping based on 1.5v input to full output.

The circuit features a signal summing path, as well as value selections to achieve a very low IMD rise without NFB. IMD is the congestion that occurs when the signal level gets very high. This involves the bass notes modifying the shape of the higher notes by the mixing actions of the tubes and components involved. IMD is mostly dissonant and is heard very fast in conventional circuits. This is the basis for the SMPTE 4:1 two tone tests, one bass note and a high note mixed, filtered, and measured.

The zero to one-watt distortion, IMD/THD, with these innovations is much less than a good quality SE 300B amp. The first watt of this design makes this a very good amplifier for high efficiency, horn loaded (97-110dB SPL) speakers. The signal from 0 to 7 watts is very clean and from there on to maximum output, is the same as any other conventional SE amp.

The given name and theory involved to achieve this low IMD rise is called ISO-LINEAR. This is the action of two signals in balance, in opposition between the driver triode and the first equivalent triode of the EL34. Meaning the cathode, grid, and screen being the first triode, the second triode being, effectively, the screen, suppressor, and plate.  The ISO-LINEAR action nulls the mixing result of the two or more signals in play by balancing at the zero axis point between both triodes. 

The cause of IMD is the result of “mixing” two or more signals in one tube at the same time. For example, a high-level signal arriving in a triode causes its amplification to change due to it increasing current so tube amplification increases and develops a modification of the signal added onto the original signal. The balancing action of both triodes, as indicated, nulls this when both driver tube and screen currents are identical nulling the added signal/s out.  

The EL-34 is two tubes in series in this circuit. The output circuit is also unique, using a special tapped output transformer that offers more power from an EL-34 than if wired for pentode operation, while also designed for use with the above description of the nulling process. The P134 offers a dual input impedance, basically just a high and low switch on each channel, to be selected by which setting gives you the loudest volume, would be the correct selection.

The P134 also offers a wide range of voice coil impedances, centered on 8 ohms, but will play from 6 ohms to over 20 ohms, another benefit from our 25 years of experience with transformer designs. Bias is fixed and is automatically set for a wide range of quality tubes. To view the schematic click HERE.

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