P134B- SE EL34, Iso-linear amplifier, circuit operation
Using a conventional, three stage, single ended, power amplifier circuit. This circuit has been re-engineered with many unconventional features. The first stage, 12AU7, offers a usable signal level for the next stage, the driver stage. This driver tube, a 12AT7 and the output tube, an EL34, pure pentode, are coupled in a loop, similar to negative feedback. This loop is referred to as a “summing loop” and is not negative feedback, it does not lower the power like NFB will do and will not add global compression and phase shift.
The operational characteristics of the 12AT7 and EL34 balance out or see each other in comparison, as the same tubes, in exactly the same operation, but opposite. The 12AT7, being a medium Mu triode and the first or lower half of the EL34 pentode, cathode, control grid and screen, operating as a triode plate, offer’s an exact signal cadence, in opposition, so signal/s at both grids and plates have no comparative change in amplitude and current draw. This is measurable. This action will not allow any part of the low frequency of the signal to modify the mid and high frequencies, which is, Intermodulation distortion, IMD.
In conventional tube circuitry, the increase of plate voltage as signal amplitude increases, adds more amplification, or Mu, (1) to that tube, changing the output waveform as the current stays the same (2). These two different events now sum/difference in the electron flow and a third signal results. This process, in radio circuitry called a superheterodyne, is two signals mixing to make a third signal (intermediate frequency). This intermediate frequency is the IMD of this type tube. These IMD (mixing) generated products, are mostly dissonant, hearable and always very irritating. This irritation is commonly mistakenly as too much harmonic distortion. Harmonic distortion, even order, is actually pleasant in most cases. The presence of IMD is measureable using SMPTE, two tone, test equipment.
ISO-LINEAR, so named, is what this balancing and nulling, in opposition, is called.
The EL34 screen, suppressor grid and plate, complete the second or upper half of the pentode, which will supply the remainder of the power generated.
The special wound output transformer allows these two series triodes, (the EL34 pentode) effectively, to add the power of the lower triode to the upper triode and the sum will provide about 11 watts of power. The developed power is extremely clean up to about 7 watts, each instrument of the music is not impaired by other instruments and no negative feedback caused compression of the original signal, is present! IMD is kept to lower than 1.5% to 3 watts and will rise to 10% at about 7 watts, then clip at 11 watts. Most conventional amplifiers with no NFB, IMD will rise to over 30% at the respective 7 watts.