Interstage Coupled PP 6B4G Amplifier
by Ralph Power
After building and enjoying my interstage-coupled single ended 300B amp for a few months, I began to wonder what advantages interstage coupling would bring to push pull triode amplification.
After studying and reading about push pull amps, it became clear to me that the phase splitting function was the most critical portion of the circuit and to a great extent determines the overall sound character of the amplifier.
This phase splitter function, which breaks the audio signal into upper and lower halves before sending it to the output tubes, requires utmost accuracy or the audio quality will suffer greatly. In conventional push pull amps, this chore is performed by an additional tube stage and a number of resistors and capacitors.
But this configuration depends on many circuit factors being in "perfect" balance and they almost never are. Also the addition of the extra components works against our "less is more" philosophy, each one changing the original signal a little more as it passes through.
We must to look back at phase splitting as it was done in the early days of audio, by using an interstage transformer to parse out the upper and lower waveforms. A transformer performs this task perfectly without the addition of other tubes and components. This also allows us to keep the parts count much lower and contaminate the original audio signal much less.
So I built my interstage-coupled push pull triode amplifier using a mix of old and new parts, vintage Amperex 5842s (a WE417A clone) driver tubes in single ended mode, driving a pair of new Lundahl 1635s interstage transformers which handle impedance matching and phase splitting. These are feeding a pair of new Sovtek 6B4Gs, which have that "single plate" magic which are driving a pair of vintage Sansui 1000A push pull output transformers with about 8 watts per channel of glorious triode sound.
What results is different kind of amplifier that has all the usual strong points of push pull, dynamics, imaging, clarity, and directness, without the usual liabilities of graininess, compression, and glare. In addition, the direct heated triodes add considerable magic to the mix, their relaxed palpability further counterbalancing these tendencies.
So you wind up with a sound that is not quite single ended and not quite push pull, but instead one that borrows the best from both worlds. The sound has great imaging, and clarity like single ended but with great drive and bass extension, unlike conventional single ended. Overall, a very well balanced bag of tricks indeed and one that might just be the answer for those who feel that single ended is not "gutsy" enough for them. In addition, it is much more friendly to "conventional" speakers with their lower impedances and elaborate crossovers and thus should give a greater chance of success in more systems.
You can read about these amps, but you can never really know what one sounds like until you hear it. So please do so at your earliest possible convenience, you won't regret it.