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DIY Tube Audio
SE 6336A Amp


Main Amp | Main Speakers | Bass system | My Preveous Systems | Design Notes | Phonostage


The output transformer is 3.3k:8 (it is actually a gapped 6.6k 20w push pull transformer with the 8 Ohm load connected to the 16 Ohm tap)
The 1.2k cathode resistor for the 6336A is rated 25w
The 15k cathode resistor for the 5687 is rated 5w
The 170k load resistor is rated 2w (actually consists of 4 680k 1/2w resistors in parallel)
The coupling network formed by the .022 capacitor, and the 68k resistor sets the amps LF rolloff to 110 Hz
The 5687 has its own heater supply to keep all the heater to cathode voltages within recommended specifications

The 6336a turned out to be a rather challanging tube to design around.  It has a much lower plate impedance than most other output triodes.  Most amps use a load impedance 2-4 times the plate impedance.  This amp has a load impedance about 10 times the plate impedance.  Because of this, demands on the power supply are much greater than they would be with a more conventional tube.  After some experimenting, I decoupled the first stage power supply from the output stage with a two stepRC filter.  The first step was designed to damp the output stage power supply.
I also chose a bass cutoff frequency that is at least 1 octave above the natural full power cutoff of the output transformer.  This allows the transformer to operat in a more linear range.  (Yes, output transformers do add distortion to the amp).  It also should reduce the demands made on the power supply.
I tried the amp with and without each of the features mentioned above.  In the original circuit, all B+ was taken from the second capacitor in the CLC pi filter, with no decoupling.  The cutoff frequency was also about an octave lower.  Each change allowed the amp to be played noticably louder with less degredation of the sound.