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Original Message

Push-pull SEX

Posted by Paul Joppa on July 26, 2007 at 11:52:08:

A few weeks ago I said I would post some information on converting a stereo SEX amp to a push-pull monoblock. This post is my first attempt. Note that I have not tried this and make no guarantees - hey, it's an experimenter's amp, so experiment!

I have here tried to come up with the smallest possible set of modifications to a stock amp, not the best amp I can think of.

We can convert the drivers to a "long-tailed pair" phase inverter fairly easily. First, connect the cathodes to each other (they are pin 6 on the 6DN7).

Second, replace each 1270 ohm bias resistor with a 10K resistor from the cathode (pin 6) to the corresponding "- bias" (T12, T32). At the same time, replace the 620 ohm 2 watt bias resistor with 680 ohms 2 watts (T12 to T15 and T32 to T35). What this does is provide a high-impedance current feed to the driver cathodes, and since the driver current no longer flows through the 620 ohm output stage bias resistors, those resistors must increase to maintain the same voltage drop.

Now the two sides are driven out of phase; you can connect an input to either RCA jack and the volume control will continue to work properly. The phase of the output can be reversed by using the other RCA input.

The outputs will be taken between the red binding posts. It will be a 16 ohm output if you have wired to the 8 om taps, and an 8 ohm output if you are using the 4 ohm taps. Note that so far, the two sides use separate transformers. It is still a parafeed arrangement, and it still uses two power supplies with independent WE style biasing.

You can partially "overlap" the transformer primaries to get closer to a single PP transformer. To do this, you must disconnect the primary common leads from ground so they can be cross-connected. For safety, they should still have a DC path to ground; I suggest a 100K resistor for each transformer. See the sketch. This will reduce the plate-to-plate impedance, allowing Class AB operation and increasing the available power output. Because of the reduced impedance, it may be desirable to increase the parafeed capacitor values.

You can use whichever taps you wish for the cross-connection - the more overlap, the lower the effective primary impedance. With no overlap, the effective plate to plate impedance is 16K; cross-connecting the entire transformer gives a 4K effective impedance.

To get a 2 ohm or 4 ohm output, you will have to similarly disconnect the secondaries from ground and cross-connect the 4 or 8 ohm taps respectively.