I devoted the majority of my audio life to single-ended DHT amplifiers. I don’t regret the path, however, I do not want a polarised perspective on amplifiers. I do like very much a good push-pull amp, though. Over time, I stumble across poor implementations or excessive use of gNFB. Local feedback (lFB) designs well implemented are music to my ears.
I played around with SPUD amps over the years, with happy results. One of them I loved over the breadboard is the 6e5p in push-pull. I will definitely build a definitive version of it. Here is the concept overall:
The input transformers is step up. It will need 1:4+4 ideally. My friend Dorin from DVB Transformers has a great device which I reviewed before and would be ideal for this amplifier. To simplify the power supplies, I used only one, which provides a bit of a strain into the source followers SF1 and SF2. If you have a good driver, you can omit them, I wouldn’t. They take care of the miller capacitance which is worsened by the step up transformer. It’s just a pair of PCBs, not big deal. RK1 is an arrangement with a pot to balance the bias. C1 is in ultrapath. I used an Oil 40uF one. T1 was a Lundahl 8K8:8Ω PP OPT.
The CCS provides a stable bias current into RBIAS1 which sets the bias voltage.
Here is the operating point I used:
The 6e5p is biased at about 40-43mA and 240V. It needs 5V bias so a cathode resistor of 58Ω should do. For 4W output in A1, you need 4Vrms, which only with a 1:2+2 step up should do. You can get about 5W in A2 or a tad more.
I loved the sound of the 6e5p as output stage (as much as I do as driver).
There are multiple variances to explore indeed
I have a pair of Toroidy PP OPTs with CFB, so would be interested in running them in tetrode instead.
