If it’s any consolation, it is actually making sense to me & believe I am following the line of reason.
That's good! There is some unstated background I'm not explaining, and the approach I've used is back-of-napkin and generally assumes the output tube is an ideal device. In all cases where the tube deviates from the ideal, the result is less-than-calculated clean output power.
A pre-requisite for understanding this stuff is basic Ohm Law, the Equation for Power, and basic properties of a.c. & d.c. voltage, current and resistance/impedance. If you know those things, you can pick up the rest over time.
I also didn’t appreciate the differences between the 6L6 variants, it seems that will really make a huge difference choosing the correct components.
Well, it may not make a huge difference to total clean power output...
I brought up the different 6L6 versions because any big device can be used at less than its full capabilities. So you
could idle a 6L6GC at 19w, and get less power output. This may matter if you want to build an amp and not care which 6L6 variant winds up in the socket. If you build to take advantage of everything a 6L6GC can do, the 19w 6L6 (and some modern Russian types sold as "6L6") or the 6L6GB/5881 (or Russian & Chinese tubes sold as 6L6's) may not fare well in the socket.
I might be overestimating the PT requirement some in my examples. Also, a PT can usually have more than the rated current drawn from a winding, but the voltage output will sag.
My examples also gloss over some of the complications, like selecting the required bias voltage and how that adds to the required B+, as well as calculating the cathode bias resistor to use. But output stage design is an iterative process, and I just worked the first-pass to see if it was worth bothering to take any particular condition further.
How does the “unbalanced DC” PT rating figure in & is that a typical spec?
Not every OT you buy will have that specified. But if you're doing something uniquely your own (or not copying a known-good circuit), you should know that rating.
Look at
this page of Hammond SE OT's. The 125FSE is a 20w SE OT with multiple secondary taps which can be loaded to give a number of primary impedances. Note the 3rd column in the table says, "Maximum D.C. Bias". That is the maximum direct current you can pass through the primary as idle current for your output tubes.
Transformers work by the magnetic energy from the power applied to one winding and using the core to transfer that energy to another winding. There is a maximum amount of magnetic energy a core of a particular size can handle & transfer; if you try to apply more energy to that core beyond the max, the core just saturates and no additional energy is transferred to the other winding.
Standing d.c. (idle bias current) uses up a chunk of that energy handling ability, and single-ended transformers are specially designed to cope with it. But they are also only designed to handle a certain, reasonable amount, hence the maximum rating.
In a push-pull OT, the B+ power is applied at a midpoint in a winding, and flows in opposite directions to either opposite end. If the tubes attached at either end have the exact same idle currents, the magnetic energy of the idle current to one end offsets the magnetic energy of the idle current to the opposite end, and none of the core's capability is used up. That's "balanced d.c."
SE OT's may be rated to handle unbalanced d.c. of up to several-hundred mA's (for a VERY big SE OT), while a push-pull OT may not be able to handle much unbalanced d.c. That's because there is a special construction feature to a SE OT for this task and they are very large for their rated power; while push-pull OT's take advantage of the presumed nearly-balanced d.c., lack the SE model's construction feature and generally use a much smaller core for the same rated power.
Topic: Another 5F2A build (Read 7027 times)