1. Will this amp work any better by having a filtering capacitor between the O/T and the power supply?
Define "better".
The Firefly shouldn't suffer from hum due to insufficient filtering because:
- The 1st filter is already 47uF and has very little drain from the output stage.
- The output is push-pull, which will cancel hum in the output stage, and
- Even if the output wasn't push-pull, triodes have a higher power supply rejection ratio (PSRR) which makes their output less sensitive to hum in the supply.
Maybe you're thinking frequency response, in which case skip to #3 below.
2. Aren't the typical nine pin tubes restricted to a maximum of about 4.5w combined, so those high wattage tubes like the ecc99, are total tube dissipation limited ...
I don't know about "typical". The
12AU7,
12BH7 and
ECC99 data sheets say 2.75w, 3.5w and 5w per triode, respectively. I'd read that as 5.5w, 7w and 10w per tube envelope, as the sheets don't state any restriction in that regard.
3. Does the high impedance chosen by the original builders help because the O/T also serves some functions as a filter see question 1?
There is little consideration given by the designers regarding any filter formed by the OT primary impedance, at least for initial choice of operating conditions. Rather, a higher primary impedance provides a load which keeps the output tube plate current from becoming excessive during operation. If Power = Current
2 * Resistance (or Impedance), then more primary impedance reduces current for the same power.
When looking at the loadlines, an infinite impedance is represented by a horizontal line on the graph; voltage could increase infinitely with no increase of current. So also a vertical line is zero impedance. When our early class A loadlines tended to be too vertical and stray into excessive plate dissipation, we increased the load impedance the flatten the loadline, reduce plate current and bring plate dissipation back down to a safe level.
As PRR pointed out earlier, the choice of primary impedance loading a triode plate is generally related to the triode internal plate resistance, in a 1-to-1 or 2-to-1 ratio, which also sets how the load and the tube will divide voltage.
The drawback of small signal triodes as output tubes is that even the 12AU7 has a plate resistance high enough that we found 22kΩ+ to be good loads for class A operation at a convenient plate voltage. In order to wind a primary with enough inductance to have a reactance very much greater than 22kΩ at the OT's lowest rated frequency (and thereby meet the OT's specs), many, many, many turns of fine wire are needed. That will increase winding capacitance within the OT (unless it is extraordinarily well-designed and manufactured), which will cause treble roll-off in the audio range.
So the primary impedance required is set by the demands/requirements of the output stage, and any impact of frequency is an undesired byproduct of a practical device. This is the reason why you see so few transformers with wide frequency response and specified high-impedance primaries. And the (well-made & uncompromised) mic-to-grid & plate-to-line transformers you do see with high impedance windings will cost as much as guitar/bass amp OTs 20-50 times bigger.
4. Does anyone have a design discussion on the split phase inverters other than what I posted?
Self-split inverters can sometimes look like something-for-nothing on paper, but they don't deliver their full promise in practical use. That's why you won't see them outside of a few oddball Gibson amps, very cheap tabletop radios, and (surprisingly to me) the Firefly.
But David wanted to venture into class AB output stage design, and self-split inversion (as well as stable cathode bias) pretty much demands class A operation for a total plate current that doesn't vary much from idle to full-tilt. It seems likely at this point that whatever output tube he chooses, the amp will likely use a conventional phase splitter circuit.
