Can someone help me understand this brilliance/mid cut function?

[tubedood]

Hi again folks… I am trying to figure out the feasibility of the little .047 cathode bypass cap I asked about last week in my tweed Princeton project. I got the idea partially from a schematic I found of a modded champ, and partly due to John McIntyre's Prince o Wails build. I liked the sound of the "Brilliance" feature on that amp. I'm curious if using the .047uf cap in the first half the 12ax7 in my 5F2-A build is the same as the POW use in the second half. I have attached the prince o wails schematic for your review. Is the "Brilliance" control just the .047uf cap or is the inductor in that area also a part of the tone of the brilliance control?

Sorry to have asked a similar question a bit back I am still trying to figure this out.

Have a great day!

Dale

[Old Tele man]

It's feeding forward a "portion" of the cathode voltage up to be injected (add/subtract) into the plate output signal feeding into the following PI input stage.

The LC form a series "resonant tank" which is switched in/off by the 10K||1Meg "LEAD/BOOST" paralleled resistors.

The 50K pot variably subtracts the two (22µF & 0.047µF) capacitor values in series, thus changing the LC resonant frequency.

[tubedood]

OK thanks for a lightning quick reply! Much appreciated. So the .047uf by itself isn't that great sound to the brilliance switch?

[frus]

It's best to try it out and see how it sounds. 47nF is commonly seen on Hiwatt brilliant channel, but also on later preamp stages. However, their brilliant channel also has some bass shaved off by a small (1nF) coupling cap before 1st and 2nd stage

[66 Kicks]

Tone-shaping cathode bypass capacitors such as the .047uF in question increase gain as the frequency increases. The gain with the .047uF in parallel with the 4.7K in the POW rises with frequency out of the abyss to +3dB at around 721Hz and continues to climb with frequency. A .047uF capacitor in parallel with the 1.5K cathode resistor in the 5F2-A will not hit +3dB until around 2.3KHz, which is kind of high for a bright feature. It is the product of the capacitance and the resistance that determines the +3dB point, so you can juggle those values to get what you want. A 0.15uF capacitor in parallel with a 1.5K cathode resistor will yield a frequency response like the POW brilliant mode.

The "inductor" part of the POW circuit is not necessary for the Brilliant control, but it interacts with it. The .047uF capacitor in series with the 600:600 transformer have a resonant frequency and at that frequency the impedance of the combination is at the minimum. The 22uF capacitor in series with the .047uf capacitor of the LC combination is still .047uf, so it has no effect on the resonant frequency. Since this is a "Mid" control, I would expect the resonant frequency to be around 400Hz and the maximum effects of the control to be around this frequency.

[66 Kicks]

At one extreme of the 50K Mid control, the wiper is connected to the 22uF cap. At the resonant frequency of the LC combination, the impedance between this 22uF cap and ground is at the minimum and it acts like a cathode bypass cap with the increase in gain. As the frequencies move away from the resonant, the impedance between the 22uF cap and ground increases and the gain drops. So without the brilliant cap, this setting results in a boost for the mid frequencies.

At the other extreme, the wiper is connected to the 10K resistor. This 10K resistor in series with the LC combination forms a variable load for the plate output. At the resonant frequency, the load has the lowest impedance and the gain is the lowest. As the frequencies move away from the resonant, the impedance of the load increases and the gain increases. So without the brilliant cap, this setting results in a cut for the mid frequencies.

[tubedood]

Thanks guys great info in there! I appreciate your time!