I am designing an output transformer for a 300B SE amplifier, and rather than copy some design, I have been designing a new configuration from scratch.
So, how many watts can you squeeze from a 300B? Well the 300B is a 40watt max dissipation tube; so theoretically you should be able to get 10 watts.
Important aspects of the SE OPT design is
Generally speaking, it is good to design exactly for the chosen circuit. Why?
I do not think there is an easy answer to this question, but broadly speaking, the smaller the transformer is, the better would be it's high frequency response - shorter copper length used to make the transformer has less capacitance, and so the high frequencies pass easier.
So if you choose a iron core for the job, it is better to use the smallest one that will still kick out bass at max volume.
You also want to keep the losses in the copper within reasonable limits (10% being typical); so to use very thin copper in the primary might get you more windings/better bass/higher primary impedance, but the DC resistance and loss of power to heat becomes a problem.
If you use thicker copper, the transformer is nice and fast, and less music is turned into heat, but now lower end response is more difficult to achieve.
The other parameter that complicates things in SE OPT design, is the air gap. Since a dc current runs through the primary, the core is already magnetized in one direction, before you even start playing music. So the music magnetizing goes on top of the dc flux. To stop early saturation on SE OPT the magnetic path is interupted by the so called air gap. A bigger air gap stops saturation, but it also limits the bass response. So ideally you want to set the gap for exactly the current you will be using, so you get better bass (higher primary impedance) for the same transformer used.
It seems that a 300B, realistically speaking, only puts out 8 watts of power. So if I design for 8 watt with the same amount of iron and copper used, I will get a better transformer than a 10watt design.
The 300B uses about 60mA current flow in the typical circuit. A 60mA air gap setting will give me wider bandwidth in the same transformer, than a 100mA air gap setting.
So it seems to me, bigger is not better in SE OPT design.
So, how many watts can you squeeze from a 300B? Well the 300B is a 40watt max dissipation tube; so theoretically you should be able to get 10 watts.
Important aspects of the SE OPT design is
- core size - so the iron do not saturate when playing loud music
- band width - typically 20Hz to 20kHz
- primary impedance - enough Henries for the bass response
- a reasonable flat impedance response over the audible spectrum.
- size of the air gap
Generally speaking, it is good to design exactly for the chosen circuit. Why?
I do not think there is an easy answer to this question, but broadly speaking, the smaller the transformer is, the better would be it's high frequency response - shorter copper length used to make the transformer has less capacitance, and so the high frequencies pass easier.
So if you choose a iron core for the job, it is better to use the smallest one that will still kick out bass at max volume.
You also want to keep the losses in the copper within reasonable limits (10% being typical); so to use very thin copper in the primary might get you more windings/better bass/higher primary impedance, but the DC resistance and loss of power to heat becomes a problem.
If you use thicker copper, the transformer is nice and fast, and less music is turned into heat, but now lower end response is more difficult to achieve.
The other parameter that complicates things in SE OPT design, is the air gap. Since a dc current runs through the primary, the core is already magnetized in one direction, before you even start playing music. So the music magnetizing goes on top of the dc flux. To stop early saturation on SE OPT the magnetic path is interupted by the so called air gap. A bigger air gap stops saturation, but it also limits the bass response. So ideally you want to set the gap for exactly the current you will be using, so you get better bass (higher primary impedance) for the same transformer used.
It seems that a 300B, realistically speaking, only puts out 8 watts of power. So if I design for 8 watt with the same amount of iron and copper used, I will get a better transformer than a 10watt design.
The 300B uses about 60mA current flow in the typical circuit. A 60mA air gap setting will give me wider bandwidth in the same transformer, than a 100mA air gap setting.
So it seems to me, bigger is not better in SE OPT design.
