Power suplies and capacitors

[traynorbassmate]

How does an amp builder decide on the ideal power supply and rectifier?

I have a single-ended 8ohm 6V6 design. Designed plate voltage is around 315-350V?
My design, which I believe is flawed, uses a 250V secondary (CT not used), bridge rectifier, and 150uf/350 for the output transformer > plate (pin3), and 20uf/350 for the grid (pin4). The two caps are separated by 1.5Kohm.

When powered up, the secondary reads 275VAC and the filter capacitors read 370VDC / 800VAC. worried.


Adam

[diagrammatiks]

How does an amp builder decide on the ideal power supply and rectifier?

I have a single-ended 8ohm 6V6 design. Designed plate voltage is around 315-350V?
My design, which I believe is flawed, uses a 250V secondary (CT not used), bridge rectifier, and 150uf/350 for the output transformer > plate (pin3), and 20uf/350 for the grid (pin4). The two caps are separated by 1.5Kohm.

When powered up, the secondary reads 275VAC and the filter capacitors read 370VDC / 800VAC. worried.


Adam

seems like either A. your wall voltages are a bit higher then what the transformer is speced for
or B. the unloaded voltage of the transformer is a bit higher then the loaded voltage

at any rate 350v is cutting it close for 315-350 plate voltage.

[traynorbassmate]

I've found out that the power transformer only performs at spec if it meets its expected load, 42mA. It has a linear response between 0-42mA.

How do I add to the load in the circuit? dang!

[Colossal]

How do I add to the load in the circuit? dang!

You can add series resistance to one leg of the PT's secondary (since you are using a bridge rectifier). This will simulate the voltage drop of a tube rectifier. This is not to say that the amp will "sag" under load as there is continuous current draw in single-ended amps; you will just get a voltage drop which will help you get your operating voltages more in line with your target.

You will need to compute the power rating of a resistor but a rough estimate of at least 10W at minimum.

[niftynev]

Looks like you're sailing pretty close to the wind with the voltage rating of the caps too. You're probly safer using 400V or even 630V caps.

[VacuumVoodoo]

If it is an older PT designed for 110-115V mains then 275V AC on secondary side is ok. You can reduce the rectified DC voltage using an "amplified Zener diode" (Look it up) connected between bridge (-) and ground.