Power transformer for tube circuits

[5v333]

Im a bit confused with power electronics and wonder what transformer to choose.
The PSU will deliver about 250 VDC, 152 mA. Everything is in class A.
If i would make a centertapped, fullwave rectified, cap coupled, mosfet regulation... and lets say i want a raw DC of about 270 VDC before regulation, should i aim for VDC = 0.45*VACsec which equals to VAC = 600ct, and since i need about 152 mA i could aim for a transformer that i s rated a little bit higher (around 170mA for ex)

I am looking at hammond 372FX and 290DEX as candidates.

 

[beatnik]

https://www.sowter.co.uk/rectifier-transformer-calculation.php

 

[scott2000]

How about this?
270 x .707 = 190.89
190.89 x 2 = 381.78
A ct full wave rectifier circuit can be loaded to the full current rating of the transformer winding..

 

[5v333]

Thanks.
Maybe 270 VDC raw DC is a bit low when thinking about fluctuations on the mains etc... 400 VACct might be a better target?
Its actually hard to find a transformer with these specs. 500VACct seems to be the next level, but that is going to result in quite alot of P across the mosfet regultor...
Have to search further.

 

[john12ax7]

Do you need regulation? Much tube gear works just fine without it.

 

[5v333]

I guess i dont. But why not do it anyway.
IRF740 is speced for 125 W so it will handle the Vds * I. Im just extra nervous when it comes to high voltage and mains stuff.
Maybe a small series resistor before regulation could be a good idea. For a little bit of dissipation and some damping of startup currents...

Hammond have two 450 VACct units speced at 150mA and 300mA. Would the 150mA unit be a bad choice since the current is basically right on the money? Or would the higher VA compensate the current with a bit lower voltage?

 

[merlin]

IRF740 is speced for 125 W so it will handle the Vds * I. Im just extra nervous when it comes to high voltage and mains stuff.
Maybe a small series resistor before regulation could be a good idea. For a little bit of dissipation and some damping of startup currents...

Hammond have two 450 VACct units speced at 150mA and 300mA. Would the 150mA unit be a bad choice since the current is basically right on the money? Or would the higher VA compensate the current with a bit lower voltage?

450Vct will give you close to 300Vdc under load. Hammonds are conservatively rated so the 150mA current rating shouldn't be an issue, expecially if you're not using the full heater capacity (less load on the heater winding means you can suck a little bit more from the B+ winding instead)

 

[5v333]

Thanks!
The heater windings, on that transformer, are def oversized for this project.

 

[ccaudle]

I guess i dont. But why not do it anyway.

It depends on the type of equipment you are building. If you want something with high headroom and low distortion, then regulation can be useful (microphone preamp for example).
Some people think that power supplies sagging under load is what gives guitar amplifiers a characteristic sound when you overload the output stage, so if you are building a guitar amp for heavy overdrive, regulating the power supply might not give the sound you expect.