how do I lover dc voltages

[robertguerrieri]

I have a dc power supply that uses 2 5u4 tubes and is putting out 570v ,after the choke , I need to drop it down to 400v any ideas . I don't know what will work the beat

[bob91343]

You need a different transformer. Any other approach will make compromises, since you want so much reduction.

I assume you are measuring with a load connected, the only proper way to test a power supply.

[robertguerrieri]

bod I am measuring it from the choke to ground and getting 575v

[robertguerrieri]

i am trying to build a simple single ended amp , i need about 450 v to the transformers

cant i just use resistors and caps to lower it (the transformer is quite large ) about 10 lbs and the choke is a 1000v 7henrie

[Enzo]

Robert, Bob was asking you if the voltages you report are with or without the rest of the circuit active? If you took the voltage readings with the other tubes not installed, the resulting rectifier-only voltages will be a lot higher than what they would be in the circuit normally.

What AC voltage are you working with?

Have you considered a choke input filter instead of capacitor?

[robertguerrieri]

thanks guys
i swapped out the power transformer and went to one 5u4 now I have 455volts comming off the 5u4 going into a 7 henrie choke 200ma (1000 volt) so at either end of the choke the voltage is 455volts . I have a pair of ubt single ended transformers that i wasgoing to use with el34 tubes , the schematic is looking for 400vdc after filters

am I on the right path . I was going to use 2 30mfd caps one in each side of the choke .................?

[VictoriaGuy]

Originally Posted by robertguerrieri

am I on the right path . I was going to use 2 30mfd caps one in each side of the choke .................?

I don't think you are paying attention to the advice you have gotten so far.
1) The voltages you are measuring are meaningless unless you have the rest of the circuit 'attached' and 'working' - as has been mentioned.
2) If you are now suggesting that you should 'add' caps - one before and one after the choke - then you don't seem to understand the difference between 'choke input' and 'capacitor input' power supply filter sections....which is what Enzo was driving at, I think.
Have a look at:
Rectifier guidelines from Hammond Transformers

and Duncan amps power supply designer

[robertguerrieri]

your right ,I dont understand the difference >

all am am trying to do is build a simple power supply . the power transformer has a 900v winding with a center tap .I grounded the center tap ,connected the 450volts to the 5u4 along with the 5v taps .Now I have 455volts dc to ground .I take the 455vdc connect it to one end of a 7 henrie choke ,the other end od the choke still reads 455vdc . all I want to do is add whatever I need to get the dc filtered and down to 400vdc .I dont understand what the rest of the circuit has to do with any of it .the schematic I have is asking for B+ 400vdc

I havent installed the 6550's yet or wired the transformers yet because I dont have the right dc voltage

am I wrong in my thinking ? but I cant wire these until the b+ is correct

thanks
ps I did this once before and it was quite easy ,just followed the schematic
in this case I cant because the power transformer is a little different

[fhs]

Hi,

Originally Posted by robertguerrieri

your right ,I dont understand the difference >

I am concerned about you working with HV power supplies... You have been advised to test the PS under load. An appropriately rated (!) resistor would do the trick if you do not want to power up your tubes.

Your circuit description is unclear; please post a diagram. You have a transformer with a center tapped secondary -- all you need is 2 half-wave rectifiers, 2 caps, 1 choke.

Warm regards,

Fred

[mastodon]

I take the 455vdc connect it to one end of a 7 henrie choke ,the other end od the choke still reads 455vdc

Because there is no load...there is no load, so there is no current flowing, so there is no voltage drop. The voltage drop across a choke will be minimal in any case, but still.

You might get away with a B+ of 450V, but that depends on the rest of the schematic (i.e. the audio section); you might need to adjust some values. You could also, as you suggested, use a series resistor to drop the voltage. Assuming you'll use two EL34's (or something similar) in SE, that means you'll probably consume somewhere between 110mA and 200mA for two channels, depending on bias. That means you'll need a resistor of 250 to 450 ohms, with a power rating depending on the exact biasing of the output tubes and the resistor value, but think of somewhere in the neighborhood of 10W to 20W. You'd need to get rid of that heat, which could become a major concern, depending on your chassis.

So the practical solution is to either live with your +/- 450 Vdc (or a bit less, under full load) and adjust the circuit where necessary to accommodate for the higher B+, or find yourself a different power transformer.