460V DC to 400V DC How?

[boombar]

I need to lower voltage from 460V DC (sometimes goes even to 470V) to 400V DC.
Need to supply up to 1A max.

It's probably possible to achieve that with some zeners and a transistor... is it?

[ante]

Hi boombar,

That’s 70W to burn away! Ouch 470VDC sounds dangerous  :o don’t you have AC nearby, it would be much easier. What are you going to use it for?

[Shahriar]

Dear boombar

I am not completelty sure but you can do it with those normal Regulator circuits. if you need a good regulation, maybe connecting some zeners in series and a suitable transistor with Vce >= 100V with high enough hfe.

if you don't need 400V fix, then you can use two 30V zeners. just change the Resistor and zener place in the following Picture.

HTH - Shahriar

[Shahriar]

By the way, your transistors will get too much Hot because of 70W Power dissipation. you should use fan and some more Transistor in parralell and some resistors for current sharing. I hope you understand what I mean!!!


HTH - Shahriar

[boombar]

Thank You Ante for Your reply.

I found an old transformer i want to use for my tube amp. It's perfect, but mussing 400V for screen grid.
I't doesn't realy need full 1A of current. Sorry.
Just to supply four 6L6GC screen grid voltages, but don't want to blow something in power supply (instead of fuze) in case of overload.

As it looks I'll probably have to buy correct

[boombar]

And thank You Shahriar too.  :)))

[Guest Decade]

Hi boombar!

Not much current needed for your screeen grids even with a quartet of 6L6,s.
Are you building something from scratch or is it just some mods?
Got 4 6L6 in my Bassman 100, some money when there's time to replace

[ante]

Hi Decade,

How much is “not much current”?

[boombar]

Yeah. Decade is right.
Screen grid need very low current. I overcalculated a bit and caused confusion. Sorry.

Maximum screen grid power dissipation for 6L6GC is 5W. So screens for all four tubes will dissipate maximum of 20W (right?), that means 0.05A on 400V. Right?
But I will not be driving tubes so hard since their lifetime is drastically decreased.

Sorry guys, I'm still on experimental phase of learning tubes...

[Guest Alun]

Try this,

Tr1 needs to be rated for Vin

Tr2 needs to be rated for Vin - Vout and will be fine providing you don't put any big capacitors on the output (>100pf or so) otherwize it needs to be good for Vin.

I haven't tested this circuit as high voltages but it works well at low voltages.

Edit:
Posting the schematic would be helpful too. ::) ;D

[Guest Decade]

Hi Ante!

That was a bit fuzzy Ante "not much"  ;D
Had a look at the 6l6 data sheet were they state 5W maximum screen grid dissipation.
With 450v applied to screen grid you get 11mA to stay safe so we can say that "not much" is 11mA  but close to "to much"  ;D
Best way to shorten tubes life is to use to high idling current.

[Guest Alun]

Just thinking,
I think Ante has hinted on this but have you considered building a custom made transformer to produce the correct peak voltage in the first place?

[boombar]

have you considered building a custom made transformer

As I mentioned earlier, I'm using an old transformer I found. Just missing 400V. And it's not easy to order custom transformer here since they're not very cheap. I will probably benefit more by putting few transistors.
That's why I chose this power supply. It didn't use transformer tap for 2 voltages.

Posting the schematic would be helpful too

There is nothing to look at. Its just rectified 325VAC and gives 460VDC.
The original circuit had a IGBT with a lots of zeners between base and ground (directly) giving about 430V all together, and resistor between collector and emiter.
I would gladly do it that way, but in that case I would have to order from Germany, and I think 100pcs in package.

Thank you all....
I believe these 2 schematics will do the job.

[ante]

Hi boombar,

I can’t see your schematics; I don’t know what’s wrong! :-\
But searching the internet I found a tube PSU article :
I thought it might give some ideas!

Tube_power.pdf

[boombar]

Thanks Ante. This is interesting.

I can