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-   -   Tube Preamp PSU Question (http://www.diyaudio.com/forums/tubes-valves/26274-tube-preamp-psu-question.html)

JojoD818 18th January 2004 07:05 AM

Tube Preamp PSU Question
 
Hi everyone,

This may seem to be a regular stupid question but I still need to ask it to be sure. Thank you for understanding.

I found a schematic of a tube preamp in the web that uses 330-0-330 trafo, a 6X4 tube rectifier and CLC componets for a B+ of 255VDC.

My question is, can I replace this tube power supply with a descrete one? I mean using 2 regular rectifier diodes or a bridge diode? Do I follow the usual VDC = VAC X 1.4142 formula used in solid state? If I do, then I need a 180VAC trafo and a single bridge right?

This is my first of many tube projects, please help me.

Thank you very much,
JojoD

analog_sa 18th January 2004 07:40 AM

Indeed, you are correct in your assumptions. Though i don't really understand how the 330v AC gets rectified down to 250v dc. Even allowing for heavy drops on the rectifiers and choke. Are you sure there are no additional resistors involved? Or is it choke input?

SHiFTY 18th January 2004 07:43 AM

Yep a 180 volt tranny with a diode bridge should be about right, or 180-0-180 with full wave diodes. Valves are pretty ok with differnt voltages, so 200-0-200 or even more would work just fine...

I usually find the multiplier ends up about 1.2 after a CLC filter BTW. 330-0-330 would end up at about 395V I think, is there a big resistor in the powr supply????

Also make sure your power tranny is rated for about double the current rating you intend to use, and put about a 0.1uF 3KV ceramic cap across the power transformer secondary to reduce noise.

A decent low R choke is good, maybe 10H or more at double your intended current...

JojoD818 18th January 2004 01:18 PM

Hi,

The preamp I am refering to is this http://diyparadise.com/preamptutorial.html

The choke I think is 595 ohms each? Could that be the cause for the voltage drop? Also, if I use a solid state psu, do I still need the chokes?

BTW, I just recently bought a cheap manual rewinding machine (the one with a turns counter) for use on passive crossover coils, can I build my own chokes?

Thanks for your patience.

JojoD

analog_sa 18th January 2004 01:37 PM

Well, it is choke input so you multiply the 330v by 0.9 and then subtract the drop across the rectifier. Replacing the 6X4 with anything sandbased won't do much favours to the sound but can still sound decent. You still need to reduce power supply ripple and here you have a choice - either build a simple emitter follower type regulator (or just cap multiplier) or start from a much higher voltage and use several RC filters in cascade. What cap values can you get?

arnoldc 18th January 2004 01:48 PM

jojo, i use duncan's excellent psu designer to make all my psu. it allows both solid state and tube rectification. pretty accurate too.

JojoD818 18th January 2004 03:40 PM

arnoldc,

Thanks for the info.

analog_sa,

I already have 2 pieces of ELNA 100uf/400V caps and more smaller capacitance, high voltage caps soon to come. I also have several 330uf/200V caps but that's for a tube preamp project that requires +/-100V rails. I once thought that these caps are useless since I needed large caps for ss.

I also have several small discarded trafos that can be recycled and rewound with fresh magnet wires for filter chokes.

You also mentioned that I can use solid state regulators, capacitance multipliers which I have some experience on. So as in ss, psu design really affects much of the overall sound of tubes?

JojoD

analog_sa 18th January 2004 04:23 PM

Quote:

So as in ss, psu design really affects much of the overall sound of tubes?
Probably more. Tube circuits generally provide very little PS rejection. IME, which likely differs from that of others, a vacuum or gas rectifier sounds better than any kind of fancy diode and choke regulation sounds better than valve regulation which in turn is preferable to SS regulation. 200uF is simply not enough to achieve acceptable level of ripple with an RC filter. Nor is it enough to give you reasonably good bass. Increase the capacitance and the bass also increases in quantity but generally suffers in quality. Keep low capacitance but include a choke or two and you get low ripple and excellent bass. Chokes are also not created equal; it's nearly as difficult to wind a great sounding PS choke as it is to make a plate choke. Core material, flux level and DC resistance all seem to play a significant role.

Most commercial tube audio does not use chokes though. Too heavy and expensive to be worth it . You generally see diode bridges, hundreds uF of capacitance and simple emitter-follower type regulators. In the more upmarket stuff you may see tube regulators and Jung-type opamp regulators. If it's good enough for them it will probably do well in a first project.

Good luck.

fdegrove 18th January 2004 11:50 PM

Hi,

Money no object PS:

Valve rectification into LCL filter, into series reg using valves followed by C, shunt reg using a valve or VRs followed by R loaded anode.

Variations on this theme are welcomed but this pretty much sums up my favourite recipe.

Cheers,;)

JojoD818 19th January 2004 02:20 AM

analog_sa,

Thank you for explaining the behaviour and effects of psu on tube circuits. As I suspected, it does affect the output and must be very careful in selecting components for that matter.

Frank,

Lovely recipe, unfortunately, valve rectifiers are so hard to find locally, I'll go ss on my psu just to try my hand on tubes and maybe later on a valve psu too. :)

JojoD


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