Tube power supply info?

[rascalseven]

Can anyone point me to a simple resource for learning the basics of tube power supplies? I want to be able to design my own power supplies instead of always asking the group, though I'm always grateful for specific assistance as well.

Currently, I have a Hammond 269AX power transformer (info can be seen at http://www.a1parts.com/transformers/Hammond_Power_Transformer_263_Series.htm ), and I'm wanting to power a pair of Jakob's SRPP output stages with it. Is there a schematic using this type of trafo to create the voltages necessary for it, or is there a site that explains the formulas for designing such a supply?

Thanks so much!

JC

 

[Gus]

IIRC duncans amp site had a power supply calculator program.

 

[PRR]

The usual DC power supply today, any voltage, is transformer- rectifier- capacitor, with optional resistor/choke- capacitor filter stages.

The raw DC voltage is 1.4 times the transformer AC voltage, possibly sagging to 1.2 times the AC voltage.

So for stages not sensitive to hum, select transformer voltage about 0.7 to 0.8 times desired DC voltage. For a 250V supply, use 175 to 200VAC.

Because rectifiers used to be expensive, many tube-culture power supplies use a center-tapped transformer with two rectifiers instead of one winding with four rectifiers. In this case the total VAC is double what you calculated: 360VAC CT will give about 250VDC. (But fashion is changing to specifying this as "180-0-180", which is clearer.)

The AC current rating must be at least twice the DC current needed. (If you use the center-tapped affair, the AC current can be equal to the DC current.)

The rectifiers must be rated for the full current of the transformer (not the load), and twice the DC voltage, and always be generous. Cap-input supplies are very hard on rectifiers, and even if you are not pulling large DC current there will be huge transient spikes on every cycle.

Use at least 1,000μFd of first capacitor for every 1 Ampere of DC current. Older tube amps often used less, because caps were expensive. These days big caps are quite cheap so be generous. I have used 80,000μFd/Amp when I needed a very simple but low-ripple supply.

When you need "clean" power for low-level stages, you add several LC or RC stages. For RC filters, plan on one RC stage for every 20-30dB of ripple reduction you need. Pick the resistors to drop about 5% to 20% of supply voltage, depending how clean you need, how cheap you are, and how little voltage you can stand. Larger resistors allow smaller caps for the same ripple reduction, but of course drop more voltage. Old hi-fis minimized filter cap costs by starting with 400V for the output stages, then dropping to 200V for the tone stage and 100V for the phono stage, huge filter resistors. But tubes run cleaner with higher voltage, so if you can afford bigger caps you will usually use smaller filter resistors and leave more voltage for the tube.

Filter cap reactance at 100/120Hz should be 1/10th to 1/30th (up to 1/100th) of the resistance, to give 20dB or 30dB (to 40dB) of ripple reduction.

 

[b3groover]

Don't know if this will help you, but I started a thread about this here:

http://www.groupdiy.com/index.php?topic=1106&highlight=

 

[CJ]

One thing to remember that if using a vacuum tube rectifier, say a 5Y3, it will chomp more voltage than a 1N4007, so adjust your math accordingly.
(i woulkd think adding ten percent to the high tension ac coming out of the secondary would get you in the park)

 

[Marik]

[quote author="cjenrick"]One thing to remember that if using a vacuum tube rectifier, say a 5Y3, it will chomp more voltage than a 1N4007, so adjust your math accordingly.
[/quote]

Chris,

Isn't it other way around???

Another thing to remember--after tube rectifier don't put a cap value more than 50Mf.

 

[CJ]

Typical forward drop on a 1n4007 might be six tenths of a volt, where as a tube rectifier, depending on how it is heated, well, lets take the 5y3 since it is so common, my book says it will eat 60 volts at 125 ma.
GZ34, much less due to direct heating.
cj

 

[Marik]

[quote author="cjenrick"]Typical forward drop on a 1n4007 might be six tenths of a volt, where as a tube rectifier, depending on how it is heated, well, lets take the 5y3 since it is so common, my book says it will eat 60 volts at 125 ma.
GZ34, much less due to direct heating.
cj[/quote]

Ha! Of course you are right! :thumb:
I couldn't find "chomp" in English-Russian dictionary and for some reason assumed that it is something like "give". My wife told me that it's like "eat".
Sorry CJ :guinness: :guinness: :guinness:

 

[CJ]

I see!
Note that as the current draw goes down, yhe tube wile "eat" maybe only 30 vac at 60 ma.
cj

 

[Guest]

Different tube rectifiers can handle different capacitor loads. Some only 8uF while others 100uF. Make sure to check the data sheet for the one you use if you have a capacitor input.