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PostPosted: 29 Aug 2008, 07:52 
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I am not any kind of expert on this so take my words with a grain of salt and consider them incomplete. I think the caps would,yes, be before the load and they will filter as well as store energy. You may need to add some bleeder resistors somewhere, or maybe a resistor/LED combo so that you can drain the caps while they are not in use.
Ri

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PostPosted: 29 Aug 2008, 08:19 
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Ok, I ran it again with 10,000uf on C1 and still can get 246v but the line is real fat now whereas before it was pretty and thin.

So two questions arrise:

1) Is 470uf enough for this power supply?
2) Is the increased capacitance worth the fat waveform?


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PostPosted: 29 Aug 2008, 09:16 
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I think I will be ok with just 470u, my VTL only has 440u in its B+ supply.

Man, this stuff is nerve racking!

So my drawing will be just like posted but without the monster caps.


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PostPosted: 29 Aug 2008, 09:56 
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Your layout looks good.

Current draw for a preamp should be rather small, so I doubt you will need all that capacitance.


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PostPosted: 29 Aug 2008, 12:00 
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Hi Bill,

On your layout, I would swap the transformers and choke around.

Email me the PSUD II file and I will take a look at it.

Cheers

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PostPosted: 29 Aug 2008, 21:45 
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Hi, It may be a bit late, but I have some suggestions on the PS. Essentially what you want is clean 250 volts and 12 volts. The 250 is really fairly easy, especially if you stick to solid state. I have no beef with PUSD, but it doesn't always give you the answer you seek. I have an Aikido PCB on the way for my next project and here is what I will use for the PS. The power transformer will be 180-0-180 similar to the one in your drawing. However I will be using a 100uf (350 volt rating for all caps) right after the rectifiers then the choke followed by a second 100uf. After that there will be a resistor (value depends on how much current you will be drawing) which in my case will be in the 470-1000 ohm range followed by another 100uf. In parallel with each 100uf there will be a .1uf audio grade cap. These will pass high frequencies that the electrolytics will have problems with. The Aikido operates at a steady current, so a "monster reserve" is not needed. the three stage filter will provide much cleaner B+ than the one stage you drew. The 1N4007 rectifiers are OK, but I find that "faster" ones work better. I have had good results with 1N4948 rectifiers (MCM and Parts Express plus others carry them at cheap prices). If you already have the others then try them. the multiple filter sections will usually clean up the noise they generate. I would also put a .01uf capacitor across the transformer (180 to 180) windings to clean up a bit of noise from the rectifiers.

For 12 volts for the heaters, I would use a 12.6 volt winding that can handle at least twice the needed current. I would use a full wave bridge rectifier in the 20 amp or greater current rating with a minimum of 50 volt rating. This would feed a large filter capacitor (I like at least 10,000uf at 25 volts or more) and then go to your regulator. After the regulator I would put a second cap in the 1000uf range. The voltage on the first cap will be approximately 1.4 times the 12.6 (minus about 0.7 volts for the rectifier). Approximately 16.5-17 volts. The minimum drop in the regulator is about 4 volts so it will bring it down to 12 for the tubes. The regulator will need to be on a pretty good sized heat sink. The use of the 12.6 transformer winding (or separate transformer) is on the close side to make this deliver 12 VDC, but all should be fine. An ideal transformer would be about 13.5 volts, but that is an unusual value and would have to be custom wound.

Now that all this is said. A simple and possibly equally good solution is to use a SMPS (switching mode power supply) such as the one from Lighthouse Electronics on the web. They have one for preamps that delivers about 40 ma at 250 volts and 12 DC. I would check the dc heater current rating though as it might not be enough for the tubes you chose.

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PostPosted: 29 Aug 2008, 21:53 
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I have a 15v 3a toroid for the heaters.

Could you possibly send me a sketch? Will the 180-0-180 get the required 250v?


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PostPosted: 29 Aug 2008, 22:15 
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Hi, The 180-0-180 will deliver 250 with that load. This is somewhat dependent on the actual transformer. Some companies skimp a bit and their windings have a lot of resistance. I prefer Edcor ones but others are fine as well. If you want a cushion go for a 200-0-200 and then increase the value of the resistor in the filter to bring the DC down to 250. This would give you about 28 extra volts. If you go much above the 200 range for the transformer then you find that the resistor will have to dissipate a lot of heat to bring it down to the 250 you want (the heat is equal to current squared times the resistance). My preference is to not generate too much excess voltage and then have to get rid of it. I try to scribble up a 12 v ps for you. The 15 volt trannie ought to be OK. It will deliver about 20 volts from a full wave bridge. You will have to dissipate about 8 volts in the regulator at 2.2 amps that is nearly 18 watts. That will get quite hot and will need to be on the outside of the chassis. I don't have the specs here on the 78T12 tp know if it will handle that load. It might make more sense to use a large NPN power transistor (like a 2N3055) and an adjustable regulator (like a LM317) to get what you need.

Good listening
Bruce

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PostPosted: 29 Aug 2008, 22:33 
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Hi, I said it would be crude.


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PostPosted: 29 Aug 2008, 22:46 
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How's this?


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