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In Reply to: RE: B+ dilemma posted by Chip647 on July 16, 2014 at 05:20:05
I may not have explained it correctly....
My pre amp has a seperate chassis power supply that uses a tube (5r4gy) rectifier and has a LCLC filter network with a 20k/50w bleeder resistor across the 2nd 15u cap. This gives me 400 vdc for the b+ for the pre amp via an umbilical to the pre amp chassis. What I want to do is tap into the b+ on the pre amp chassis and take 250 vdc to the phono pre amp chassis for it's b+.
I was trying to model this on psuII but could not see any way to do it. Not sure how you would set it up.
It wasn't a RC filter that wasn't recommended, but rather dropping b+ voltage with a resistor. It was said that this would adversly affect sound quality from what I could find.
Not quite sure how you would set up what you described on PSUII either.
Since this appears to be a simple solution to the problem I would really like to see a drawing of how to implement it. Do you know where one can be found on the net?
Edits: 07/16/14 07/16/14 07/16/14Follow Ups:
You don't say how much extra current your PS can supply, but you can make a very nice 255V shunt regulator for the phono pre using a CCS, a 0D3 and a 0C3 VR tube. The CCS feeds the series-connected 0D3 and 0C3 tubes from the 400V at the current needed for the stage plus the current you want to shunt, say a total of 20-30mA, the two tubes in series will regulate your voltage to 255V.
I do this for my 26 preamp by dropping a 400V supply down to 225V with a CCS, 0D3 and 0A3. I use one regulator per channel to ensure excellent channel separation, but you could also do it with one regulator for both channels. Plus they look really cool when they are lit up!
Who did the silkscreen/paint on your panels?
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Buy Chinese. Bury freedom.
Front Panel Express. It's engraved and infilled.
Edits: 07/18/14
Are the edges of those panels silver (raw aluminum), rather than black like the faces? They don't quite look like that in the photos, which is why I asked. FPE cuts panels after they're anodized, so edges aren't finished.
--------------------------
Buy Chinese. Bury freedom.
Sounds interesting. Do you have a schematic you would be wllling to share?
My power supply transformer says it has 0.165A available. My preamp uses 2 76's, 2 6sl7's and the phono pre will have 2 6dj8s.
Edits: 07/16/14
0.165A equals 165 milli-amps (mA), not "micro-amps" as you stated up above. I wondered how any kind of tube circuit could require less than 1 mA of current and still work.
Your question provoked a lot of experienced guys to provide a variety of interesting ways to go, which is very educational for the rest of us too. Thanks.
Milli and micro will make me crazy yet!
That's one of the things that is so neat about this site; it can be very educational.
You have to add up the current demands of all parts of the two units, then make sure you have enough current from your PT and that your chokes can handle the higher current as well.
Here's a link to an article by Gary Pimm about CCS-fed VR tubes. I think you'll be able to figure it out from there.
I was hoping for a simple way too just reduce the b+, but it is beginning to look like I may have to go back to the seperate power supply for the phono preamp.
Someone else mentioned this, but the simplest way to use your existing supply is to drop voltage with an appropriately sized and watt-rated R in series, followed by a C in parallel, for a dollop of added isolation between the two stages. I don't see why that wouldn't work, but perhaps I've missed something in the discussion. All you need to know is the current draw of your phono stage, in order to calculate R.
Is there a drawing anywhere of how this would be done? I am not sure I understand how it would go together. I am all for simple; as long as it doesn't make a space heater out of my phono stage. : )
If there is anything wrong with doing it this way, anyone can jump in and let us know.
I had the impression that your B+ for the linestage is already regulated upstream. If so, you could tap voltage for the phono stage off the last node that feeds the linestage circuit, which should be at 400V, if memory serves. Then you need to have at least a ballpark idea of how much current is needed for the phono stage. Let's say it's 50mA, just for the sake of calculation. And let's say you want to drop the voltage to 250V. So, you need a resistor to go in series between the 400V node in the linestage PS to a node that feeds the phono stage. This resistor must drop 150V. Here is where Ohm's Law comes in handy. Voltage (V) = current (I, in Amperes) times resistance (R, in ohms). Rearranging the equation to solve for R, we have R = V/I or R = 150/.05 = 3000 ohms.
You also need to take into account the power, P (in Watts)= I*V = .05A*150V = 7.5W. Use at least a 10W resistor; I have used 12W Mills resistors in a similar situation. They get hot; don't worry.
Then you could place an additional filter cap in parallel with the resistor, on the phono stage side. This will afford some additional isolation of the phono stage PS from the linestage PS. 50uF or 100uF would be acceptable values, for example. More or less capacitance could also be used with various consequences.
A possible negative consequence of doing it this way is that you are now pulling that extra 50mA from the same source that you want to supply 400V to your linestage. This might drop the voltage for the linestage below 400V. What will happen depends upon the relative current demands of the two stages. But you probably could live with a slight drop in the linestage V.
I just checked and the power supply on the line stage is unregulated. Where does that leave us?
except that leaves you the possibility of inserting a constant current source/voltage regulator in between the linestage 400V and the phono stage 250V. The 150V you need to drop can best be used to allow the needed headroom for the VR. See above responses by others; there were several suggestions for how to implement a VR at the linestage/phono stage interface. Putting a CCS before the VR will also effect isolation between the two, an added bonus.
I am beginning to think that getting rid of the hum on the separate power supply for the phono might be the easiest way to go after all. Going to have to mull all this over. Thanks!
Edits: 07/17/14
Thanks for the post. I will digest all this and give it a try. Right now I can still go either way, it's just swapping a feww things around.
Not sure if my ps is regulated on the line stage, I will have to check.
Edits: 07/17/14
K and K audio sells a nice high voltage shunt regulator kit that has fully adjustable output voltage. They would work great in you application, as far as I know. They come with full documentation on how to set up for your application.
I have used them with success. I have no affiliation with them.
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