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Old 2nd July 2005, 06:40 AM   #1
Hamish is offline Hamish  Australia
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Default Help with linestage power supply

Hi,
i've been playing about with some old tubes i had lying around during the holidays (some old 6aq5's. Thanks to the guys here that helped me get a basic circuit ) and i've decided to go and do something decent. The sound of tubes was impressive, however that particular tube sucked. Alot. Very microphonic and i could hear what sounded like a spring reverb.....

Anyhoo.... i bought a transformer about a year ago and i was hoping i could get some help in using it for some circuits i've decided to build (don't try changing my mind on tubes, as they are bought already). Problem is i need to burn off alot of voltage, as it's a 350-0-350, and the line stage needs 250, and the phono 305 (i want to run both channels and stages off the one power supply. Don't get me wrong, i love dual mono, but the last one sent me poor). Any help would be much appreciated.

Here are the line stage i want to use:
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Old 2nd July 2005, 06:41 AM   #2
Hamish is offline Hamish  Australia
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And the phono stage:
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Old 2nd July 2005, 06:42 AM   #3
Hamish is offline Hamish  Australia
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sorry, here:
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Old 2nd July 2005, 06:57 AM   #4
PRR is offline PRR  United States
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The idea is "Electronic Design", not "Electronic pot-luck".

> i bought a transformer .... Problem is i need to burn off alot of voltage, as it's a 350-0-350

350VAC * 1.414 = 500V DC. You can't easily buy 500V power capacitors. So already you are in trouble. Not to mention that a "350V" winding may be 385 or even 400V at light load. You can either buy special 500V caps, stack 300V caps, or add a Choke (and a bleeder to keep the voltage from soaring before the 6SN7 warms up).

You probably wanted a 200-250VAC winding, not a 350VAC winding.

Adapting that plan to work with 300, 400, even 500VDC is no big deal. Above 300V I would change the 47K resistor to 100K, maybe change the 820 to 1K. All the caps should be rated for the supply voltage, although actually you can probably stick with the 250V output cap because there is no way that cathode is going to get above 100 or 150V. (The 0.033uFd must be rated full no-load supply voltage: it will get that much at every switch-on.) But as long as you have excess voltage you should put some of it in the power supply filtering. 5K in the power line will drop 100V.
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Old 2nd July 2005, 09:57 AM   #5
Hamish is offline Hamish  Australia
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Thanks. I sort of started to work it out. I was having problems with psu designer, but lowering cap values sorted it out. I'm still having a bit of trouble working out the total current drawn on each of the stages, so if anyone can help, i'd appreciate it. It looks like about 5mA for the phono stage (both channels), but i'm not sure about the line stage. Around 15 for both?
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Old 2nd July 2005, 10:45 AM   #6
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Default Coupling cap

The coupling cap from the sch. of the first post should be 0.33uF not 0.033uF!

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Yugovitz
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Old 2nd July 2005, 01:56 PM   #7
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Time out, please.

Dropping Volts in a resistor wastes energy as heat. IMO, there is an ELEGANT way to deal with the problem. Use choke I/P filtration. Neglecting losses, the 350 V. RMS from the power trafo comes out as 315 VDC. That's much, much, better!

Choke I/P filtration has additional advantages too. The O/P voltage is well regulated. As long as an appropriate bleeder resistor (10 KOhms for a 10 H. choke) is placed across the 1st filter cap., 450 WVDC filter caps. will not be over stressed. The available DC current is slightly greater than the AC RMS current; that's quite different from the 0.5 RMS available with cap. I/P filtration.
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Old 2nd July 2005, 02:24 PM   #8
SY is offline SY  United States
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Tanstaafl, Eli. A choke input is a very good option with all the advantages you point out, but it has downsides, too. One biggie is the minimum current draw requirement- when you go below that, it stops acting as a choke-input filter and the voltage rises. You can:

hang a resistor across the output sized to guarantee minimum current draw (but there's the heat and inefficiency demon biting again), OR

depend on the timing of rectifiers and signal circuitry (good luck), OR

rate all components to withstand a high voltage (oops, there goes that advantage!).

There's no panacea for starting with bad engineering criteria, and trying to match a random transformer to a random circuit is a bad engineering criterion. PRR said it a bit more kindly, I think
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Old 2nd July 2005, 02:50 PM   #9
jlsem is offline jlsem  United States
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Quote:
The idea is "Electronic Design", not "Electronic pot-luck".
Quote:
There's no panacea for starting with bad engineering criteria, and trying to match a random transformer to a random circuit is a bad engineering criterion. PRR said it a bit more kindly, I think
Well, the 700VCT transformer, as it turns out, is the PERFECT transformer for your needs. Do what Eli suggested using a 10-15 henry choke and a 6X5GT rectifier and you are good to go.

John
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Old 2nd July 2005, 05:31 PM   #10
SY is offline SY  United States
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Quote:
Originally posted by jlsem

Well, the 700VCT transformer, as it turns out, is the PERFECT transformer for your needs. Do what Eli suggested using a 10-15 henry choke and a 6X5GT rectifier and you are good to go.

John, I figure about 25mA as a minimum current draw for a 15H choke. Is that correct?
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