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Old 20th April 2003, 09:16 PM   #1
mig-ru is offline mig-ru  Russian Federation
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Default DC filaments setup

playing around with PSUDII the other night I decided to try setup a 6.3V PS for filaments. playing around and trying different setups including many from headphone amps on www.headwize.com, I couldn't get 6.3V or even close on the output, i tried different filter setups including just a capacitor. also the AC ripple seems somewhat enormous. I really doubt that if most of these designs are actually used, that they provide these low voltages. So my question, is PSUDII not able to function accurately at low voltages? Or perhaps I am doing something wrong?
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Old 20th April 2003, 09:30 PM   #2
dhaen is offline dhaen  Europe
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Dmitri,

It works well enough for low voltages. What may be wrong is that the default values for series R of the transformer chokes and caps are unrealistic.
Try 0.5 ohm for the transformer, and 0.1 ohm for the caps as starters.

Good luck,
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Old 20th April 2003, 11:49 PM   #3
EC8010 is offline EC8010  United Kingdom
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I find PSUD tends to be a bit pessimistic about output voltage (and yes, I use as accurate data as I can), it's not much of an error, 5% at worst. However, why worry, why not design for 9V, and add a regulator such as a 317 to give precisely 6.3V, even when the mains moves up and down?
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Old 21st April 2003, 12:12 AM   #4
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Hi,

While I agree that the LM317 is a fine regulator...sometimes a good regulator with limited bandwidth can be used to advantage:

A prime example is the 78/79xx series of regs, their limited bandwidth has a positive effect in that it keeps mainsborne/rectifier crud away from the tubes/valves.

While it doesn't allow for a spot on 6.3VDC/12.6 voltage (well not without the use of a zener noise source anyway) but gives you a 6/12VDC output this will not affect performance in any way...

Unless EC8010 is onto something I'm not aware of yet?

Cheers,

P.S. I hear people getting fine results by regulating both filament voltage AND current...the why for this is still beyond me but like everyone else I'm learning.
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Old 21st April 2003, 12:28 AM   #5
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I have read various things on AC versus DC on heaters... does anyone know the real deal that can explain it? I would think that using a perfectly stable DC voltage appropriate to the tube would be best, but I keep reading that AC is better for the tubes or some such thing (and it sounds better). Also, if it is so much better to use a different transformer for the heaters from the B+ transformer, why is it not used more often? Just cost? Is there a benefit from running different heaters from different trannies? I guess what I am saying is why not use 1 tranny for B+, and a seperate one for all the heaters using solid state rectification and regulation?
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Old 21st April 2003, 12:30 AM   #6
EC8010 is offline EC8010  United Kingdom
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Default Differential vs common-mode

I've always found 78/79xx series to be noisier than 317. If you look at the manufacturer's data sheets, they show poorer power supply rejection as a consequence of reduced (internal) op-amp gain-bandwidth product. Whether this translates into any real difference, I don't know, after all, we're talking about noise at the 20mV level as viewed by a something MHz oscilloscope on a heater that would otherwise have had 6.3VAC on it.

I think that when it comes to noise, regulation is a bit of a red herring. Regulators reduce differential noise, to which the valve is not particularly sensitive. However, the valve is sensitive to common-mode noise.
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Old 21st April 2003, 12:41 AM   #7
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Hi,

Quote:
Also, if it is so much better to use a different transformer for the heaters from the B+ transformer, why is it not used more often?
Cost it is.

Quote:
Is there a benefit from running different heaters from different trannies?
Yes, especially when current demands are varying wildly.Capacitive coupling once again but we're splitting hairs here.

Quote:
I guess what I am saying is why not use 1 tranny for B+, and a seperate one for all the heaters using solid state rectification and regulation?
SS regs for the heaters, you don't have much of a choice here anyway.
Tube regs for the B+ and make any stage as independent as possible from the other unless you don't mind IMD.

Not to mention that you need independent PSUs for each channel too of course.

Boy, do I love PSUs...
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Old 21st April 2003, 12:45 AM   #8
EC8010 is offline EC8010  United Kingdom
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Mach Y,

your question comes directly back to the issue of common-mode and differential noise. Regulators imply rectification, which generates noise. Thus, if you were unlucky, and used a poor transformer with noisy diodes and poor layout, you could generate more common-mode noise than if you had simply used AC.

In theory, DC shortens the life of directly-heated valves because one end of the filament/cathode emits more electrons than the other. AC heating balances this out. However, if you wire the filament one way round on one amplifier, and the other way round on the other, you can simply periodically swap the valves over and rebias. For a push-pull amplifier, it's even easier.
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Old 21st April 2003, 12:51 AM   #9
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Talking ON PAPER...

Hi,

Quote:
I've always found 78/79xx series to be noisier than 317.
Yes, they are but not significantly so to abolish them for heater use.

Tip of the day: balanced heaters using a set of 7806/7906s on a 12.6 centertap?
This can be used on any heater as long as a balance is obtained for maximum CMR...useful for DAC stages.

SET folks are you listening?

Back to you EC,
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Old 21st April 2003, 01:04 AM   #10
EC8010 is offline EC8010  United Kingdom
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Default Re: ON PAPER...

Quote:
Originally posted by fdegrove
Tip of the day: balanced heaters using a set of 7806/7906s on a 12.6 centertap?
Interesting idea. But negative regulators are almost always noisier than positive ones (I can't remember the details, but it's to do with the fact that it's easier to make NPN transistors than PNP, so the differential pair at the input of the op-amp is implemented with NPN, and their common-mode performance suffers).

However. One possibility is to use a 317 for the rail to rail regulation, and add a rail-splitter to couple each rail to ground. This is all becoming a bit theoretical because it's hard to reliably measure these sorts of problems. I suppose one could measure it by fitting a 100R, 100R 0.1% potential divider across the rails, and then measure the noise from the centre tap to ground.

Back to you, Frank
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