Elevated 6.3V AC supply with 6.3V DC regulated supply?

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FYL
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Re: Elevated 6.3V AC supply with 6.3V DC regulated supply?

Post by FYL »

FYL, The drawing you posted is one that I drew. Just so you know. :)
Thank you.
:)
paulster
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Re: Elevated 6.3V AC supply with 6.3V DC regulated supply?

Post by paulster »

FYL wrote:Have you tried fixed 5V LDO regs with a 1.3V zener between ground and the GND terminal? Should give 6.3V with a minimum part count.
I haven't but it should work quite nicely. The key requirements are the dropout voltage on the regulator and the Vf on the diodes, as you know, as these are the limiting factors for the amount of voltage headroom you'll end up with.

I designed my board based around an adjustable regulator so I could use it for a multitude of different purposes. I can get 6.3Vdc heaters off a 6.3Vac supply for retro-fit purposes, or I can use it for 12.6Vdc heaters off a custom winding. That said, I've found it works so well off a 6.3Vac supply that I don't bother specifying a higher voltage filament tap these days.
jgrg1
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Re: Elevated 6.3V AC supply with 6.3V DC regulated supply?

Post by jgrg1 »

Interesting stuff! Thanks for all your help. Was thinking about implementing a slow-start circuit into it as well.
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jjman
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Re: Elevated 6.3V AC supply with 6.3V DC regulated supply?

Post by jjman »

What is the expected DC voltage from the preamp section of Jana's drawing?
If it says "Vintage" on it, -it isn't.
Jana
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Re: Elevated 6.3V AC supply with 6.3V DC regulated supply?

Post by Jana »

If memory serves me right (a rarity), it is about 6.5. When I get time I can pull an amp and measure if need be.
jgrg1
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Re: Elevated 6.3V AC supply with 6.3V DC regulated supply?

Post by jgrg1 »

So with the raised 6.3V DC levels in Jana's diagram, would it be possible to feed these into a LDO voltage regulator? So instead of tying the regulator to ground, it's tied to the -ve line on the right of the diagram where it says "DC Heaters"?

Also, so why would you not tie the elevated DC to the centre tap on the 6.3V windings instead of going through pair of 100R resistors?
paulster
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Re: Elevated 6.3V AC supply with 6.3V DC regulated supply?

Post by paulster »

jgrg1 wrote:So with the raised 6.3V DC levels in Jana's diagram, would it be possible to feed these into a LDO voltage regulator? So instead of tying the regulator to ground, it's tied to the -ve line on the right of the diagram where it says "DC Heaters"?
Yes, absolutely.

The debate is whether DC elevation will serve any purpose whatsoever though for the power amp, when you've already regulated your preamp tubes.

In my experience, in a push-pull amp, I can't get any hint of hum through the power section running it on AC when I've fully quieted my preamps through regulation.

Going off at a tangent there is also a school of thought that the tone and feel of the amp can change by using DC elevation. I can't say for certain whether this is genuinely the case or not since it's not a technique I use, but I'd certainly do some experimentation in this field if I was going to consider using it.
jgrg1 wrote:Also, so why would you not tie the elevated DC to the centre tap on the 6.3V windings instead of going through pair of 100R resistors?
Presumably this is a design that could be deployed on a transformer with no centre tap. Since this design ties the virtual AC and DC grounds together it is important that they are very closely balanced, so using 100R resistors may result in a closer match than the centre tap of the secondary. As I said before, it is only necessary to reference one side to ground, so you could make this the AC side only using the centre tap, or the DC side only using the 100R resistors, or just taking the negative output of the bridge rectifier.
jgrg1
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Re: Elevated 6.3V AC supply with 6.3V DC regulated supply?

Post by jgrg1 »

Apologies to resurrect this, I've just started looking at it all again. With the SPX29302/MIC29302 LDO regulators, what values of R1/R2 did you use. The equation specified in the datasheets is:

Vout = 1.24 * (1 + R1 / R2)

The example in the datasheet uses R1 = 158k. Is this a sensible value? There is a statement in the datasheet that the total series resistance should be small enough to pass the minimum regulator load current (which is 7mA for the 2930x series).

I'd be powering 3 or 4 ECC83's so total current draw will be 1.2A.

http://docs-europe.electrocomponents.co ... adbbbe.pdf
paulster
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Re: Elevated 6.3V AC supply with 6.3V DC regulated supply?

Post by paulster »

I used 39K and 1K8 to get 40K8, and 10K.
diagrammatiks
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Re: Elevated 6.3V AC supply with 6.3V DC regulated supply?

Post by diagrammatiks »

how do you get enough voltage off of a 6.3vac centertapped transformer

isn't that only putting out 4.41vdc after rectification?
paulster
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Re: Elevated 6.3V AC supply with 6.3V DC regulated supply?

Post by paulster »

diagrammatiks wrote:how do you get enough voltage off of a 6.3vac centertapped transformer

isn't that only putting out 4.41vdc after rectification?
Use a bridge rectifier across the entire 6.3V secondary (ignore the centre tap) and then you have, in theory 8.9Vdc. Use diodes with very low forward voltages, a large capacitance bank to minimise ripple and a low-dropout regulator and you can comfortably get regulated 6.3Vdc.
tictac
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Re: Elevated 6.3V AC supply with 6.3V DC regulated supply?

Post by tictac »

About the schematic....

If you're floating the center-tap on a DC reference voltage you don't need the bridge rectifier, the second pair or 100 ohm resistors and all that.
Just wiring the tubes 6.3VAC. The DC reference will provide the quieting you're looking for. 50 to 75VDC I believe is the range to go for...

TT
jgrg1
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Re: Elevated 6.3V AC supply with 6.3V DC regulated supply?

Post by jgrg1 »

After all that, having checked the power transformer, it looks like I won't have enough juice to power the 2xEL34s + 4xECC83 (and relays) anyway, so I'm going to install a second power transformer to get 12.6v DC for the preamp tubes.

I planned to DC elevate the 6.3v heaters anyway, but just out of curiousity, could I power this from a completely dedicated rectified DC source instead of from the tail end of the HT line? What would be the current draw on that DC source - assuming that the DC was fed into two 100R resistors connected across the 6.3V lines?
diagrammatiks
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Re: Elevated 6.3V AC supply with 6.3V DC regulated supply?

Post by diagrammatiks »

ya you could of course.

but then you'd need another winding somewhere just for that elevation voltage.
husky
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Re: Elevated 6.3V AC supply with 6.3V DC regulated supply?

Post by husky »

You could use a switching supply for 12.6V DC and forget elevating it. They are $26 and you are done.
I have really never heard a hum advantage to speak of elevating the tap. But....it is easier to keep the heater to cathode voltage in check depending on if it is an amp with a cathode follower. Sometimes I will do a hum balance and elevate but again more for the heater to cathode limitation on cathode follower driven tone stacks.
Switchers can even run the power tubes if you have two and run them in series.
They work fine and wont draw on your transformer. I use one that is 5.4A and adjustable. 100~250 input VOF-65-12, fits fine on the internal side wall of most chassis.
http://parts.digikey.com/1/parts/727069 ... 65-12.html
Last edited by husky on Wed Oct 05, 2011 5:50 pm, edited 8 times in total.
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