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Elevated 6.3V AC supply with 6.3V DC regulated supply?
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jgrg1



Joined: 28 Apr 2010
Posts: 10

PostPosted: Wed Apr 28, 2010 10:53 pm    Post subject: Elevated 6.3V AC supply with 6.3V DC regulated supply? Reply with quote

Hi,

I've got a question for people more knowledgeable than I regarding tube heater power supplies. I'm looking to power the preamp valves with 6.3V DC, and the power valves with DC elevated 6.3V AC. Is this possible from a single 6.3VAC centre-tapped transformer winding - as shown in the attached image?

So the 6.3VAC centre tap is connected to a 50V DC source, and the DC side of the bridge rectifier tied to ground. Or is this monumentally stupid?

Regards
Justin
[/img]
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paulster



Joined: 10 Oct 2006
Posts: 1299
Location: Los Angeles & London

PostPosted: Wed Apr 28, 2010 11:14 pm    Post subject: Reply with quote

If you're using AC and DC off the same winding you can only have one ground reference (that's not 100% true but I'll come back to that).

You can either reference the centre-tap to ground, or the DC ground, but not both.

The reason is that the centre-tap will sit at 3.15Vdc relative to the 0Vdc side, so if you have the centre-tap tied to ground, your DC voltages will be plus and minus 3.15V. If you try to tie the DC side to ground as well you'll see your bridge rectifier crack and get that lovely burnt silicon smell very quickly as it is shorted out.

You could reference your centre-tap to 50Vdc, and not connect the DC 0V side to anything at all. This will DC elevate your AC side, and also DC elevate your DC side with respect to the chassis, but there will still be 6.3Vdc between the DC rails so the tubes won't care.

In my experience, however, of using combined AC and DC supplies, I've always found that a) the power tubes aren't sensitive and b) that the preamps are quietest when the ground reference is on the DC side.

To that end you might find is quietest to tie your 50Vdc point to the negative side of the bridge rectifier and cap off the AC centre tap.

That's how I'd do it.
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jjman



Joined: 06 Oct 2007
Posts: 658
Location: Central NJ USA

PostPosted: Wed Apr 28, 2010 11:28 pm    Post subject: Reply with quote

With PP output there's really no need for elevating the heaters on the output tubes. Whatever heater hum may be there cancels since PP is humbucking. Just wire the heaters at the same polarity on the pins. Even when my SFDR was wired wrong on the 6v6 heaters, there was no output hum.

DC on just the preamp tubes makes since because they draw much less heater current. I think I've read that 6.3VDC is difficult to get from a 6.3vac winding.

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paulster



Joined: 10 Oct 2006
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Location: Los Angeles & London

PostPosted: Wed Apr 28, 2010 11:44 pm    Post subject: Reply with quote

jjman wrote:
With PP output there's really no need for elevating the heaters on the output tubes. Whatever heater hum may be there cancels since PP is humbucking. Just wire the heaters at the same polarity on the pins.

I agree wholeheartedly with this. If you've got 4 power tubes you can swap the polarity between inner and outer pairs too, e.g. - + + - so that you aren't relying on the balance of the OT primaries for common-mode rejection.
jjman wrote:
DC on just the preamp tubes makes since because they draw much less heater current. I think I've read that 6.3VDC is difficult to get from a 6.3vac winding.

Basic 6.3Vdc is easy to get off a 6.3Vac winding but can end up being noisier than the AC would have been because the level of ripple can be high and it is also pretty much a sawtooth wave which is really nasty sounding.

Properly regulated 6.3Vdc is quite difficult to get off 6.3Vac but can be done with very careful component selection. This is what I do in all of my builds.

You need Schottky diodes with a very low forward voltage, a regulator with a very low dropout voltage and plenty of filtering to get as high a ripple-free DC voltage before the regulator as possible. Do that and you can have regulated 6.3Vdc off a 6.3Vac supply, even with a low wall voltage.
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jgrg1



Joined: 28 Apr 2010
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PostPosted: Thu Apr 29, 2010 9:18 am    Post subject: Reply with quote

Ah, I thought I was being stupid. Still, you have to ask. So the only difference really is that I ditch the centre-tap on the transformer winding.

Awesome stuff, thanks all. I could just as easily power the power amp valves with 6.3V DC as well?
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paulster



Joined: 10 Oct 2006
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PostPosted: Thu Apr 29, 2010 10:19 am    Post subject: Reply with quote

You'll have a hard job powering the power tubes off 6.3Vdc.

Depending what power tubes you're using you can say that you'll be drawing 3A for a pair of EL34s, 6A for a quad of EL34s, 1.8A for a pair of 6L6es or 3A for a quad of EL84s.

It's not easy to find a regulator that will supply that current plus the typical 0.9A for the preamp, that will also have a low enough dropout voltage to be usable. Not to mention one that won't get upset by the cold filaments effectively presenting it with a dead short at startup.

You'll also be drawing significantly more current, which means that you'll have much more DC ripple reducing the voltage headroom at the input to your regulator. You'll need significantly more filtering to ensure that the regulator input voltage remains above 6.3V plus the dropout voltage at all times, even with a slightly low wall voltage. Otherwise your regulator will drop out of regulation and you'll suddenly have a load of noise.

It can be done but in my experience there's little to be gained in a push-pull output stage because a) the gain of the power tubes is comparatively low, so any hum is not going to be amplified much, and b) because you can use the common-mode rejection of the OT to effectively cancel it out.
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FYL



Joined: 17 Feb 2006
Posts: 654

PostPosted: Thu Apr 29, 2010 11:46 am    Post subject: Re: Elevated 6.3V AC supply with 6.3V DC regulated supply? Reply with quote

Got this a while ago. Dunno who the author is so I can't credit him/her.
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jgrg1



Joined: 28 Apr 2010
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PostPosted: Thu Apr 29, 2010 12:32 pm    Post subject: Reply with quote

Hi, do you know if it works?
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jgrg1



Joined: 28 Apr 2010
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PostPosted: Thu Apr 29, 2010 12:35 pm    Post subject: Reply with quote

Hi Paulster, do you have a list of components that you used and a schematic?
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FYL



Joined: 17 Feb 2006
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PostPosted: Thu Apr 29, 2010 12:53 pm    Post subject: Reply with quote

Quote:
Hi, do you know if it works?


It should. Will it work well? Maybe - filtering seems low, even for a 900 mA load.
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paulster



Joined: 10 Oct 2006
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PostPosted: Thu Apr 29, 2010 1:13 pm    Post subject: Reply with quote

jgrg1 wrote:
Hi Paulster, do you have a list of components that you used and a schematic?

I use MS503 Schottky diodes and a Sipex SPX29302AU5-L regulator in my boards (see below).

I can describe it in more detail but since it's a 5-pin regulator in a TO-220 package you'll need to be able to fabricate a PCB in order to be able to use it.

FYL wrote:
Quote:
Hi, do you know if it works?

It should. Will it work well? Maybe - filtering seems low, even for a 900 mA load.

I agree that the filtering is low, in fact way low I'd say. A ballpark figure is 10,000uF per Amp drawn for 10% ripple, so the ripple on this in a 3-tube preamp will be significant. Sure, it's then elevated, but it's a 120Hz sawtooth wave that could easily be noisier than the AC it was replacing.

Functionally it's a sound design that should work but I'd use a lot more filtering if I was going to implement this, and you don't need to reference both the AC and the DC sides. The imbalance between the components could make it noisier than only referencing one side.
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FYL



Joined: 17 Feb 2006
Posts: 654

PostPosted: Thu Apr 29, 2010 1:26 pm    Post subject: Reply with quote

Quote:
I use MS503 Schottky diodes


Time to switch: the MS503 is obsolete and out of production. No problemo as there are many alternatives.

Quote:
I can describe it in more detail but since it's a 5-pin regulator in a TO-220 package


http://www.exar.com/Files/Documents/sipex/datasheets/spx29300.pdf
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paulster



Joined: 10 Oct 2006
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PostPosted: Thu Apr 29, 2010 1:43 pm    Post subject: Reply with quote

FYL wrote:
Time to switch: the MS503 is obsolete and out of production. No problemo as there are many alternatives.

True, and I've tested the SR503 as a replacement part. The specs aren't quite as good for Vf as a MS503 though (0.55V vs. 0.49V @ I=5A), so that still remains my preferred part.

If we're going to split hairs then it's worth mentioning that there isn't a Sipex SPX29302AU5-L any longer and that the Exar MIC29302WT is the replacement part.
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Jana



Joined: 07 Sep 2008
Posts: 1183
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PostPosted: Thu Apr 29, 2010 2:10 pm    Post subject: Reply with quote

FYL, The drawing you posted is one that I drew. Just so you know. Smile
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FYL



Joined: 17 Feb 2006
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PostPosted: Thu Apr 29, 2010 2:16 pm    Post subject: Reply with quote

Quote:

True, and I've tested the SR503 as a replacement part. The specs aren't quite as good for Vf as a MS503 though (0.55V vs. 0.49V @ I=5A), so that still remains my preferred part.


I use SB540's - 0.55V @ 5A and 0.48V @ 3A. Works nicely on an unused 5V winding, giving close to 7V under a 1A load, just add a series diode and you've got 6.3V...

Quote:

If we're going to split hairs then it's worth mentioning that there isn't a Sipex SPX29302AU5-L any longer and that the Exar MIC29302WT is the replacement part.


Very Happy

Same part, different reference AFAICT, Exar even points to the old Sipex datasheet...

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.
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