Big SS power transformer?

[Blair]

I have a friend that does audio repair. Every couple months I go over and have a few beers and I usually dig through his "discards" for either non repairable, or not worth repairing pieces. Yesterday, I came home with a Proceed preamp I plan to use for a preamp of my own, and a Crown DC300A.

I have no plans at all to repair the Crown though it is doable. It has a 1KVA power transformer that produced +/- 60V rails. So, I was thinking that a quadrupled would give me 480VDC at over 2.5A.

Is this a practical use at all, or will the quadrupled just give me noise, etc. that is not worth dealing with at all?

Thanks!

Blair

[Geek]

You can get away with voltage multiplying, but they are lossy and need HUGE capacitances (compared to FWCT or bridge) to get that kind of current with any regulation, plus strain the transformer with high peak currents. So don't count on 2.5A, but derate significantly.

Give it a bench test and see what you can get away with

[EWBrown]

I am currently testing out a full wave quadrupler power supply, using a 65VCT 2A power trannie extracted from a dead "dumpster carcass" Sony receiver.
The quadrupler uses four 1N5408 rectifiers, and four of the 220 uF / 250V electrolytics, as were used in the early A and B versions of the ST35, and Ike boards.

I added more filtering, with a C354 choke and a 470 uF 400V cap, and the B+ ripple is about half a volt, under the 200 mA loading.
in an actual amplifier design, the current consumption would be less, probably 100-150mA, maximum.

Using a 40W 230VDC light bulb as a load, I get 295VDC at 200 mA into the bulb (yeah, it runs very bright, acts like it thinks it's a 60 watter).
At the input side of the choke, the DC voltage is approx 305VDC, which is correct considering the choke's DC resistance (53 ohms)and appx 200 mA current,
for a 10VDC drop, thus the 295 VDC output, under load conditions... No load DC output is appx 350VDC (just a 270K 2W bleeder resistor load)

Right now, it's just a "breadboard" prototype , until I can determine that it will hold up to long-term operation without premature cap failures.
So far, I have found no problems withthe two "input" caps heating up much above room ambient temperature.

The full wave quadrupler was adapted / modified from the circuitry shown inthe 1956 - 1960 ARRL Amateur Radio Handbooks.

In its simplest form, it looks like this:

With the four electrolytics, the positive side is the top, I used 220 uF / 250V caps, and 1N5408 rectifiers.



This, in turn feeds into a C354 / Hammond 156R choke, and a 470 uF / 400V cap, with a 270K 2W bleeder resistior.

For filament power, I'll use a Rat Shark 12.6VAC, CT, 3A transformer that I have on hand.

It's a work, still in progress

Update 1-20-2013: I mounted it all onto a bamboo 6.25 X 13.5 inch "breadboard", found at the local "dollar store". Same as I used for the "Uglytron" 6SL7 / 6L6GC
"super-champ" guitar amp experiment. No more "flying" leads, clip lead jumpers, and impending shock hazards

If all of this works out, then I have a really huge 72VAC CT & 12VAC CT power trannie which I found for a few bucks at a hamfest about ten years ago,
it's about 12 pounds of heavy iron and copper, at least. I have some hefty, good quality electrolytics which could work with it, as well as some beefy
10 Amp "stud" type rectifiers, for the beast-zilla version of my prototype quadrupler PSU.

With your big 1 KVA Crown power trannie, the quadrupled DC output voltage, under load would achieve up to 480VDC, but the current will be significantly
less than 2.5 amps, because of the losses and conversions of the voltage multipliers. I'd SWAG no more than one amp peak, perhaps 500-600 mA continuous duty,
which is still pretty darn impressive. And somewhat dangerous, be careful...

No load output voltage could exceed 670VDC (120VAC X 1.414 X 4), so plan accordingly with the electrolytic capacitors' DC voltage ratings.

/ed B

[EWBrown]

I also built up a smaller version of this quadrupler, using four 47 uF / 250V caps, and four UF4007 rectifiers, with a 25.2V, 2A filament trannie,
I get 120VDC out, at about 50 mA. With no loading, the AC input is 29.4VAC, and the DC voltage is approx 158VDC.



With a 48 to 60V 0.5A trannie, this would be just about perfect for the B+ supply a linestage or perhaps even a stereo 12AU7 / 6S4A SE amp,
or use a small 12VAC trannie, and use it for the basis of a negative voltage grid bias supply .

What I really need to do with this circuit, is to connect a 10 ohm resistor in one leg of the AC secondary feed, so I can measure the actual current going into the
quadrupler, as compared to the DC output current loading.

/ed B

[Geek]

Thanks for the footwork, Ed!

[Tom Bavis]

Use a true RMS meter for current measurement, as it's FAR from a sine wave. And RMS current is the important number for transformer and capacitor ratings. The two input caps have to carry this current at 60 Hz, so may have to multiple caps in parallel.

[EWBrown]

I rAan a uick current test today with the smaller quadrupler, with a 120V 4 W bulb load, and 29VAC input, the DC output current load was 32.6 mA@ 123VDC,
the input AC current was 153.2 mA, approximately 4.8X the DC output, whih is pretty close to the 5X DC loading that I had figured on.

I'll be assembling another "heavy duty" HV quadrupler soon, and I'll set it up for easier current monitoring, rather than having to "hawire" it for the testing.

THis one will be using the hefty 72VAC trannie I mentioned earlier, and 1200 uF / 250V caps, and higher current rectifiers, at least 10 amp rated.

Stay tuned....

/ed B

[Blair]

Thanks guys!

Not unfortunately, but I will not be using this transformer in this configuration anymore. A viewer saw my post and want to save the amplifier from being cannibalized. They made a generous trade offer and I'm mailing it off as soon as my house gets over the flu.

Blair