Voltage Multipliers on the Move

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Voltage Multipliers on the Move

Postby EWBrown » Fri Jul 27, 2007 12:27 pm

This started under the ST70 topic, but I'll start its own section over here

Just today I read in AudioXpress magazine an article about voltage quadruplers where that autor suggests can be used to get the recycled solid state PT suitable for the tube projects.

In theory sounds ok, not sure what the heck it would look like in practice.



Voltage Quadruplers aren't all that efficient, basically they consist of two "half-wave" voltage doublers, cinnected in series. They're OK for light current loads, but pretty much NG for power amp levels of current loading, as they tend to drop off rather rapidly with increasing current demand. The PS tranny needs 4X or greater of the expected delivered DC HV current.

Sometimes these were used with direct AC line voltage. NOT a good idea... This always delivers a HOT chassis

The basics of voltage doubling and quadrupling can be found here:

http://www.nmu.edu/www-sam/tlac/spotlig ... pliers.pdf

and a few more ideas here: http://www.voltagemultipliers.com/html/multcircuit.html

And if you're into some serious arcs and sparks, check ouut this Cockcroft-Walton HV multiplier:

http://members.tm.net/lapointe/Cockcroft_Walton.htm

Back to the real world, the full wave voltage doubler is a tried and true performer, and with SS rectification and hefty electrolytics, it's a good way to get usable B+ voltages out of isolation and other low voltage (100-150 VAC) power trannies.

Remember that the primary current capability has to be at least twice (three X is better) that odf the delivered DC B+.


HTH

/ed B in NH
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Postby parabellum » Fri Jul 27, 2007 3:58 pm

Yeah I knew something had to be wrong when I read it otherwise everyone would be doing it Yellow_Light_Colorz_PDT_12
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Postby erichayes » Sat Jul 28, 2007 12:34 am

Hi All,

I'm the first to admit that I use voltage doublers in low current (line amp, bias, etc.) applications when it's warranted, but I am adamant in my opposition to VDs in power amps.

The doubler was essentially a parlor trick until the early '50s, when television manufacturers wanted to eliminate the cost of a power transformer. Series string heater tube types were developed, and a half-wave doubler was used to get the roughly 300 VDC needed to run the sets. Half-wave doubling allowed the negative end of the output to be combined with one side of the AC line, something that a full-wave doubler can't achieve. The downside was that it created a "hot" chassis that could knock you on your ass if it was plugged in wrong and you got between it and Earth ground. But it worked.

For some reason, the Hi Fi manufacturers picked up on doubling, but decided to retain the power transformer and go with full-wave. This is where the logic escapes me. As Ed pointed out, you need at least around 150% of power going into a voltage multiplier to get the desired power out. There's no smoke and mirrors involved.

With a conventional full-wave rectifier, be it center tapped or bridge, the power transformer is supplying energy 100% of the time, and filtering capacitance can be in the 20 to 40 µF range for the first leg. With full-wave voltage doubling, the power transformer has to have twice the current rating (but only half the voltage), the rectifiers (for power amps) have to be solid state, and the input caps (plural) have to be at least 200 µF in order to emulate a conventional full wave configuration.

Voltage doubling provides relatively poor regulation, as it's relying on capacitors to provide the final B+ potential, rather than the power transformer. Poor regulation translates into flabby bass response and ill-defined instrument placement . . . not what you want in a true Hi Fi power amp.
Eric in the Jefferson State
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Postby Tom Bavis » Sun Jul 29, 2007 12:45 pm

Misconceptions... the doubler and full-wave bridge are identical in efficiency and regulation EXCEPT for one factor - the input capacitors carry more ripple current in the doubler, with a 60 Hz component in each. With modern capacitors, this isn't a problem. The full-wave center-tap lags FAR behind, needing a 40% larger transformer to deliver the same current and regulation.

For todays exercise, we have a transformer with 120V primary, 10 Ohms DCR, four 120V secondaries, 40 Ohms each. Three possible circuits to deliver 300V at 200 mA- FWB, FWCT, doubler, using the secondaries in series-parallel, series or all in parallel. Three caps are provided, 2 each 470 @ 250V, and 330 @ 400V.

First person to simulate and print the three circuits (with currents) will get the three caps so that they can try the circuits for real. You'll have to buy your own transformer - sorry!
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Postby soundmasterg » Mon Jul 30, 2007 11:23 am

I've got a Conn organ power amp that I've modified into a guitar amp and added a preamp chassis. THere are 3 power amps one the Conn, and three preamps on my preamp chassis, so 3 completely seperate amps on one head except for the sharing of the power transformer. The PT is using a voltage doubler and was from the beginning. The OT's are large but not hi-fi large. The amp has plenty of bass response and I haven't noticed any sagging audibly. It had a half wave doubler stock and I've changed it to a full wave doubler setup. I don't mind using a voltage doubler setup at all...it seems to work quite well.

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Postby EWBrown » Mon Jul 30, 2007 12:47 pm

Fred Nachbaur used a voltage quadrupler in his "6X8" amp:

Image


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Postby mhuss » Wed Aug 22, 2007 11:01 am

The Fender PS300 (300 watt bass amp) used a voltage doubler circuit of sorts to produce the 0...350...700 volt supply needed. http://mhuss.com/PS300/Schematic.pdf

Humongous PT.
Image

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Postby mesherm » Wed Aug 22, 2007 1:27 pm

That is a serious chunk of iron. I wonder how big an equivalent switching power supply would be? Probably no larger that a 450 watt computer supply I bet. Certainly would weigh a lot less.
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Postby TerrySmith » Wed Aug 22, 2007 3:53 pm

Would there be any side effects of using a voltage multiplier on the 6.3v heater winding for a bias supply?
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Postby Shannon Parks » Thu Aug 23, 2007 5:41 am

TerrySmith wrote:Would there be any side effects of using a voltage multiplier on the 6.3v heater winding for a bias supply?


Since the load remains constant - the filaments, that is - there doesn't seem to be a fluctuating load that would cause havoc to the regulation. So this seems OK to me. The start up sag will probably be no problem, too.
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