|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
24.17.48.137
In Reply to: RE: A particular Marshall and "Voltage Doubling" posted by TubeAcolyte on July 21, 2014 at 19:26:12
It was potentially a bit easier for them to source a transformer with the lower voltage windings.
Voltage doublers work nicely in a class A amp, but don't offer terrific regulation, so when you transition to class B, you'll get quite a bit of sag. (Maybe part of the design intention?)
Doublers also let you get away with lower radiated magnetic field from the power transformer.
As far as how the doubler works, there are a lot of articles online with neat little animations. If you connect what would be the ground lead of one of the supplies to the hot side of the other, then you have one high voltage supply.
Follow Ups:
Properly executed voltage doublers do provide excellent regulation. They have the lowest DC resistance of any voltage comparable power supply. There is a reason McIntosh, HK Citation and other top of the line vintage amps used voltage doublers, and with modern diodes and electrolytics they get still better.
"They have the lowest DC resistance of any voltage comparable power supply. "
I've never heard that. What's the reasoning behind it? Is this a reference to the resistance of the secondary winding of the transformer?
--------------------------
Buy Chinese. Bury freedom.
Yes (partly) Since the transformer only has to generate about half the voltage the secondary is much shorter and since the transformer has to generate twice the amperage the secondary winding is of heavier gauge copper. Also solid state diodes DC resistance is much lower if compared to a tube rectifier.
Assuming proper transformers of the same VA rating, a voltage doubler and full-wave bridge will perform exactly alike: same output current, same supply impedance, same regulation, ripple voltage, etc. A full-wave center-tap rectifier will lose out on current rating or regulation unless it uses a transformer about 30% larger. Still, it's difficult to use anything else with a tube rectifier.
Cost of rectifiers and limited PIV ratings were a factor in the past that may have pushed designers to the doubler circuit. Stacked supplies were sometimes used to use lower PIV diodes.
vintage amps like the Marantz 8 and 9, Heathkit W-6. Don't think cost was a major consideration with them
The Marantz 8, in all of its file cabinet sheet metal glory was definitely cost constrained. The high PIV diodes back then were not like they are now. Getting a diode to stand up reliably to high voltage was hyper-expensive back then. The Model 8B, was first offered for sale in 1961 or '62, for $250. $250 in 1962 has the same buying power as $1,973.04 in 2014.
With any manufacturer cost is always a consideration. Back in the late 50's and early 60's solid state rectifiers and large capacity electrolytics were expensive. Their cost was second only to the transformers. Voltage doubler power supplies were the most expensive power supply to make back then. They were used because they yeiled superior results. Today they have gotten still better with modern components.
This is all assuming that you are ignoring choke input filters.
Yes, there's no way to implement choke input with the voltage doubler (there is a "current doubler" configuration with two chokes that cuts voltage in half though - not likely to be useful for high voltage).
Choke input with a full-wave bridge will have better regulation and/or current capacity than with full-wave center tap with same size transformer. Choke input with solid-state rectifiers (why did my spell-checker try to change it to "sordid state"?) will require a snubber of some sort - cap, RC, MOV - or diodes will be zapped sooner or later.
I believe that the 100 watt Marshall lead amp (model 1959) ran a network of four 50mfd filter caps (parallel and in series) in the first filter stage. This stage supplied power to the OPT and output tubes. Like most Fender amps, the choke was applied prior to the gain stage taps.
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: