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In Reply to: RE: A particular Marshall and "Voltage Doubling" posted by FenderLover on July 21, 2014 at 19:34:31
Oh, the amp works beautifully. Brings a man to tears!
But what baffles me is that many transformer designs (including the 3B Super100) have PT's which pump out some 550V+ for the same current.
That said, there's something funny about how this thing "delivers". I had some time with it with the preamp at 7 (remember, no master V) and a Strat (that's it lol). There's something about how it delivers this sorta strained-out current out of the power section, which is only slightly further restrained by a low-ohm choke. BUT! I think there's a lot of voltage sag under a fair load. So, in a flash, as you're about to seemingly drive the amp into oblivion, B+ drops, and that chokes kicks in and cleans it up. BUT!!! On top of that, there's this constant drive and grit thanks to the higher voltages down the line in the preamp when idle. So, in the middle of a mid-rift crunchy bit, you can drop this clean deep E out of nowhere!
I don't know if that makes any sense... It's indescribable...
So, I believe there's magic in the transformer. But how does that voltage doubling work? Are there other documented examples of this practice which I can study?
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May your tubes be lively and long-lasting. Holy be thy heater.
Follow Ups:
During this stage of Marshall's history, RS Components supplied Marshall with most of the OUTPUT transformers. Their "De-luxe" (most potent, at the time) model could really only handle 30-40 watts. So, Marshall made several prototypes using two OPT's. Two GZ34's. And four 6L6GC's.According to Doyle, these twin OPT Marshall lead amps had a very distinct tone versus later models with a single 100-watt OPT (1959 100-watt Lead).
Not much (read: nothing I could find) about two PT version of this amp.
Edits: 07/21/14
Don't misunderstand! ^^ I'm referring to the power transformer, not the output. And its true, '65 Marshall amps had some funny dual-OPT models at first.
Anyway, it's not that it uses two transformers, it uses two seemingly identical windings instead of one main winding to step up AC voltage for rectification. And like the article says, the windings deliver 95VAC each (which are then rectified and joined again into a single B+ line at 530V or so).
What a weird setup...
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May your tubes be lively and long-lasting. Holy be thy heater.
With floating outputs, you can stack the voltage. If you have two 250 volt supplies, you can hook the bottom of the first supply onto the top of the second and get 500 volts. Each of the 95VAC supplies could be doubled and then stacked to come up with +500v. The supply would be pretty springly. I would not doubt it if this was done because Marshall got a super deal on a pallet of 95 volt transformers.
"I would not doubt it if this was done because Marshall got a super deal on a pallet of 95 volt transformers. "
I've been tempted to do the same in my amplifier designs, simply because isolation transformers (120V:120V) sometimes go on sale at ridiculously cheap prices. Only thing is, you'd have to get a super deal on electrolytics at the same time!
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Buy Chinese. Bury freedom.
Yes, I know you stated the amp has two PT's. And only one OPT. I thought it was interesting that the same amp (early 100 watt Leads) are noted for a few with two OPT's and one PT.
Be interesting to see if there are any 1959 prototypes with two OPT's and PT's.
From the photos in the link, looks like one PT and one OPT side-by-side. If these are both PT's, where is the OPT? I think that the tranny on the top is a choke.
Thanks!
No, there was this weird variation as well with the two small power transformers
I'm sorry, but you're mistaken. Those are the two output transformers. The hidden one under the metal shroud is the power transformer. It's the same setup as pictured in the link below. It's a well-documented amp. In my OP, in that link, look for the Version 2 (Mid-Late 1965) amp (right at the top of the page). The amp you pictured is that amp. Hands down without a doubt.
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May your tubes be lively and long-lasting. Holy be thy heater.
Ya, like Alpha Al said, it's two windings in the one PT. I'm only beginning to understand transformers in terms of design, and this threw me through a bit of a loop. Not to mention the fact that I knew nothing about bridging transformers like that. I'm still scratching my head a little, but I get the general principle.
Thanks for the awesome replies! The AA always provides!
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May your tubes be lively and long-lasting. Holy be thy heater.
He stated elsewhere it's one PT, but with two identical windings, each with it's own doubler.
Al,
Have you worked on any of these Marshall amps? I've never seen one with dual windings on the PT. At least, not one where I had to replace this PT. Are they still being wound?
Thanks, again.
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