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Old 16th September 2006, 08:41 PM   #1
Bryan is offline Bryan  United States
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Default Help me understand the topologies here...

Hey Guys,

I've taken a break from building for alittle while, and now want to begin building a few guitar amps.

I've been looking at some different amp schematics, and trying to wrap my hands around push-pull phase splitters.

So, in the two diagrams below (Fender Tweed Deluxe and a Vox AC30) I am unsure about how the slpitters work. The Fender diagram seems fairly straight forward where the first half of the 2nd 12AX7 drives the 2nd half and this is where the two opposing phases are taken from to feed the power tubes...

But, with the second diagram, I don't see how the phase is split.... Any help here would be great.

Thanks,

Bryan
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Old 16th September 2006, 08:49 PM   #2
Bryan is offline Bryan  United States
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and the second schematic...
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Old 16th September 2006, 08:56 PM   #3
SY is offline SY  United States
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It's a so-called "long tail pair." Basically, the signal is coupled via the cathodes of the phase splitter- the 47k resistor can be thought of as a crude current source. As the current in one tube goes down, the current in the other goes up, forced by the current source.
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Old 16th September 2006, 09:07 PM   #4
Yvesm is offline Yvesm  France
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It's a clasiical so named Long Tail Phase splitter.
Both triode share tne same cathode load resistor.
You may consider the top one as a usual common cathode inverting stage, and the bottom one as a grounded grid cathode driven non inverting one.

Ideally, the common cathode load should have an infinite impedance and is sometime replaced by a constant current source.

This way, another view is that, if the sum of the current in the two triodes is forced to be constant, then, when the current of the top one changes (because the signal at its grid) in any direction, the current in the bottom one must changes too in the oposite direction to keep current constant.

Yves.

Cross post !
Hi SY, no journey planned for "Vendanges en Ardeche"
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Old 17th September 2006, 12:44 AM   #5
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Exactly as Yves says - a good explanation. [BTW, a long-tail split definitely tastes better with currant sauce ]

The first schematic is questionable use of a concertina splitter, very much like the Dynaco ST70. It would be better to have a driver stage between the splitter and the OP tubes, like Williamson did, to provide enough headroom so that the splitter doesn't overload before the OP tubes.
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Old 17th September 2006, 02:06 AM   #6
Bryan is offline Bryan  United States
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Interesting...

I'm learning a bunch here as usual!

I think the overload is desired in this application, as it is a guitar amp known for it's early break-up.

The strange part of this all is going from SET designs optimized for HiFi, compared to guitar amps which are "down and dirty."

BK
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Old 17th September 2006, 06:53 AM   #7
poobah is offline poobah  United States
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The Vox AC30 had a very popular option. I can't recall the name, whether this option had an additional chassis, or built in etc... I do recall that is was additional early gain stage; the purpose of which was to get some overdrive and crunch.

If you google for schems, you'll find this mod/option. I've seen it in several places. If not, holler, and I'll retrace my steps and find the info.

Not sure about this , but I don't think an LTP (diff stage) is really the right beast for overdrive.

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Old 17th September 2006, 10:40 AM   #8
SY is offline SY  United States
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Quote:
Hi SY, no journey planned for "Vendanges en Ardeche"
Malheureusement, non. At the moment, I'm in Spain.

Will you be at ETF this year? If so, we can try an interesting VDP from a friend of mine in Ampuis.
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Old 18th September 2006, 07:08 AM   #9
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Quote:
Originally posted by SY


Malheureusement, non. At the moment, I'm in Spain.
Hi SY.

As you are near, if you decide to come to Portugal , I will be pleased to invite you for a dinner and show you some good Portuguese wines...
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