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Old 14th December 2002, 10:58 PM   #1
G is offline G  United States
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Default Need help with circuit operation. Please.

Hi Guys,

I'm interested in using this schematic but I don't follow how the gain stage/phase splitter is operating. As far as I can tell the gain stage/phase splitter does not have a DC reference on the cathodes only an AC reference. I see a 100 ohm resistor between the cathodes with a 100uF cap from one side of the resistor to ground. Is this a mistake in the drawing or do I not understand what I'm looking at (most likely)? Can anyone explain the operation of this stage? I would appreciate it much.

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Old 15th December 2002, 12:11 AM   #2
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Default THE WHOLE PICTURE....

Hi Gavin,

Yes,circuits like this can get a bit confusing....
Thing is you see grounding is taken,with B+ for the preamp to a separate chassis.
So as long as you don't see these two as a pair it won't make much sense.

The amp as shown get its signal straight into the phase splitter and that drives the output stage.

Cheers,
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Old 15th December 2002, 05:28 AM   #3
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Gavin,

Yes, there is a dc reference to the cathodes through the 1000ohm feedback resistor and the secondary of the transformer.

cheers

peter
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Old 15th December 2002, 11:00 AM   #4
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Good eyes. So I guess that it is workable as drawn assuming that you are using a preamp.

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Old 15th December 2002, 04:42 PM   #5
Joel is offline Joel  United States
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No preamp needed. 2V straight in from the CD will definitely get you full power from this amp. Just ignore the arrow extending the B+ to the preamp.
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Old 16th December 2002, 03:38 AM   #6
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Default SHORTSIGHTED?

Hi,

When that preamp is not connected B+ is bound to rise at the amp.
You better look at how this combo behaves and write down the voltages.

Cheers,
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Old 16th December 2002, 02:29 PM   #7
Joel is offline Joel  United States
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Frank, I doubt the increase will be anything to worry about - in fact, they probably chose the values of PS resistors assuming there would be no additional draw. Anyway, a pre (from that era) is just a bunch of 12AX7's, and draws only a few mA.
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Old 16th December 2002, 02:35 PM   #8
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Default FAIR ENOUGH

Howdy,

True,it won't prevent the amp from functioning.
OTH it won't hurt to play it safe either.

The 470K input R can easily be replace with a 500K volpot and you're set.

I must say I quite like the elegant simplicity of that amp.

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